Answer:

(a)

No. The statement is not true.

There are many forces acting on an object at rest. The weight of the object is always downwards and many other forces like normal force, frictional force, push or pull may act on it. According to Newton's second law of motion, the object remains at rest when the net force acting on it is zero.

(b)

No. If the net force is zero, then according to Newton's second law of motion, the acceleration of the object is zero. Thus, the object may move with a constant velocity or be at rest. Therefore, the net force acting on the object is zero doesn't conclude that it is at rest. It can be in motion with a constant velocity.

Answer:

The weight of an object on Earth is

and its weight on Moon is

.

Mass of a body is a numerical measure of inertia. This gives that a massive body offers more inertia, or more resistance to a change in motion than a less massive body. Hence, the inertia does not depend on the weight of the body. So, the object on the Earth and on the Moon has same inertia.

Answer:

From the Newton's second law of motion, the net force acting on the system is equal to rate of change in momentum.

In other word, the net force acting on the system is equal to the product of the mass of the system and the acceleration of the system.

The expression for the net force acting on the system is as follows:

Here, m is the mass of the system.

(a)

Case-1: The hockey puck is sliding freely in the horizontal direction.

In the case-1, there is no net force acting on the puck in vertical direction.

Hence, the normal force is equal to the weight of the puck.

Case-2: The hockey puck is at rest.

In the case-2, there is no net force acting on the puck in vertical direction.

Hence, the normal force is equal to the weight of the puck.

Therefore, the upward force acting on the hockey puck is same in the both cases.

Thus, option (3) is correct.

(b)

Calculate the upward force acting on the hockey puck in the both cases as follows:

In both cases, the upward force on the puck is same i.e., normal force.

Hence, the normal force acting on the puck in the both cases is nothing but weight of the puck.

The weight of the puck is as follows:

Substitute 0.50 lb for m and

for g in the equation

and solve for W.

Therefore, the upward force acting on the hockey puck in the both cases is

.