Physics Essentials: 20 MCQs with answers

Newton

The SI unit of force is the newton (N), defined as kg·m/s². Joule is energy, watt is power, pascal is pressure.

$F = ma$

Newton's second law states that the net force on an object equals the product of its mass and acceleration: $F = ma$.

Energy

Energy has magnitude only, no direction, making it a scalar. Velocity, force, and acceleration are vectors.

A stretched spring

A stretched spring stores elastic potential energy. The others involve kinetic or radiant energy.

The rate of doing work

Power is the rate at which work is done or energy is transferred, measured in watts (J/s).

$PE = mgh$

Gravitational potential energy near Earth's surface is $PE = mgh$, where $h$ is height above a reference point.

It is the same for all objects regardless of mass

In free fall, all objects experience the same acceleration due to gravity ($g \approx 9.8$ m/s²) in the absence of air resistance.

True

Power is defined as the rate of doing work, so 1 W = 1 J/s.

Velocity changes because direction changes continuously

In uniform circular motion, speed is constant but velocity (a vector) changes because its direction continuously changes, so there is centripetal acceleration.

Perpendicular to displacement

Work $W = Fd\cos\theta$. When $\theta = 90°$, $\cos 90° = 0$, so work is zero.

4 m/s²

Acceleration $a = \frac{\Delta v}{\Delta t} = \frac{20 - 0}{5} = 4$ m/s².

There is no horizontal acceleration

In projectile motion without air resistance, the only acceleration is due to gravity, which is vertical. Thus horizontal acceleration is zero, so horizontal velocity is constant.

No external force acts on the system

Momentum is conserved when the net external force on a system is zero. Internal forces do not change total momentum.

9 J

$KE = \frac{1}{2}mv^2 = \frac{1}{2} \times 2 \times 3^2 = 9$ J.

Action and reaction forces are equal in magnitude and opposite in direction

Newton's third law states that for every action, there is an equal and opposite reaction. They act on different objects, so they don't cancel.

20 J

Work $W = Fd\cos\theta$. Here $\theta = 0°$, so $W = 10 \times 2 = 20$ J.

Total mechanical energy is constant

When only conservative forces (like gravity) act, the sum of kinetic and potential energy remains constant.

100 J

$PE = mgh = 5 \times 10 \times 2 = 100$ J.

True

Speed is scalar and constant, but velocity is a vector that changes direction continuously, so it is not constant.

$KE = p^2 / (2m)$

Since $p = mv$, $KE = \frac{1}{2}mv^2 = \frac{p^2}{2m}$.