Question

Describe how a non-contact force could change the speed of an object? Give one or two
examples.

Answer 1

A non-contact force acts at a distance (no physical touch) and can change an object’s speed by doing work on it — i.e. by exerting a force with a component along the object’s velocity so it accelerates.

Key points:
- Newton’s 2nd law: F = ma. A net force with a component along the motion produces acceleration and changes speed.
- Work–energy: W = ∫F·ds = ΔK. If the force does positive work (F·v > 0), the object’s kinetic energy and therefore its speed increase. Instantaneous power = F·v.

Two examples:
1. Gravity on a falling apple: The Earth’s gravitational force (non‑contact) acts downward. As the apple falls, gravity does positive work, converting gravitational potential energy into kinetic energy and increasing the apple’s speed.
2. Electric field accelerating an electron: A charged particle released between oppositely charged plates feels an electric force. If the force has a component along the particle’s motion, it does work and the particle’s speed increases (used in cathode‑ray tubes and electron guns).

(As a contrast: a magnetic force is non‑contact but always perpendicular to velocity, so it changes direction but not the speed.)