## Topic outline

• ### Force and Pressure

##### Q37. What do you understand by state of Motion?

Ans. The state of motion of an object is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed. An object may be at rest or in motion; both are its states of motion.

##### Q38. Why the tools meant for cutting and piercing always have sharp edges?

Ans. The cutting edges of tools like blades, knives etc., are provided with sharp edges to cut the things easily because the smaller the area, larger the pressure on a surface for the same force.

##### Q39. Explain why, the tip of a sewing needle is sharp.

Ans. The tip of the sewing needle is sharp so that due to its sharp tip, the needle may put the force on a very small area of the cloth, producing a large pressure sufficient to pierce the cloth being stitched.

##### Q40. “When a ball is dropped from a height, its speed increases gradually.” Name the force which causes this change in speed.

Ans. The force of gravity causes all the objects to fall towards the earth. Thus, when a ball is dropped from a height, its speed increases gradually because of the force of gravity acting on it.

##### Q41. Explain why, porters place a thick, round piece of cloth on their heads when they have to carry heavy loads.

Ans. By this they increase the area of contact of the load with their head. So, the pressure on their head is reduced and they find it easier to carry the load.

##### Q42. Explain now why shoulder bags are provided with broad straps and not thin strap?OrWhy the straps of school bags are made broader?OrWhy is it comfortable to lift a school bag with broad straps than thin straps?

Ans. Shoulder bags are provided with broad straps and not thin strap so that the weight of the bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder.

##### Q43. A small device pulls iron nails from a distance. Which type of force is involved in this process?

Ans. Magnetic force is involved in this process. The magnetic force acts even from a distance. The magnet exerts a magnetic force on objects made of iron, steel, nickel and cobalt.

##### Q44. Write one example where force changes the direction of a moving object.

Ans. In cricket, a batsman plays his or her shot by applying a force on the ball with the bat. In this example the speed and the direction of the moving ball change due to the application of a force.

##### Q45. Explain why, frictional force is said to be a contact force.

Ans. The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion. Since the force of friction arises due to contact between surfaces, it is an example of a contact force.

##### Q46. In a tug of war; when the two teams are pulling the rope, a stage comes when the rope does not move to either side at all. What can you say about the magnitudes and directions of the forces being applied to the rope by the two teams at this stage?

Ans. If there is no movement on either side, the force applied from both side is equal in magnitude.

##### Q47. Explain why the walls of a dam are thicker near the bottom than at the top.

Ans. The walls of a dam are much thicker at the bottom than it is at the top. This is because the pressure of the water is much greater deeper down and the dam needs to be thick at the bottom so that it is strong enough to withstand this larger pressure.

##### Q48. Give three examples of non-contact forces.

Ans. The examples of non-contact forces are:

i.        Magnetic force

ii.        Electrostatic force

iii.        Gravitational force

##### Q49. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Ans. The two forces acting on the rocket are the force of gravity, which pulls the rocket towards the ground, and the force of friction due to earth's atmosphere, which opposes its motion.

##### Q50. What is meant by gravitational force (or force of gravity)? Give its one example.

Ans. Every object in the universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force. Example: Water begins to flow towards the ground as soon as we open a tap.