Help with an exam question?

  • 0 votes

The diagram shows a small, radio-controlled, flying toy.  A fan inside the toy pushes air downwards creating the lift force on the toy. The weight of the toy is 0.6N.
When the toy is hovering in mid-air, the fan is pushing 1.5 kg of air downwards every 10 seconds.  Before the toy is switched on, the air is stationary.

Usiing these equations, work out the velocity of the air when the toy is hovering.

momentum  =  mass  ×  velocity

force    =     change in momentum / time taken for the change

Thanks for any help given :)

Posted Sun 13th January, 2013 @ 15:56 by :) PurpleJaguar (: - Team GR
Edited by :) PurpleJaguar (: - Team GR on Sun 13th January, 2013 @ 15:57

23 Answers

  • 1 vote

Well 

force = change in mom/time

Rearrange this to get force x time = change in mom

Therefore mass x velocity = Force x time

so 1.5 x v = 0.6 x 10

So 1.5v = 6

6 / 1.5 = v

v = 4ms-1

Hope this helps :)

Answered Sun 13th January, 2013 @ 16:56 by Tilly - Team GR
Edited by Tilly - Team GR on Sun 13th January, 2013 @ 16:57
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You're amazing, thanks so much :):)

Answered Sun 13th January, 2013 @ 17:12 by :) PurpleJaguar (: - Team GR
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That's ok.

That question actually took me a long time to work out. It's been a while since I did momentum.

Answered Sun 13th January, 2013 @ 17:13 by Tilly - Team GR
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I find these type of questions really hard :(

Answered Mon 14th January, 2013 @ 16:45 by :) PurpleJaguar (: - Team GR
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You just have to look at what numbers you've been given, put them into the equations you've been given and then see what you don't have (and what you want to get). Then you have to re-arrange things until you get what you're looking for. It takes time for you to be able to easily do it. Don't worry - I used to struggle on Qs like that too. :)

Answered Mon 14th January, 2013 @ 17:30 by Tilly - Team GR
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Yeah im gonna keep practising as much as i can before my exam :S

Answered Mon 14th January, 2013 @ 19:19 by :) PurpleJaguar (: - Team GR
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This website might help you with momentum. It explains the topic and gives you practice questions.

Click here to view the website.

Answered Mon 14th January, 2013 @ 20:48 by Tilly - Team GR
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Thanks this website is really useful :)

Answered Tue 15th January, 2013 @ 18:27 by :) PurpleJaguar (: - Team GR
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If you go on the chemistry page I have put up GCSE chemistry websites that might help you with chemistry. 

Answered Tue 15th January, 2013 @ 18:47 by Tilly - Team GR
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I'm not struggling with chemistry at the moment so i'm really trying to focus on physics.

Out of all the past papers i have done for chemistry ive been getting things like 87-92% in them. But in the physics ones im getting 67-74% :(

But thanks anyway. 

Are they for chemistry as a whole because if i struggle with unit 3 chem i can have a look at them then? ;)

Answered Tue 15th January, 2013 @ 19:19 by :) PurpleJaguar (: - Team GR
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Yeah for chemistry as a whole. i will also add some GCSE physics to the discussion page later tonight, so check the chemsitry discussion pages.

Answered Tue 15th January, 2013 @ 19:28 by Tilly - Team GR
Edited by Tilly - Team GR on Tue 15th January, 2013 @ 19:28
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okay will do :) 

would you mind helpingme with a question quickly? Dont worry if you dont want to 

Answered Tue 15th January, 2013 @ 19:38 by :) PurpleJaguar (: - Team GR
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Yeah sure what is it?

Answered Tue 15th January, 2013 @ 19:45 by Tilly - Team GR
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Its another momentum question;;;

A ball of mass 12 kg is thrown at a velocity of 5.5 m/s to a person of  mass 60kg who is standing at rest on ice. The person catches the ball and then slides with the ball across the ice. 

Calculate the velocity of the person and the ball after the collision.

I think I know the answer but...

Thankyou! :) **

Answered Tue 15th January, 2013 @ 19:49 by :) PurpleJaguar (: - Team GR
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Momentum = mass x velocity

Momentum of ball before = 12 x 5.5 = 66 kgms-1 (I think they're the units)

Momentum of person before = 60 x 0 = 0

Momentum before = momentum after

72 x ? = 66

66 / 72 = 0.92ms-1

I think this is right.

Answered Tue 15th January, 2013 @ 19:54 by Tilly - Team GR
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yaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay!

Thats what i got!!!!

That's the first momentum question ive got right

:DDDDD Soooo Proud :) 

Answered Tue 15th January, 2013 @ 20:00 by :) PurpleJaguar (: - Team GR
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Thanks for answering that! :)

Answered Tue 15th January, 2013 @ 20:00 by :) PurpleJaguar (: - Team GR
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Well done!!!! Practice is the best way to get better!

Answered Tue 15th January, 2013 @ 20:01 by Tilly - Team GR
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Thankyouuu!!

I know, i'm gonna practise some more over the next couple of days, my exam is next friday :O

Answered Tue 15th January, 2013 @ 20:09 by :) PurpleJaguar (: - Team GR
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I used to help people in my year with their GCSE work for most of my subjects last year, and the year before. I got used to teaching it to other people in a way that they understood it. I still explain most of the work in my AS classes to my friends too.

good luck with your exam.

Answered Tue 15th January, 2013 @ 20:13 by Tilly - Team GR
Edited by Tilly - Team GR on Tue 15th January, 2013 @ 20:15
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Thankyou :) and Thanks for your help :)

Answered Tue 15th January, 2013 @ 20:17 by :) PurpleJaguar (: - Team GR
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That's ok. Ask again if you need anymore help!

Answered Tue 15th January, 2013 @ 20:20 by Tilly - Team GR
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Will do,   thanks again

Answered Tue 15th January, 2013 @ 20:47 by :) PurpleJaguar (: - Team GR