A. Fill in the blanks
- An unbalanced force, acting on a moving object, may change its speed and/or its direction.
- In the game of cricket, the ball will fall when the bat strikes it. This is an example of an unbalanced force.
- The force responsible for raising our body hair, when we try to take off a synthetic (terylene or polyester) shirt in dry weather conditions, is an electrostatic force.
- The atmospheric pressure decreases as we move towards higher altitudes.
B. State True or False
- The effect of a force may change by changing the direction of the applied force. — True
- A force can be specified both in terms of its magnitude as well as its direction. — True
- Frictional force is an example of a non-contact force. — False
- A gas-filled balloon moves up. The upward force acting on it is smaller than the force of gravity. — False
- We can reduce the pressure of a given force by increasing the area over which the force acts. — True
- Inside a bottle filled with water, pressure is least at the bottom and greatest at the surface. — False
C. Tick (✓) the correct answer
1.Force may not move the object but may cause a change in its shape.
✓ True
2.Out of the three, the contact forces are:
A. Gravitational force
B. Muscular force
C. Magnetic force
D. Frictional force
✓ B and D
3.To help Rohini move the box faster in the same direction, her friend should:
✓ Push the box in the west direction with a force of any magnitude greater than F.
4.Pressure at different points in water:
Point A (deepest) > Point B > Point C
✓ Pressure at A > Pressure at B > Pressure at C
5.To exert maximum pressure, the bottle should be placed on:
✓ Position B
6.When the dropper is released, water rises in it due to:
✓ Atmospheric pressure
7.The pressure exerted is maximum when the:
✓ Length and breadth sides form the base.
D. Answer the following questions in brief
1. In the following situations, identify the agent exerting the force and state the effect of the force.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
- Agent exerting force: Fingers (muscular force)
- Effect: The lemon changes shape and juice comes out.
(b) A person diving into a swimming pool.
- Agent exerting force: Earth’s gravity
- Effect: The person moves downward into the water.
(c) A labourer moving a loaded cart.
- Agent exerting force: Labourer’s muscles
- Effect: The cart starts moving.
(d) A car coming to rest once its engine is switched off.
- Agent exerting force: Frictional force
- Effect: The car slows down and stops.
(e) An athlete making a high jump.
- Agent exerting force: Athlete’s muscles
- Effect: The athlete moves upward.
2. 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. A net non-zero force changes the speed or direction of an object. Therefore, it cannot move with constant speed in a straight line.
3. Distinguish between
(a) Balanced and Unbalanced Forces
| Balanced Forces | Unbalanced Forces |
|---|---|
| Equal in size and opposite in direction. | Unequal forces acting on an object. |
| Do not change the state of motion. | Change the state of motion. |
| Net force is zero. | Net force is not zero. |
(b) Contact and Non-contact Forces
| Contact Forces | Non-contact Forces |
|---|---|
| Act only when objects touch each other. | Act without physical contact. |
| Example: Frictional force. | Example: Gravitational force. |
4. When we throw a ball upwards, what happens to its speed while it is going up?
Answer: The speed of the ball gradually decreases because gravity pulls it downward.
5. The figure shows a man with a parachute.
(a)
Answer: Gravity pulls the man downward. It is a non-contact force because it acts without physical contact.
(b)
Answer: Air resistance (friction) acts upward and slows down the fall, so the man does not fall at the same speed without the parachute.
6. How would pressure exerted by a force change if
(a) The area is doubled while keeping the thrust constant?
Answer: Pressure becomes half.
(b) Thrust is doubled while keeping the area constant?
Answer: Pressure becomes double.
7. Why will a diver experience greater pressure 100 m below the surface of water than 200 cm below the surface?
Answer: Water pressure increases with depth. Since 100 m is much deeper than 200 cm (2 m), the diver experiences much greater pressure at 100 m depth.
E. Answer the following questions
1. Define force. Is it necessary for two bodies to be in direct contact for a force to exist between them?
Answer:
Force is a push or a pull that can change the motion, speed, direction, or shape of an object. No, two bodies need not be in direct contact because forces like gravitational and magnetic forces act without contact.
2. State three effects of a force.
Answer:
- A force can move an object.
- A force can change the speed or direction of an object.
- A force can change the shape of an object.
3. Give reasons for the following statements.
(a) Snow shoes have a large surface area so that less pressure is exerted on snow and the person does not sink.
(b) Porters place a large round cloth on their heads to increase the area of contact and reduce pressure.
(c) A sharp knife cuts better because it has a smaller area and exerts greater pressure.
(d) A school bag is easier to carry with broad straps because pressure on the shoulders is reduced.
(e) Water enters the balloon because atmospheric pressure pushes water into the balloon.
(f) Airplanes have pressurised cabins because atmospheric pressure is very low at high altitudes.
4. Two rods A and B are placed vertically on a table. Which exerts more pressure?
Answer: Rod A exerts more pressure because it has a smaller area of contact with the table.
5. Three holes A, B and C are made in an empty can.
Answer: Water from hole A travels the shortest distance because pressure is least near the top. Water from hole C travels the farthest because pressure is greatest at the lowest point.
6. Activity Conclusion
Answer: Water flows out faster from the lower holes because water pressure increases with depth. The deepest hole experiences the greatest pressure.
7. Atmospheric pressure and altitude
Answer: Atmospheric pressure decreases as altitude increases because the density of air becomes lower at higher altitudes. Temperature also affects atmospheric pressure; warm air is lighter and creates lower pressure, while cold air is heavier and creates higher pressure.
E. Answer the following questions
1. Define force. Is direct contact necessary for force to exist?
Answer:
Force is a push or a pull that can change the motion, speed, direction, or shape of an object. No, direct contact is not always necessary. Some forces, like gravitational and magnetic forces, act without contact.
2. State three effects of a force.
Answer:
- A force can move an object.
- A force can change the speed or direction of an object.
- A force can change the shape or size of an object.
3. Give reasons for the following statements.
(a) Snow shoes are more effective than ordinary shoes for walking on snow.
Snow shoes have a larger surface area, so they exert less pressure on snow and prevent sinking.
(b) Porters place a large round piece of cloth on their heads while carrying heavy loads.
The cloth increases the area of contact and reduces pressure on the head.
(c) A sharp knife cuts vegetables and fruits more effectively than a blunt knife.
A sharp knife has a smaller edge area and exerts greater pressure, making cutting easier.
(d) School bags are provided with wide straps.
Wide straps spread the weight over a larger area and reduce pressure on the shoulders.
(e) It is easier to burst an inflated balloon with a needle than with a finger.
A needle has a very small tip, so it exerts more pressure and bursts the balloon easily.
(f) Airplanes have pressurised cabins while flying at high altitudes.
Atmospheric pressure is very low at high altitudes, so cabins are pressurised for passengers’ comfort and safety.
4. Two rods A and B are placed vertically on a table.
Answer:
Rod A exerts more pressure because it has a smaller area of contact with the table.
5. Three holes A, B and C are made in an empty can and filled with water.
Answer:
Water from hole C will travel the farthest because water pressure is greatest at the lowest hole. Water from hole A will travel the shortest distance because pressure is least near the surface.
6. Conclusion of the bottle experiment.
Answer:
Water comes out faster from the lower holes because water pressure increases with depth. The deepest hole experiences the maximum pressure.
7. Discuss the variation in atmospheric pressure with altitude.
Answer:
Atmospheric pressure decreases as we go higher above sea level because the amount of air above us becomes less. Therefore, places on mountains have lower atmospheric pressure than places at sea level
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