Balanced and Unbalanced Forces

Back to Physics Grade 9/10

Description

This is a basic physics tutorial that is targeted at GCSE (grade 9 and grade 10) standard, considering how balanced and unbalanced forces influence the movement of a bike and biker. The associated video tutorial is on its way.





What are Balanced and Unbalanced Forces?

By studying Newton's First Law of Motion, we've learned that

Every object in a state of uniform motion tends to remain
in that state of motion unless an external force is applied to it.



This means that unless a force is applied to an object it will continue as is - either remaining at rest, or moving at a steady velocity. But what happens when more than one force is applied to an object? Take a person on a bike for example.



They are applying a force to move the bike forward. But there are other forces acting in the opposite direction - the friction force between the tyres and the ground, and between the biker/bike and air (also known as air resistance).

If the forces forward and backward are equal in magnitude, they are described as being balanced i.e. they cancel each other out. If the forces are different in magnitude, they are described as being unbalanced.



What do balanced forces mean for the biker?

If the forces acting on a system are balanced, then the overall force acting on the system is 0 N. That is, it's as if there's no force acting on the system. This means it will continue to move in uniform motion.

In the case of the bike and biker, balanced forces will mean that


What do unbalanced forces mean for the biker?

Unbalanced forces will make the bike and biker accelerate, either positively or negatively.

In any bike ride, the forces will switch between being balanced and unbalanced, depending on the slope of the road, the energy exerted by the biker, etc.

Balanced and unbalanced forces on a bike ride

Getting started

Initially you need the the force exerted by the biker to be greater than the forces acting in the opposite direction. That is, unbalanced forces are needed to get the bike moving from rest i.e. accelerating.

The force pushing the bike forward is greater than the sum of those working in the opposite direction (e.g. friction and air resistance); this results in the bike accelerating forward.


Continued acceleration

If the biker continues to exert a greater force on the bike than the forces acting in the opposite direction, the unbalanced forces in their favour will result in continued acceleration i.e. they'll continue to increase their velocity.

Constant speed

If the biker reduces the force which they're exerting in the forward direction, and the forces eventually balance out, they will continue to move forward, but at a constant velocity i.e. no acceleration.

The force pushing the bike forward is equal to the sum of those working in the opposite direction; this results in the bike and biker moving at a constant velocity.


Slowing down and coming to a stop

If the biker starts to get tired, and the force they exert in the forward direction is less than the forces acting in the opposite direction, the unbalanced forces working against them will mean that they will start to slow down i.e. they will undergo negative acceleration (or deceleration).

The force pushing the bike forward is less than the sum of those working in the opposite direction; this results in the bike and biker slowing down.



If the unbalanced forces continue to act in this manner, the biker and bike will eventually come to a stop. Let's very briefly cover work, energy and power now.