# NCERT Physics Class 11 Exemplar Ch 5 Laws Of Motion Part 6

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Q. 31. A gun fires a ball of horizontally from a cliff of height . It falls on the ground at a distance of 400m from the bottom of the cliff. Find the recoil velocity of the gun.

Answer:

Q. 32. Figure shows diagram of a particle moving in 2-dimensions

If the particle has a mass of , find the force (direction and magnitude) acting on the particle.

Answer:

y-axis.

Q.33. A person in an elevator accelerating upwards with an acceleration of , tosses a coin vertically upwards with a speed of . After how much time will the coin fall back into his hand?

Answer:

## Long Answer

Q.34. There are three forces acting on a body, all acting on a point P on the body. The body is found to move with uniform speed.

(a) Show that the forces are coplanar.

Answer: Since the body is moving with no acceleration, the sum of the forces is zero be the three forces passing through a point. Let be in the plane A (one can always draw a plane having two intersecting lines such that the two lines lie on the plane). Then must be in the plane A. Since is also in the plane A.

(b) Show that the torque acting on the body about any point due to these three forces is zero.

Answer: Consider the torque of the forces about P. Since all the forces pass through P, the torque is zero. Now consider torque about another point 0. Then torque about 0 is

Torque

Since