Unbalanced Forces

An object at rest stays at rest and an object in motion stays in motion, with the same speed and in the same direction unless acted upon by an unbalanced force.

PERSONALISED 1-TO-1 TUTORING

Need more help on this topic? Get 1-to-1 support

Get support with clear explanations, practice and feedback (Ages 5 to 16).

  • DBS-checked tutors
  • Flexible times
  • Parents can be updated

Want an ad-free experience? Subscribe to remove ads

Illustration showing Newton's First Law of Motion:

Top row: A rock at rest, a person pushing the rock with no movement (balanced force), two people pushing the rock causing it to move (unbalanced force).
Bottom row: A soccer ball at rest, a kick changing the ball's speed and direction (unbalanced force), the ball in motion, the ball hitting the net (unbalanced force changing speed and direction).

When we have unbalanced forces acting on an object:

  • A moving object will change speed or direction
  • A stationary object (an object that is not moving) will start to move

When the two forces acting on an object are not equal in size, we refer to them as unbalanced forces.

In this example, to determine the resultant (overall) force acting on the truck, we need to find the difference between the two forces.

100N – 60N = 40N

The resultant force on this object is 40 N, and it is acting towards the right.

If the driver increases the acceleration, there will be a greater force (in N) propelling the car forward, relative to the friction. This means that the truck will increase in speed. If the driver eases off the acceleration, there will be a smaller force (in N) propelling the truck forward, compared to the friction. As a result, the truck’s speed will decrease.

When the forces are balanced, the force forward from the engine is equal to the friction and air resistance pushing the truck backwards. This doesn’t mean that objects won’t move; rather, it means they will move at a constant speed, neither accelerating nor decelerating. In this context, it means that the truck is moving at a steady pace.

The change in the motion of an object is dependent on:

  • The direction of the resultant force
  • The size of the resultant force

Below is an example illustrating all the opposing forces acting on a moving car.

Flashcards
Flashcards are a fun way to make sure you really remember what you've learned. Just click the button below to start reviewing with flashcards and master your knowledge on this topic!
View flashcards for this topic

You’ve used 1 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up