Question 1

What is Newton's second law and how is it used?

Accepted Answer

Newton's second law states that the acceleration of an object equals the net force on it divided by its mass, written F = ma. To use it, you add every force on the object as a vector to find the net force, then divide by the mass to get the acceleration. The same push gives a light cart a large acceleration and a heavy truck a small one, because acceleration is inversely proportional to mass.

Question 2

What is the difference between mass and weight?

Accepted Answer

Mass is the amount of matter in an object and stays the same everywhere, measured in kilograms. Weight is the gravitational force pulling that mass down, measured in newtons, and equals mass times g (about 9.81 m/s² on Earth). A 5 kg bag always has a mass of 5 kg, but it weighs about 49 N on Earth and far less on the Moon, where g is smaller.

Question 3

Why do passengers lurch forward when a car stops suddenly?

Accepted Answer

Passengers lurch forward when a car stops suddenly because of inertia, described by Newton's first law. The car decelerates, but the passengers' bodies keep moving forward at the original speed until an outside force slows them. That force is the seatbelt; without it, the body keeps going until it hits the dashboard or windscreen.

Question 4

What is a free-body diagram and what forces belong on it?

Accepted Answer

A free-body diagram is a sketch of one chosen object showing only the external forces acting on it, each drawn as an arrow from the object. Typical forces are weight (down), the normal force (perpendicular to the surface), applied pushes or pulls, friction (opposing sliding) and tension (along strings). Internal forces and forces the object exerts on other things are never drawn, because the diagram analyses just that one body.

Question 5

How does friction decide whether an object moves?

Accepted Answer

Friction decides whether an object moves by comparing your applied force with the maximum static friction, which equals the static coefficient times the normal force. While your push is smaller than this maximum, static friction grows to exactly cancel it and the object stays still. Once your push exceeds the maximum, the object breaks free and slides, and the usually smaller kinetic friction takes over.

Question 6

What forces act on an object resting on an inclined plane?

Accepted Answer

An object on an inclined plane feels three main forces: its weight pulling straight down, the normal force pushing perpendicular to the slope, and friction acting along the slope. The weight is split into a component down the slope, mg·sin(θ), and a component into the slope, mg·cos(θ). The normal force balances mg·cos(θ), while mg·sin(θ) is what tries to slide the object down.

Question 7

What is Newton's third law and what are action-reaction pairs?

Accepted Answer

Newton's third law states that for every action force there is an equal and opposite reaction force. Action-reaction pairs always act on two different objects: when you push on a wall, the wall pushes back on you with the same size of force in the opposite direction. Because the two forces act on different bodies, they never cancel each other out.

Question 8

How do you calculate the normal force on a flat surface versus an incline?

Accepted Answer

On a flat horizontal surface with no vertical push, the normal force equals the object's weight, N = mg, because it must balance gravity. On an incline at angle θ, the normal force only has to balance the perpendicular part of the weight, so N = mg·cos(θ). As the slope gets steeper, cos(θ) shrinks and the normal force falls, reaching zero at a vertical wall.

Question 9

Why can a rocket accelerate in empty space?

Accepted Answer

A rocket accelerates in empty space because of Newton's third law. It throws hot exhaust gas backward at high speed, and the gas pushes the rocket forward with an equal and opposite force called thrust. The rocket does not need air or ground to push against — the reaction to expelling its own fuel is enough — which is exactly why rockets work in the vacuum of space.

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