# Class VIII Physics Force and Pressure

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1. Are these two forces equal? [MRK]

Ans: No, both the forces are different as they are acting in different directions.

2. What is a force? How can you test for sure if a force is acting or not? [2 MARKS]
Ans: A push or a pull on an object is called a force.
If a body changes its state of motion, then we can say for sure that a net force was acting on the body.

3. In which direction will the rope move if
a. Girl’s team is winning
b. Both the teams apply an equal force? [2 MARKS]

Ans: a. The rope will move towards the right as net force is towards the right.

b. The net force on an object is zero if the two forces acting on it in opposite directions are equal. If both teams apply equal force, the red pointer on the rope doesn’t move from its position and no team wins.

4. Give two examples where force changes the direction of motion. [2 MARKS]
Examples where force changes the direction of motion:

1) While playing cricket the batsman hits the ball and changes the direction of the ball.

2) While playing volleyball the players hit the ball either towards their teammates or towards the opposite team thus changing the direction of motion.

5. Is it more comfortable to have thin straps for school bags or broad? Why is that? [2 MARKS]

Ans: Thick strap bag would be more comfortable than the thin strap ones.

SThe surface area of broad straps is more than the thin straps so the same force will apply less pressure on the shoulders in case of broad straps.

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What characterises the state of motion? Explain for cases of the object at rest, uniform and non-uniform motion. [3 MARKS]
The state of motion of an object is described by its speed and the direction of motion. i.e its velocity.

Speed/Velocity = 0 => Rest
Speed/Velocity = constant => Uniform Motion
Changing speed/velocity => Non-uniform Motion

7. What is pressure? What is atmospheric pressure? [3 MARKS]

The force acting on a unit area of a surface is called pressure.

The atmospheric air extends up to many kilometres above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure.

8. A closed can of soda is thrown into the sea. After a certain depth, the can starts getting crushed. What is the reason behind this phenomenon? If we take it to Mount Everest, what will happen? [3 MARKS]

Pressure is different at different depths. With the increase in depth, the water column above increases so its pressure also increases. That’s why the can gets crushed.
At Mount Everest, the exact opposite will happen. Now, outward pressure is less than inward pressure. So can inflate.

9. Explain the following :

a. Why can’t we hammer a nail by its head on the wood?

b. A sharp knife cuts better than a blunt knife. Why? [3 MARKS]
a. The head of the nail has a larger area than the tip so the pressure is reduced in case of hammering the head instead of the tail.

b. A sharp knife has a lesser surface area than blunt one, so, a sharp knife applies a greater pressure when used for cutting.

10. What is a cyclone? What is the reason for the formation of cyclones? [3 MARKS]

Cyclone refers to any spinning storm that rotates around a low-pressure centre. The low-pressure centre is also referred to as the ‘eye’ of the storm, which is well known for being eerily calm compared with the areas under the spinning ‘arms’ of the storm. Cyclones are basically created by the differences in air pressure. A cyclone is formed when a system of winds moving in circular motion closes in toward an area of the sea with low atmospheric pressure.

a. After combing hair when you bring your comb near pieces of paper it attracts them. What kind of force is in play here?

b. What kind of force is acting between two animals in the jungle? [3 MARKS]

a. After combing hair the comb acquires electrostatic charge and hence it attracts the paper pieces. Electrostatic is a non-contact force.

b. Gravitational force is acting between two animals in the jungle. Every object in the universe, having mass, exerts a gravitational force on every other object.

12. Observation 1: Try to stop a ball coming towards you, so as to save a boundary, by applying a force of some magnitude in the opposite direction of the motion of them all.

Observation 2: In the same case, you apply the force of the same magnitude but in the same direction of the motion of the ball.

Observation 3: In the same case, you apply the force in the opposite direction of the motion of the ball but its magnitude is less.

Ans: In which case/cases you will stop the boundary. Why is that? [3 MARKS]

Only in the first case boundary will be saved. The rest two cases will fail to save the boundary.
In the below two observations it is clear that the change in direction of force in the second observation and less magnitude in the third observation made the team cost four runs.
So, either a change in direction or magnitude of a forces causes a change in its effect.

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What changes can a force bring in an object, when applied to it. Will a force always bring a change in the state of motion of an object? [5 MARKS]
A force

– may make an object move from rest.

– may change the speed of an object if it is moving.

– may change the direction of motion of an object.

– may bring about a change in the shape of an object.
A force may not always bring a change in motion. For instance, if you push the walls of the room with your hands, the walls do not move. The net force is required.

A. If you are on a sandy beach then in which case will you make a deeper impression on the sand

a) while you are standing straight; or

b) while you are lying down

B. If you are on a sandy beach then in which case will you make a deeper impression on the sand

a) while you are standing straight; or

b) while you are walking [5 MARKS]

A. You will make a deeper impression while standing straight as compared to lying down as when you are standing the area of contact is lesser compared to lying down. On the same time, the force applied by you is same. So the pressure in the case of standing is more than lying down.

B. You will make a deeper impression while walking as compared standing straight as when you are standing the area of contact is more as you have two feet’s area compared to walking when only one foot is on the ground at a time.

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