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
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
Q40. “When a ball is dropped from a height, its speed increases
gradually.” Name the force which causes this change in speed.
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?
Why the straps of school bags are made broader?
Why 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?
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
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
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?
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.
examples of non-contact forces are:
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.
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.