Chapter 4 : Science - Class 8
Topic: Force and Pressure
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A. Fill in the Blanks
1. For a force to come into play, the two concerned bodies must interact with each other.
2. To draw water from a well, we have to pull the rope.
3. An unbalanced force, acting on a moving object, may change its speed and/or its direction.
4. In the game of cricket, the 'bails' fall when the ball strikes them. This is an example of a/an contact force.
5. 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 electric force.
6. The atmospheric pressure decreases as we move towards higher altitudes.
B. State True or False for the following statements
1. The effect of a force may change by changing the direction of the applied force. - TRUE
2. A force has to be specified both in terms of its magnitude as well as its direction. - TRUE
3. Frictional force is an example of a non-contact force. - FALSE
4. 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). - FALSE
5. We can reduce the pressure of a given force by increasing the area over which the force acts. - TRUE
6. Inside a bottle filled with water, pressure is least at the bottom and the greatest at the surface of water. - FALSE
C. Tick the correct option.
1. When two forces, applied on an object, are equal and opposite, then these forces—
i. move the object in the opposite direction
ii. cancel each other and bring the object to rest
iii. do not move the object but may cause a change in its shape ✅
iv. make the object float in air
2. The following are the names of some of the well-known forces
A. gravitational force
B. muscular force
C. magnetic force
D. frictional force
Out of these, the 'contact forces' are the forces labelled as—
i. A and C
ii. A and D
iii. B and D ✅
iv. B and C
3. 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—
i. push the box in the east direction with a force of magnitude 'F'
ii. pull the box in the east direction with a force of magnitude 'F'
iii. push the box upward with a force of magnitude 'F'
iv. pull the box in the west direction with a force of magnitude 'F' ✅
4. 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?
i. pressure at A > pressure at B > pressure at C
ii. pressure at B > pressure at A > pressure at C
iii. pressure at C > pressure at B > pressure at A ✅
iv. pressure at A = pressure at B = pressure at C
5. 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—
i. position A
ii. position C
iii. position B ✅
iv. all positions exert same pressure
6. 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—
i. gravitational pull
ii. suction pressure created by water
iii. magnetic force
iv. atmospheric pressure ✅
7. 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?
i. the pressure exerted is same in all three cases
ii. the pressure exerted is maximum when the 'length' and 'breadth' sides form the base
iii. the pressure exerted is maximum when the 'length' and 'height' sides form the base
iv. the pressure exerted is maximum when the 'breadth' and 'height' sides form the base ✅
D. Answer the following questions in brief.
1. 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: The agent of force is the fingers. The effect of the force is that the shape of the lemon changes and juice comes out.
(b) A person diving into a swimming pool.
Answer: The agent of force is the person's muscles. The effect of the force is that the person starts moving and changes position.
(c) A labourer moving a loaded cart.
Answer: The agent of force is the labourer. The effect of the force is that the cart starts moving or its speed changes.
(d) A car coming to rest once its engine is switched off.
Answer: The agent of force is the friction between the tyres and the road. The effect of the force is that the motion of the car slows down and it finally stops.
(e) An athlete making a high jump to clear a bar kept at a certain height.
Answer: The agent of force is the athlete's muscles. The effect of the force is that the athlete moves upward into the air.
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, if a net force is acting on the object, its speed must change. This is because a non-zero force means the object is either speeding up or slowing down.
3. Distinguish between:
(a) Balanced and unbalanced forces.
Answer: Balanced forces are when the total force on an object is zero. They do not change the motion of the object but can change its shape. Unbalanced forces are when the total force is not zero. They can change the motion, speed, or direction of the object and may also change its shape.
(b) Contact and non-contact forces.
Answer: Contact forces are forces that act only when two objects touch each other. For example, muscular force and friction. Non-contact forces act even when objects are not touching each other. For example, gravitational force, magnetic force, and electric force.
4. When we throw a ball upwards, what happens to its speed while it is going up? Give reason for your answer.
Answer: When we throw a ball upwards, its speed decreases as it goes up. This happens because gravity pulls the ball downwards and air also pushes against its motion.
