Glossary of Terms-
Force–
Definition– A push or pull upon an object resulting from its interaction with another object.
Explanation– Force causes a change in the state of motion or shape of an object. It can be applied through various actions such as pushing, pulling, lifting, etc.
Muscular Force–
Definition– The force exerted by the muscles of living beings when they push, pull, or lift objects.
Explanation– Humans and animals use muscular force for everyday tasks like moving objects or walking.
Frictional Force–
Definition– A force that opposes the relative motion or tendency of such motion of two surfaces in contact.
Explanation– It is the force that resists the motion of a moving object across a surface. The magnitude depends on the nature of the surfaces involved.
Inertia–
Definition– The natural tendency of objects to resist any change in their state of rest or motion.
Explanation– An object will continue to stay at rest or in uniform motion unless acted upon by an unbalanced force.
Balanced Force–
Definition– Forces that are equal in magnitude and opposite in direction, resulting in no change in the object’s motion.
Explanation– Balanced forces do not cause an object to accelerate or change its state of motion.
Unbalanced Force–
Definition– Forces that are not equal and opposite, leading to a change in the object’s motion.
Explanation– Unbalanced forces cause objects to accelerate or decelerate.
Contact Forces–
Definition– Forces that occur when two objects are in physical contact.
Examples– Muscular force, friction, air resistance, etc.
Non-Contact Forces–
Definition– Forces that act on an object without direct physical contact.
Examples– Gravitational force, magnetic force, electric force.
Gravitational Force–
Definition– The force of attraction between any two objects with mass.
Explanation– The Earth’s gravity pulls objects towards its surface, making things fall when dropped.
Magnetic Force–
Definition– The force exerted by magnets, which can attract or repel objects made of iron.
Explanation– Magnets exert a force without being in direct contact with other objects.
Electric Force–
Definition– The force between charged particles, which can attract or repel each other.
Explanation– Positive and negative charges attract, while like charges repel.
Pressure–
Definition– The force exerted per unit area of surface.
Explanation– It is the effect of force distributed over an area. Pressure is measured in pascals (Pa).
Thrust–
Definition– The force exerted by a body in a specific direction, usually perpendicular to the surface.
Explanation– Thrust is often discussed in the context of fluids, such as in the force applied by a jet engine.
Hydrostatic Pressure–
Definition– The pressure exerted by a stationary liquid on the walls of its container or on an object submerged in it.
Explanation– This pressure depends on the depth of the liquid and its density.
Atmospheric Pressure–
Definition– The pressure exerted by the weight of the Earth’s atmosphere on objects at the surface.
Explanation– Atmospheric pressure decreases as we ascend to higher altitudes, due to a reduction in the height of the air column.
Unit of Force (Newton)–
Definition– The unit of force in the International System of Units (SI), denoted by N.
Explanation– One newton is the amount of force required to accelerate a 1 kg mass by 1 m/s².
Surface Area–
Definition– The total area of the surface over which a force acts.
Explanation– Pressure increases as the surface area decreases for a given force.
Elastic Force–
Definition– The restoring force exerted by an object when it is stretched or compressed.
Explanation– Examples include the stretching of a rubber band or compression of a spring.
Law of Inertia–
Definition– An object at rest will stay at rest, and an object in motion will stay in motion, unless acted upon by an external force.
Explanation– This law explains why objects resist changes to their state of motion.
Spring Balance–
Definition– A device used to measure force, consisting of a coiled spring that stretches in proportion to the force applied.
Explanation– The stretching of the spring is calibrated to give a reading of the force in newtons.
Hydraulic Pressure–
Definition– Pressure transmitted through a liquid in a confined system.
Explanation– Hydraulic devices such as jacks and lifts use this principle to exert large forces through small amounts of liquid.
Pressure in Fluids–
Definition– The force per unit area exerted by a fluid at rest.
Explanation– Fluid pressure increases with depth and depends on the density and height of the liquid column above the point of measurement.
Application-based Definitions-
Caterpillar Tracks–
Definition– Special tracks used on tanks and bulldozers to distribute their weight over a larger area, reducing pressure on the surface.
Explanation– These tracks prevent the vehicle from sinking into soft ground.
Sharp Knife–
Definition– A knife with a small surface area at the edge, increasing the pressure to easily cut through materials.
Explanation– A sharp edge exerts higher pressure, making it easier to cut.
Syringe–
Definition– A device that uses atmospheric pressure to draw liquid into or expel it from a container.
