Give reasons for the following statements.

(a) Two bodies must interact with one another for force to act.

(b) Force can change the speed of a body.

(c) Friction depends on the weight of a body.

(d) Static friction is more than kinetic friction.

(e) Rollers and ball bearings are used to reduce friction.

(a) A force is a push or pull exerted by one body on another. Without interaction, no force exists. Even non-contact forces—gravity, magnetism—are interactions at a distance: Earth attracts objects; magnets attract iron. Hence, interaction is necessary for force.

(b) A force is a push or pull that changes speed. Put a toy car on the floor. Give it a strong push, it speeds up quickly. Give it a small push, it speeds up slowly. Now push it gently from the front (opposite its motion), it slows down and can stop.

(c) Friction is the force that opposes motion between two surfaces in contact. A heavier body presses harder against the surface, making the rough spots of the two surfaces interlock more tightly, increasing friction. Hence, the greater the weight of a body, the greater the friction it experiences.

(d) Static friction acts when objects are at rest. The rough spots of the two surfaces interlock tightly so a bigger force is needed to start the motion. Once the object slides, the rough spots don't lock as well, so kinetic friction is smaller. For example, it’s harder to start pushing a heavy box than to keep it moving.

(e) Rollers and ball bearings reduce friction because rolling friction is much smaller than sliding friction. The balls/rollers turn so the surfaces don’t rub and interlock as much—only tiny points/lines touch. This cuts heat and wear and makes movement easier. For example, moving a suitcase with wheels is easier than dragging it without wheels.