# Quiz 4: Force and Motion

Physics & Astronomy

(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.

The density is the ratio of the mass to the volume. The expression for the density is, Here, m is the mass and V is the volume. Calculate the mass of water as follows: Rearrange the equation (1) for m as follows: Substitute for V and for d. Calculate the mass of aluminium as follows: Substitute for V and for d in the expression . Calculate the ratio of the mass of aluminium and water as follows: So, the of aluminium has greater inertia as inertia is directly proportional to the mass.

Weight: The gravitational force acting on the object due to Earth is equal to the weight of the object. Here, F is the gravitational force acting on the object, m is the mass of the object, and g is the acceleration due to gravity. Inertia: The moment of the inertia is the product of the mass and the square of the perpendicular distance to rotation axis. Here, m is the mass of the small object, and r is the perpendicular distance of the object from rotation axis. Density: The density of the object is mass divided by the volume of the object. Here, m is the mass of the object and V is the volume of the object. From the above definitions, it is concluded that the mass is related to density, inertia and weight. Therefore, option (d) is correct answer.