Explanation– By creating low pressure inside the syringe, atmospheric pressure forces liquid into it.
Something To Know
A. Fill in the blanks:
- For a force to come into play, the two concerned bodies must with __________ each other.
Answer – interact
- To draw water from a well, we have to __________ the rope.
Answer – pull
- An unbalanced force, acting on a moving object, may change its__________ and/or its
Answer – speed, direction
- In the game of cricket, the ‘bails’ fall when the ball strikes them. This is an example of a/an __________ force.
Answer – impact
- The force, responsible for raising our body hair, when we try to take off a synthetic (terylene or polyster) shirt in dry weather conditions, is an __________ force.
Answer – electrostatic
- The atmospheric pressure __________ as we move towards higher altitudes.
Answer – decreases
B. State True or False for the following statements.
- The effect of a force may change by changing the direction of the applied force.
Answer – True
- A force has to be specified both in terms of its magnitude as well as its direction.
Answer – True
- Frictional force is an example of a non-contact force.
Answer – False
- A gas filled balloon moves up. The upward force acting on it is smaller than the force of gravity, i.e. (the downward acting force).
Answer – False
- We can reduce the pressure of a given force by increasing the area over which the force acts.
Answer – True
- Inside a bottle filled with water, pressure is least at the bottom and the greatest at the surface of water.
Answer – False
C. Tick (✅) the correct option.
- When two forces, applied on an object, are equal and opposite, then these forces-
may move the object.
may stop a moving object.
may move the object and also cause a change in its shape.
do not move the object but may cause a change in its shape.
Answer – do not move the object but may cause a change in its shape.
- The following are the names of some of the well-known forces
- gravitational force
- muscular force
- magnetic force
- frictional force
Out of these, the ‘contact forces’ are the forces labelled as—
A and B
B and C
B and D
C and D
Answer – B and D
- Rohini is pushing a box towards the west direction with a force of magnitude F. To help her move the box faster in the same direction, her friend should-
push the box in the east direction with a force of magnitude ‘F’.
pull the box in the west direction with a force of magnitude ‘F’.
push the box in the west direction with a force of any magnitude,
push the box in the east direction with a force of any magnitude.
Answer – push the box in the west direction with a force of any magnitude.
- A container is filled with water as shown in the figure below.
Which of the following statement is correct about the pressure of water in this container?
pressure at A > pressure at B > pressure at C
pressure at C > pressure at B > pressure at A
pressure at A = pressure at B = pressure at C
pressure at A < pressure at B > pressure at C
Answer – pressure at A = pressure at B = pressure at C
- An empty metal bottle has to be placed on a table so that it exerts the maximum pressure on the surface of table?
For this, the bottle need to be placed in the position(s) labelled as—
position A
position C
position B
position A and C
Answer – position A
- When we press the bulb of a dropper, with its nozzle kept under water, air in the dropper is seen to escape in the form of bubbles. If we then release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to-
pressure of water in the container
gravity of the earth
weight of the bulb
atmospheric pressure
Answer – atmospheric pressure
- The length, breadth and height, of a given rectangular box, are 100 cm, 50 cm and 20 cm, respectively. This box is kept on ground in three different ways, one by one. Which of the following statements is the correct statement about the pressure exerted, by this box, on the ground?
the pressure exerted is maximum when the ‘length’ and ‘height’ sides form the base.
the pressure exerted is maximum when the ‘length’ and ‘breadth’ sides form the base.
the pressure exerted is maximum when the ‘breadth’ and ‘height’ sides form the base.
the pressure exerted remains the same irrespective of which of its ‘sides’ form the base.
Answer – the pressure exerted is maximum when the ‘length’ and ‘height’ sides form the base.
D. Answer the following questions in brief:
- In the following situations, identify the agent exerting the force. Also, state the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
Answer – Agent: Fingers (human hands).
Effect: The force causes the lemon to compress, which results in the juice being extracted.
(b) A person diving into a swimming pool.
Answer – Agent: The person’s body.
Effect: The force of gravity causes the person to fall towards the water, and the body experiences downward motion.
(c) A labourer moving a loaded cart.
Answer – Agent: The labourer.
Effect: The labourer applies force to move the cart in the direction of the force.
(d) A car coming to rest once its engine is switched off.
Answer – Agent: Frictional force (friction between the tires and road).
Effect: The frictional force slows down the car and eventually brings it to rest.
(e) An athelete making a high jump to clear a bar kept at a certain height.
