Effects of Force

Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)

What Forces Do

A force is a push or pull acting on an object
Forces can:
- change the size or shape of an object (e.g. stretching a spring, squashing a ball)
- change the speed of an object (make it speed up or slow down)
- change the direction of motion of an object
An object either remains at rest, or continues moving in a straight line at constant speed, unless a resultant force acts on it
A resultant force may change the velocity of an object by changing its speed, its direction, or both

A force causing an object to change shape, a force changing the speed of a car, and a force changing the direction of a satelite

Resultant force is the single force that has the same effect as all the individual forces acting on an object combined
To find the resultant of forces acting along the same straight line:
- forces in the same direction - add them together
- forces in opposite directions - subtract the smaller from the larger; the resultant acts in the direction of the larger force

Resultant force in three scenarios: same direction, opposite direction, and equilibrium

If two opposing forces are equal in size, the resultant is zero and the object is in equilibrium

Worked Example

Question: Mweembe pushes a box with 20 N to the right. Friction acts with 8 N to the left.
What is the resultant force?

A pushed box with friction: 20 N push, 8 N friction, 12 N resultant

Answer

Resultant = 20 - 8 = 12 N to the right

If resultant force = 0: the object remains at rest, or continues moving at constant speed in a straight line
If resultant force ≠ 0: the object accelerates - its velocity changes (in speed, direction, or both)

Balanced driving force and friction giving zero resultant and constant speed

Extended Only

Formula:
- F = ma
Where:
- F = resultant force (N)
- m = mass (kg)
- a = acceleration (m/s²)
The resultant force and the acceleration are always in the same direction
Rearrangements:
- a = (F/m) - a greater force or a smaller mass gives a greater acceleration
- m = (F/a)

Worked Example: F = ma

Question: A resultant force of 2400 N acts on a car of mass 1200 kg. Calculate the acceleration.

Answer

a = (F/m) = (2400/1200) = 2 m/s^2 in the direction of the force

Solid friction is the force between two solid surfaces in contact that may impede (resist) motion
- friction also produces heating at the surfaces in contact
- it converts kinetic energy into thermal energy (heat)
Drag in a liquid: friction acts on any object moving through a liquid, opposing its motion
Drag in a gas (air resistance): friction acts on any object moving through a gas, opposing its motion

Friction on a solid surface, drag in a liquid, and air resistance in a gas

Interactive revision notes, videos and practice questions load below.