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Year 7

Balanced and unbalanced forces

I can use the idea of resultant force to explain whether a change of speed occurs or not.

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New
New
Year 7

Balanced and unbalanced forces

I can use the idea of resultant force to explain whether a change of speed occurs or not.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. Two forces of equal size, acting in opposite directions on an object, are balanced.
  2. The resultant force is the sum of the forces acting on the object, taking into account their direction.
  3. When forces are balanced there is no resultant force and no change in speed.
  4. When there are two forces acting in opposite directions, the resultant force acts in the direction of the bigger force.
  5. The greater the resultant force on a given object, the greater or faster the change in speed.

Common misconception

The direction of the (resultant) force shows the direction of motion.

The direction of the resultant force shows the direction of the change (not the direction of motion). Forces make things change.

Keywords

  • Balanced forces - Two forces of equal size acting in opposite directions on an object, are balanced. They fully cancel each other out so no change is caused.

  • Unbalanced forces - Unbalanced forces do not fully cancel out.

  • Resultant force - The ‘force left over’ is called the resultant force. Resultant forces cause changes in speed.

Another prevalent misconception is "if there is no (resultant) force, there can be no motion". Provide examples of balanced forces acting on moving objects and ask what force is left over to change the speed.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
What do forces cause?
Correct answer: changes
motion
stillness
Q2.
Forces make things change. A larger force will cause a bigger or __________ change.
smaller
Correct answer: faster
slower
Q3.
Which option is the best description of what it means when two things are balanced?
An image in a quiz
Something is bigger on one side than the other.
Two things acting in different directions are adding up to have a bigger effect.
Correct answer: Two things are equal in some way but are cancelling each other out.
Q4.
A person walks 9 metres straight forwards, then 4 metres straight backwards. The result of doing this is they are metres from where they started.
Correct Answer: 5, five
Q5.
Forces can change an object's , direction or shape.
Correct Answer: speed, velocity
Q6.
Are the forces on this car balanced or unbalanced?
An image in a quiz
Correct answer: balanced
unbalanced
there is not enough information to decide

Exit quiz

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6 Questions

Q1.
Which image shows balanced forces?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q2.
The resultant force on this car is N.
An image in a quiz
Correct Answer: 5, five, 5 N, five newtons
Q3.
Which pictures show a resultant force of 12 N? Tick as many as you think.
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
Q4.
Which would cause an object to change speed?
If the forces are balanced.
Correct answer: If the forces are unbalanced.
Correct answer: If there is a resultant force.
If there is no resultant force.
Correct answer: If the forward force is bigger than the backward force.
Q5.
The diagram only shows the forces acting on this car, not whether it is moving or not. Which of the following could be happening to this car?
An image in a quiz
Correct answer: With these forces acting, the car could be stationary and remaining stationary.
Correct answer: With these forces acting, the car could be moving forwards at steady speed.
With these forces acting, the car could be moving forwards and getting faster.
With these forces acting, the car could be moving but will now immediately stop.
Correct answer: With these forces acting, the car could be moving backwards at steady speed.
Q6.
This car is moving forwards. What happens next to this car, with these forces acting?
An image in a quiz
It keeps moving forwards at steady speed.
It keeps moving forwards but speeds up.
Correct answer: It keeps moving forwards but slows down.
It immediately starts moving backwards at steady speed.
It immediately starts moving backwards and speeding up in that direction.