Answer – Agent: The athlete’s legs and muscles.
Effect: The athlete applies an upward force to propel themselves over the bar.
- An object experiences a net non-zero force. Is it possible for the object to move with a constant speed along a straight line?
Answer – No, an object experiencing a net non-zero force will not move with constant speed. Instead, it will accelerate in the direction of the force, which means its speed will change, either increasing or decreasing.
- Distinguish between:
(a) balanced and unbalanced forces.
Answer – Balanced Forces: Forces that are equal in magnitude and opposite in direction, and they cancel each other out, resulting in no change in the object’s motion (i.e., the object remains at rest or moves with constant velocity).
Unbalanced Forces: Forces that are not equal in magnitude or not opposite in direction. They result in a change in the object’s motion, causing it to accelerate or decelerate.
(b) contact and non-contact forces.
Answer – Contact Forces: Forces that require physical contact between two objects (e.g., friction, muscular force, tension).
Non-contact Forces: Forces that act at a distance without physical contact (e.g., gravitational force, magnetic force, electrostatic force).
- When we throw a ball upwards, what happens to its speed while it is going up? Give reason for your answer.
Answer – The speed of the ball decreases as it goes up. This is because the force of gravity acts in the downward direction, opposing the upward motion of the ball and gradually reducing its speed until it reaches the highest point, where the speed becomes zero.
- The figure shows a man with a parachute.
(a) Name the force which is responsible for his downward motion. Is it a contact force or a non-contact force?
Answer – Answer: The force responsible for his downward motion is gravitational force. It is a non-contact force.
(b) Will the man come down with the same speed without the parachute? Explain.
Answer – Answer: No, the man will not come down with the same speed without the parachute. Without the parachute, the air resistance (drag) is much smaller, and the gravitational force would cause him to fall faster. The parachute increases air resistance, slowing down his descent.
- How would the pressure (exerted by a force on an area) change if—
(a) the area is doubled keeping the thrust constant?
Answer – The pressure would halve. Pressure is inversely proportional to the area when the force is constant. So, if the area is doubled, the pressure is reduced by half.
(b) thrust is doubled keeping the area constant?
Answer – The pressure would double. Pressure is directly proportional to the force (thrust) when the area is constant. So, if the thrust is doubled, the pressure would also double.
- When will a diver ‘experience a greater pressure’—100 cm below the surface of water or 200 cm below the surface of water? Justify your answer.
Answer – The diver will experience greater pressure 200 cm below the surface of water. This is because pressure in a fluid increases with depth. The pressure at a greater depth is higher due to the weight of the water column above, and since the depth at 200 cm is greater than at 100 cm, the pressure at 200 cm will be higher.
E. Answer the following questions.
- Define force. Is it necessary for the two bodies to be in direct contact for a force to exist between them? Explain with examples.
Answer – Force Definition: A force is a physical quantity that causes an object to accelerate, change its motion, or deform. It is described in terms of its magnitude and direction.
Contact for Force to Exist: It is not necessary for the two bodies to be in direct contact for a force to exist. Forces like gravitational force, magnetic force, and electrostatic force can act over a distance without physical contact between the objects. For example, the Earth’s gravitational pull on an object or the attraction between magnets occurs without physical contact.
- State three effects of a force. Give suitable examples.
Answer – Change in Motion: A force can change the state of motion of an object, making it start moving, stop, or change direction.
Example: A car accelerates when the engine applies force.
Change in Shape: A force can cause an object to deform, change shape, or stretch.
Example: A rubber band stretches when pulled.
Change in Speed: A force can increase or decrease the speed of an object.
Example: A soccer ball slows down when friction (a force) acts on it after being kicked.
- Give reasons for the following statements-
(a) ‘Snow shoes’ are more effective than ordinary shoes for walking on snow.
Answer – Reason: Snow shoes distribute the weight of the person over a larger surface area, reducing pressure on the snow. This prevents the shoes from sinking too deep into the snow. Ordinary shoes exert more pressure on the snow due to their smaller surface area, causing the person to sink.
(b) Porters place a large round piece of cloth on their heads when they carry heavy loads.
Answer – Reason: The large cloth increases the surface area on the head, reducing the pressure exerted on the skin. This makes it easier to carry heavy loads without causing injury.
(c) A sharp knife cuts vegetables and fruits more effectively than a blunt knife.
