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Understanding Power

In this worksheet, students will understand the term power and how to use it as well as developing knowledge of the equation.

Worksheet Overview

QUESTION 1 of 10

What do we mean when we say something has a lot of power? It can mean a lot of things, right? Your teachers might have a lot of power to set you endless homework and exams and make your work in silence or someone who is lifting weights to get Hench might have a lot of power. But this is physics, and when we say power, we mean something specific. In this activity, we will be defining power and how we can use it as a concept to help us understand machines. 

Power in physics is a really useful concept. It is also SUPER simple, so simple it is scary! Power is how much energy something converts per second. So, if you put 100 J of kinetic energy into walking every second then you have a power of 100 Watts. ‘Watt’ is the unit of power, there’s a silly joke in there, but I can’t seem to find it! We normally write it as W, and 1 W is 1 Joule per second. That’s it! That’s all you need to understand! 

As you can imagine, it is really useful to know who much power something uses as it lets you prepare a set amount of energy for that thing. If you know that a device uses 10 Watts of power and you are going to use it for 10 seconds, you know you are going to need 100 J of energy to power it. And we can work this out by using this equation:

INSERT EQUATION HERE

Or this one!

INSERT QUESTION HERE

So, let’s have a go at working out some powerful ideas (get it?).

Q – A device uses 15 J of energy over 20 s. Calculate the power. 

Step 1 – highlight all of the number in the equation:
A device uses 15 J of energy over 20 s. Calculate the power. 


Step 2 – Write out the numbers next to their symbols:
P = ?
E = 15 J
t = 20 s


Step 3 – put the numbers into the equation:
Q = 15/20


Step 4 – do the maths and write your answer:
Q  = 0.75 W

Don’t forget your units!

Let's try some questions on that! 

Match the value to the unit below

Column A

Column B

Energy (E)
Joule (J)
Power (P)
Second (s)
Work Done (w)
Watt (W)
Time (t)
Joule (J)

Write down the definition of a Watt. 

A robot does 1000 J of work when it is moving. It takes the robot 10 s to travel this distance. Calculate the power of the robot. 

A crane takes 120 s to lift a mass up using a total of 5,400 J of energy during the lift. Calculate the power of the crane. 

Calculate the power of a jet engine that is able to produce 30,000 J of energy in a period of 1 minute. 

A toy car is pushed up a hill in order to calculate the energy its internal mechanisms can deliver. It is calculated this is gained a total of 15 J of energy to get up the slope by using the equation E=mgh. The toy was also timed to take 12 seconds before it stopped. Calculate the power of the toy car.

An object with a mass of 1 kg is lifted 5 m into the air in a time of 5 seconds. Calculate the power.

 

Equations to use:

E = mgh

P=E/t

Gravitational field strength = 10 N/kg

Explain why the value you have just worked out is lower than the actual power of the toy car (2 marks). 

Describe how to work out power (4 marks)

Write down an equation that links power, energy and time using the symbols only.  

  • Question 1

Match the value to the unit below

CORRECT ANSWER

Column A

Column B

Energy (E)
Joule (J)
Power (P)
Watt (W)
Work Done (w)
Joule (J)
Time (t)
Second (s)
EDDIE SAYS
So, you might have got this one wrong if you put the wrong Joule with the wrong value - there are two of them, but they are the same, so don't worry! For this question, it is all about making sure that you have the correct values so you can spot them in a question! If you can find them in the question then you can work with them to find the answer.
  • Question 2

Write down the definition of a Watt. 

CORRECT ANSWER
EDDIE SAYS
This is one of those definitions that you need to know, but it will also help you to work out the answer to maths questions. If you know the answer is in Watts and you are given joules and a time - you know that one joule per second is a Watt, so you know that if you divide the energy by the time then you will get your answer!
  • Question 3

A robot does 1000 J of work when it is moving. It takes the robot 10 s to travel this distance. Calculate the power of the robot. 

CORRECT ANSWER
100
EDDIE SAYS
Okay, simple maths questions: What are the values? P = ? E = 1000 J t = 10 s Put them into the equation: P = E/t P = 1000/10 P = 100 W
  • Question 4

A crane takes 120 s to lift a mass up using a total of 5,400 J of energy during the lift. Calculate the power of the crane. 

CORRECT ANSWER
45
EDDIE SAYS
Another simple one - this time with the energy equation instead of the work equation, but it's basically the same thing! Find the values: P = ? E = 5,400 J t = 120 s Put them into the equation: P = E/t P = 5,400 / 120 P = 45 W
  • Question 5

Calculate the power of a jet engine that is able to produce 30,000 J of energy in a period of 1 minute. 

CORRECT ANSWER
500
EDDIE SAYS
This one might be a little more complicated! You can see here that the time is not in seconds, but in minutes! So how are we going to do this? The first thing that we need to do is to convert the time from minutes into seconds. All you need to do for this is multiply it by 60 (the number of seconds in 1 minute). 60 x 1 = 60 s. Write down the numbers: P = ? E = 30,000 J t = 60 s Put then numbers into the equation: P = E/t P = 30,000/60 P = 500 W
  • Question 6

A toy car is pushed up a hill in order to calculate the energy its internal mechanisms can deliver. It is calculated this is gained a total of 15 J of energy to get up the slope by using the equation E=mgh. The toy was also timed to take 12 seconds before it stopped. Calculate the power of the toy car.

CORRECT ANSWER
1.25
EDDIE SAYS
This question has a lot of words and formulas there to try and distract you from the actual question. Always look at the last sentence first - this is going to tell you what the question is asking, then you can read through the rest knowing what to look out for - in this case, energy and time because it is a power question. So - once you have done that you should have got the numbers: P = ? E = 15 J t = 12 s P = E/t P = 15/12 P = 1.25 W
  • Question 7

An object with a mass of 1 kg is lifted 5 m into the air in a time of 5 seconds. Calculate the power.

 

Equations to use:

E = mgh

P=E/t

Gravitational field strength = 10 N/kg

CORRECT ANSWER
10
EDDIE SAYS
This is the type of question you might get in an exam - it will expect you to use more than one equation in working out the answer. Here we have given you the equations to use, you just needed to find the way of solving it (the hardest part!) You should realize that you can't work out the power until you have energy - so how do you work out the energy? You use the equation of energy that we have given you! E = mgh m = 1 kg g = 10 N/kg h = 5m E = 1 x 10 x 5 E = 50 J Now you have this you can put it into the equation for power: P = E/t E = 50 t = 5 s P = 50/5 P = 10 W
  • Question 8

Explain why the value you have just worked out is lower than the actual power of the toy car (2 marks). 

CORRECT ANSWER
EDDIE SAYS
This is harking back to your knowledge of energy! You needed to realize that energy losses where something that happens whenever there is an energy change, and this case is not any different. We can\'t work out the energy loss using this model, so there is no way of knowing how much energy has been lost so there is no way of knowing the real power of the toy car.
  • Question 9

Describe how to work out power (4 marks)

CORRECT ANSWER
EDDIE SAYS
This involved you thinking about everything that you have done so far and putting it into a handy helpful guide to yourself. This will help you remember how to work out power and all other equations that you need to know. 1 -Find the values 2 - Write them down 3 - put them into the equation 4 - do not forget the units!
  • Question 10

Write down an equation that links power, energy and time using the symbols only.  

CORRECT ANSWER
P = E / t
P=E/t
EDDIE SAYS
Power = P Energy = E time = t Did you get the capitalization correct? It is quite important! It should have looked like this P = E/t Beautiful right? Fed up with power yet? I know I am!
Try it ---- OR ----

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