Energy transfer

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Description

This is a basic physics GCSE tutorial exploring in simple terms what the repercussions are of energy transfers in different scenarios. Video tutorial to come.

Energy Transfer

We've learned that when work is done on an object, energy is transferred. Let's look at some calculation scenarios to consolidate our understanding. These are three useful formulas you might like to keep in mind for these calculations (click on them to access the relevant tutorials).

Force = mass x acceleration


Work done = force x distance


Speed = distance ÷ time


You may also find it useful to review the tutorials on Newton's Laws of Motion and Balanced and Unbalanced Forces.

Scenario 1

Part A

A 2 kg block is moved 5 m horizontally along a friction-less surface by a force of 10 N to the right. How much work is done on the block?

This is a relatively simple calculation because we have all of the information given to us:

We simply put these numbers into the second equation above:
Work done = force x distance
Work done = 10 N x 5 m
Work done = 50 N m or 50 J

Part B:

What is the resultant acceleration of the block?

Once again, we have all of the information at our fingertips

We simply put these numbers into the first equation above:
Force = mass x acceleration

This equation can be rearranged as:
Acceleration = force ÷ mass
Acceleration = 10 N to the right ÷ 2 kg
Acceleration = 5 m/s2 to the right

Scenario 2

Part A

A 2 kg block is moved along a wooden table surface by a force of 10 N to the right, at a constant speed of 10 cm/s. How much energy is used to move the block for 50 seconds?

Energy transferred = work done. So this question is actually asking us how much work is done in this scenario. We know from above that:
Work done = force x distance

We don't know distance, but we can easily work that out using the third formula above, rearranged as:
Distance = speed x time

And we have the following information (remember to make sure the units are all in correct):

Distance = speed x time
Distance = 0.1 m/s x 50 s
Distance = 5 m

So in 50 seconds, the block has been moved 5 m. Now let's work out how much work was done in this scenario:
Work done = force x distance
Work done = 10 N x 5 m
Work done = 50 N m or 50 J

Part B

If the same work (50 J) is done on an object of the same mass (2 kg), why does it accelerate in one scenario, and not in the other?

In the first scenario, the block was being moved across a frictionless surface. That is, all of the energy being transferred to the block was converted into kinetic energy, and so the block was able to accelerate.



In the second scenario, the block was being moved across a surface that is not frictionless - a wooden table. In this case, the same amount of energy is being used to move the block. The difference is that there is a counteracting force: friction. As the movement forward is at a constant speed, we know that the forward and backward forces must be balanced.

Small objects travelling at speeds of 10 cm/s will only experience a very small amount of air resistance. For the purpose of this scenario, we will assume all of the backward force is due to contact friction with the timber surface. This means that the friction force must also be 10 N in magnitude, but it will be to the left (as depicted by the blue arrow below).


Part C

What would happen in each scenario if the pushing force is removed?
We need to think about how the different forces are balanced or unbalanced in each scenario:

Scenario 1: If the pushing force was removed in the first scenario, the block would continue moving to the right at a constant velocity. Why is this?

Removing the forward pushing force will mean that there are no external forces acting on the block. According to Newton's First Law of Motion, this means that the object will continue in a uniform state of motion. As it is already moving to the right before the force is removed, it will continue to travel to the right at a constant velocity once it is removed. It will not speed up because there is nothing pushing it forward; but it will also not slow down either because there is nothing to counteract its movement forward (to the right).

Scenario 2: If the force was removed, the block would undergo negative acceleration and come to a stop. This is because in this scenario, once the pushing force is removed, there is nothing to overcome the contact forces between the block and the wooden table surface.


Quite commonly people ask about the relative energy and work needed to run or walk a set distance. If you have wondered about this, check out this side tutorial - otherwise, let's move on to exploring what power is.