5. 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: The force responsible is gravity. It is a non-contact force because it acts without touching.
(b) Will the man come down with the same speed without the parachute? Explain.
Answer: No, without the parachute he will fall much faster. The parachute creates a lot of air resistance which slows him down.
6. How would the pressure (exerted by a force on an area) change if:
(a) the area is doubled keeping the thrust constant?
Answer: If the area is made double but the thrust stays the same, then the pressure will become half.
(b) thrust is doubled keeping the area constant?
Answer: If the thrust becomes double but the area stays the same, then the pressure will also become double.
7. 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: A diver will feel more pressure at 200 cm depth. This is because pressure in water increases with 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? Explain with examples.
Answer: Force is a push or a pull experienced by an object when it interacts with another object. No, it is not always necessary for two bodies to be in direct contact for a force to exist. Some forces can act from a distance. For example, a magnet can attract an iron nail without touching it, and gravity pulls objects towards the Earth without direct contact.
2. State three effects of a force. Give suitable examples.
Answer: The three main effects of a force are:
- A force can change the state of motion of an object. Example: Kicking a stationary football causes it to move.
- A force can change the direction of motion. Example: A batsman hitting a cricket ball can change its direction.
- A force can change the shape of an object. Example: Pressing a balloon between your palms can change its shape.
3. Give reasons for the following statements:
(a) 'Snowshoes' are more effective than ordinary shoes for walking on snow.
Answer: Snowshoes are broader, which increases the surface area in contact with snow. Since pressure is inversely proportional to area, the greater area reduces the pressure on the snow, preventing the wearer from sinking in.
(b) Porters place a round cloth pad on their heads while carrying loads.
Answer: The cloth pad increases the contact area between the load and the head. This reduces pressure on the head, making it more comfortable to carry heavy loads.
(c) A sharp knife cuts better than a blunt one.
Answer: A sharp knife has a smaller surface area at its edge, which increases the pressure applied for the same force, allowing it to cut more effectively.
(d) School bags have wide straps.
Answer: Wide straps increase the contact area with the shoulders, thereby reducing the pressure and making the bag more comfortable to carry.
(e) A balloon bursts more easily with a needle than with a finger.
Answer: A needle has a much smaller contact area, which increases the pressure at the point of contact, making it easier to puncture the balloon.
(f) Mountaineers may suffer from nosebleeds at high altitudes.
Answer: At high altitudes, atmospheric pressure is lower than the body's blood pressure. This imbalance may cause delicate blood vessels in the nose to burst, resulting in nosebleeds.
(g) Aircraft cabins are pressurized.
Answer: Atmospheric pressure is very low at high altitudes. To ensure passenger comfort and safety, aircraft cabins are pressurized to maintain a suitable internal pressure.
4. Two rods, A and B, of the same weight and length, but different thicknesses, are held vertically on sand. Which one will sink less? Explain.
Answer: Rod B, which has a greater thickness and hence a larger surface area, will sink less. This is because pressure is inversely proportional to surface area. For the same force (weight), a greater area means less pressure on the sand.
5. A can has three holes—A, B, and C—at different heights. The holes are sealed, and the can is filled with water. What happens when the tape is removed?
Answer: Water will gush out from all three holes. The hole at the bottom (A) will eject water the farthest due to the highest pressure. The hole at the top (C) will eject water the least distance because it has the least pressure acting on it.
6. Two identical holes are made at the same height on the sides of a water-filled plastic bottle. What will you observe when the holes are opened?
Answer: Water will come out from both holes with equal force and reach the same distance. This is because both holes are at the same height and experience the same pressure.
7. How does atmospheric pressure vary with altitude? Does temperature also affect atmospheric pressure?
Answer: Atmospheric pressure decreases with increasing altitude because the column of air above becomes shorter. Yes, temperature changes also affect atmospheric pressure. Uneven heating of the Earth's surface causes differences in air temperature, which in turn causes variations in atmospheric pressure and results in wind movement.