Answer – Reason: A sharp knife has a smaller edge surface area, which increases the pressure applied to the food, allowing it to cut more easily. A blunt knife has a larger edge, reducing pressure and making cutting more difficult.
(d) School bags are provided with wide straps to carry them.
Answer – Reason: Wide straps distribute the weight of the bag over a larger area, reducing the pressure on the shoulders and making it more comfortable to carry heavy bags.
(e) It is much easier to burst an inflated balloon with a needle than (directly) with the finger.
Answer – Reason: A needle has a very sharp tip, concentrating the force on a tiny area, which increases pressure and easily bursts the balloon. A finger has a much larger surface area, so the pressure is spread out and is not enough to burst the balloon.
(f) Mountaineers may suffer from nose bleeding at high altitudes.
Answer – Reason: At high altitudes, the atmospheric pressure is lower, which can cause the blood vessels in the nose to rupture, leading to nosebleeds.
(g) Airplanes have pressurised cabins for passenger safety while flying high in the atmosphere.
Answer – Reason: At high altitudes, atmospheric pressure is much lower. Without pressurization, passengers would experience difficulty breathing, and the body could suffer from oxygen deprivation. Pressurized cabins ensure a safe and breathable atmosphere.
- Two rods, ‘A’ and ‘B’, of the same weight and equal length, have different thicknesses. A boy, while playing with them on a beach, held them vertically on the surface of sand as shown in the figure. Which one of them will sink less? Explain.
Answer – Rod ‘B’, the thicker rod, will sink less into the sand. This is because the thicker rod exerts less pressure on the sand, as pressure is inversely proportional to the area. A larger cross-sectional area (thicker rod) spreads the weight over a larger surface, reducing the pressure exerted on the sand and preventing it from sinking as much.
- Three holes, A, B and C, are made in an empty can at different levels, one over the other, as shown in the figure. They are (temporarily) closed with an adhesive tape. The can is now filled with water. What will you observe, when the tape (on all of them) is pulled out? Justify your answer.
Answer – When the tape is pulled out, water will start flowing out from all three holes. However, water will flow with greater force from the hole that is at the greatest depth (C), and with the least force from the hole at the shallowest depth (A). This happens because the pressure in a fluid increases with depth. The deeper the hole, the greater the pressure at that level, causing more water to flow out.
- Make two tiny holes, of the same size, at equal heights from the bottom in a plastic bottle. Cover them and fill the bottle with water. Let the ‘covers’ be now removed. State the ‘observations’, and the resulting ‘conclusions’, based on this activity.
Answer – Observations: Water will start flowing out from both holes when the covers are removed. However, the water from the hole on the side will flow out with more force than the water from the hole at the bottom of the bottle.
Conclusion: Water flows out more forcefully from the hole that is at a greater depth. The pressure exerted by a fluid increases with depth, which is why water flows more quickly from the hole at a lower height.
- Discuss, in brief, the variation in atmospheric pressure with altitude. Do changes in temperature also cause a change in atmospheric pressure?
Answer – Variation with Altitude: Atmospheric pressure decreases with increasing altitude. This is because as you go higher, the density of air decreases, and there is less air above pushing down, resulting in lower pressure.
Effect of Temperature on Atmospheric Pressure: Yes, changes in temperature can cause changes in atmospheric pressure. When the temperature increases, air expands, becoming less dense, which lowers the pressure. Conversely, cooler air is denser and increases the pressure.
Value Based Questions
On coming back from School, Anudit asked his mother to explain to him the concepts related to the addition/subtraction of forces acting along the same line. Mother explained to him the reason for adding such forces when they act along the same direction and of subtracting, one from the other, when they act along mutually opposite directions. She went on to say that, in a similar manner, we get an ‘added up and enhanced effect’, when we use our strength, hard work and attention, ‘all together’, for achieving our desired goal.
- State the ‘values’, conveyed by Anudit’s mother.
Answer – Values conveyed by Anudit’s mother:
The importance of combining strength, hard work, and focus in order to achieve a desired goal.
The concept of teamwork, where efforts in the same direction lead to enhanced results.
The value of persistence and determination in overcoming challenges, just as forces are added or subtracted based on their directions.
- Write down the magnitude and direction of the net force in the following situations.
(a) In tug of war, Team A is applying a force of 100 N along the east direction and Team B is applying force of 200 N along the west direction.
Answer – Magnitude and direction of the net force:
(a) In the tug of war:
Team A applies 100 N towards the east and Team B applies 200 N towards the west. Since the forces are in opposite directions, the net force is:
Net force = 200 N (west) – 100 N (east) = 100 N towards the west.
