## Topic outline

• ### Force and Pressure

##### Q20. Why do balloons expand when filled with air?OrWhy does the balloon get inflated?OrWhat makes a balloon get inflated when air is filled in it?

Ans. Air exerts pressure on the inner walls of balloon causes it to expand and get inflated.

##### Q21. Explain why, a wide steel belt is provided over the wheels of an army tank.

Ans. The wide belts increase the area and reduce the pressure. This avoids sinking of the tank into the ground.

##### Q22. What is pressure?

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

Pressure = force / area on which it acts

##### Q23. Which force makes a rolling ball stop on its own?OrWhy a ball rolling along the ground gradually slows down and finally comes to rest?OrWhat makes a ball to stop when it is rolling?OrWhy does a moving ball stop?

Ans. It is the force of friction between the surface of the ball and the ground that brings the moving ball to rest.

##### Q24. What is muscular force? Give one example.

Ans. The force resulting due to the action of muscles is known as the muscular force. Example: lifting a bucket of water.

##### Q25. What is described as a change in its state of motion?

Ans.  A change in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion.

##### Q26. What is meant by a contact force? Give an example.

Ans. Force that can be applied only when it is in contact with an object, it is also called a contact force. Example: lifting a bucket of water.

##### Q27. Explain, why magnetic force is said to be a non-contact force.

Ans. A magnet can exert a force on another magnet without being in contact with it. The force exerted by a magnet is an example of a non-contact force.

##### Q28. Why do the shape and size of balloon change when filled with air or water?

Ans. The high pressure produced by the gas or water molecules on the walls of balloon causes it to expand and get inflated.

##### Q29. Why water comes out more slowly from an upstairs tap than from a similar tap downstairs?

Ans. This is so because water pressure is less in the upstairs tap due to lesser depth.

##### Q30. Why are we not crushed by atmospheric pressure?OrIf the pressure due to atmosphere is so great, then why are we not crushed by it?

Ans. We do not get crushed because the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside.

##### Q31. What is frictional force?OrWhat is friction?

Ans. The force that always acts on all the moving object and its direction is always opposite to the direction of motion is called frictional force (or friction).

##### Q32. Why does a sharp knife cut better than a blunt knife?

Ans. Lesser the area, larger the pressure for the same force applied. Therefore it is easier to cut with sharp knives than blunt ones, with same force.

##### Q33. What is meant by atmospheric pressure?

Ans. 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.

##### Q34. Give two examples of contact forces.

Ans. The examples of contact forces are:

i.        Muscular force

ii.        Frictional force

##### Q35. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Ans. When a blacksmith hammers a hot piece of iron, he uses his muscular force. This muscular force changes the shape of the iron.

##### Q36. What is meant by a non-contact force? Give an example.

Ans. A force that can be exerted by an object even from a distance is called a non-contact force. The force exerted by a magnet is an example of a non-contact force.