Question

If only one external force acts on a particle, does it necessarily change the particle’s (a) kinetic energy? (b) Its velocity?

Short answer

(a) The kinetic energy of the particle may or may not change when one external force acts on a particle.

(b) The velocity of the particle changes when one external force acts on a particle.

Step-by-step solution

Given information

One external force act on a particle.

Newton’s second law

According to Newton’s second law, when a force is applied on a particle of certain mass, the acceleration of the particle takes place.

The relation between the external force and the particle’s acceleration is given below:

F = ma

Here, F is external force applied, m is particle’s mass and a is particle’s acceleration.

Particle’s kinetic energy

(a)

The kinetic energy of the particle relies upon two factors only, the particle’s speed and its mass. So, if there is a change in the particle’s speed due to external force then the particle’s kinetic energy changes but if there is only change in the direction of the velocity then kinetic energy does not change.

Hence, the kinetic energy of the particle may or may not change when one external force acts on a particle.

Particle’s velocity

(b)

It means that, the velocity of the particle changes when a force is applied to it.

Hence, the velocity of the particle changes when one external force acts on a particle.