(b) A boy is pushing a box with a force 100 N towards left by overcoming a force of friction that has a magnitude of 50 N.
Answer -(b) In the case of the boy pushing a box:
The boy applies 100 N towards the left, and the frictional force opposes the motion with 50 N towards the right. The net force acting on the box is:
Net force = 100 N (left) – 50 N (right) = 50 N towards the left
- Have a group discussion in which students give examples of ‘combined’ and similarly directed efforts of a number of individuals, producing enhanced/better ‘positive results’.
Answer – Group discussion on combined efforts:
Examples of ‘combined’ and similarly directed efforts producing enhanced results can include:
A group of athletes training together to break a record.
A team working collaboratively on a project, where each member’s effort leads to greater success.
A community coming together to clean a neighbourhood, where the collective efforts achieve a cleaner environment faster.
A group of friends planning a surprise event, with each one contributing their best, resulting in a highly successful surprise.
These examples illustrate how teamwork and focused efforts, much like forces acting in the same direction, lead to more effective outcomes.
Additional Questions and Answers
Short Questions and Answers-
Question – What is a force?
Answer- A force is a push or pull that can change the state of motion of an object or its shape.
Question – What happens when two equal and opposite forces act on an object?
Answer- The object will not move, but its shape may change.
Question – How does friction affect motion?
Answer- Friction opposes the motion of objects and slows them down.
Question – What is the direction of the force in a tug of war?
Answer- The direction of the force is opposite for the two teams—one pulls in one direction and the other in the opposite direction.
Question – What is atmospheric pressure?
Answer- Atmospheric pressure is the force exerted by the weight of the air above us on the surface of the Earth.
Question – What is the effect of increased area on pressure?
Answer- Increasing the area decreases the pressure for a given force.
Question – What happens to an object when an unbalanced force acts on it?
Answer- The object will accelerate in the direction of the unbalanced force.
Question – Why do mountaineers wear oxygen masks at high altitudes?
Answer- Because the atmospheric pressure is lower at high altitudes, leading to a decrease in oxygen levels.
Long Questions and Answers-
Question – Explain the difference between contact and non-contact forces.
Answer – Contact forces are those forces that require physical contact between objects to act. Examples include frictional force, muscular force, and normal force.
Non-contact forces are those that act even when the objects are not in direct contact. Examples include gravitational force, magnetic force, and electrostatic force.
Explanation- Contact forces occur due to physical interaction between objects, such as when a person pushes a box or when friction occurs between a moving object and the surface. Non-contact forces, on the other hand, can act over a distance. Gravitational force, for instance, acts between the Earth and an object without any physical contact.
Question – What is the effect of a force on a moving object?
Answer – A force can change the speed, direction, or shape of a moving object.
Explanation- If an unbalanced force is applied to a moving object, it can alter its velocity by speeding it up, slowing it down, or changing its direction (e.g., a car turning left or right due to steering). It can also change the shape of the object if the force is large enough, such as bending a metal rod.
Question – How does pressure change with depth in a fluid?
Answer – Pressure increases with depth in a fluid.
Explanation- As we go deeper into a fluid, the weight of the fluid above increases, resulting in an increase in pressure. This is why pressure is greater at the bottom of the ocean compared to the surface. For example, at 200 cm below the surface of water, the pressure is higher than at 100 cm due to the increased weight of the water above.
Question – Explain how force is involved in the action of a parachute.
Answer – Explanation- When a person jumps from a plane, gravity pulls him downwards. However, the parachute creates air resistance, which counteracts the force of gravity. The force of gravity is pulling the person down, but the air resistance from the parachute slows the fall, leading to a safe landing. This is a combination of gravitational force (a non-contact force) and air resistance (a contact force).
The parachute reduces the speed of descent by increasing the surface area that interacts with the air, thereby enhancing the opposing force to gravity.
Question – Describe the concept of combined efforts in achieving better results.
Answer – Explanation- Just like forces can add up when acting in the same direction, human efforts can be combined to produce better results. For example, when multiple people work together towards a common goal, the result is often better and more efficient than if each person works alone. Similarly, when individual strengths, skills, and focus are combined, the overall outcome is amplified.
Example- In a team sports game like football, each player’s effort contributes to the overall success of the team. Similarly, when a group of workers collaborates on a project, the combined skills and efforts lead to the successful completion of the task faster and more effectively.