# Mechanics Work Done

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#1
A car driver brakes on a horizontal road and slows down from 20m/s to 12m/s. The mass of the car and it’s occupants is 1,150kg.

a) Find the loss in kinetic energy
b) Given that the Work Done against resistance to motion is 50KJ, find the Work Done by the brakes.

I’ve done part a fairly comfortably but I’ve been looking at part b for over 30 minutes and I just don’t know what to do. I’m just looking for equations that I can rearrange and plug into another but I’m struggling so much with this topic
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2 years ago
#2
(Original post by Y12_FurtherMaths)
A car driver brakes on a horizontal road and slows down from 20m/s to 12m/s. The mass of the car and it’s occupants is 1,150kg.

a) Find the loss in kinetic energy
b) Given that the Work Done against resistance to motion is 50KJ, find the Work Done by the brakes.

I’ve done part a fairly comfortably but I’ve been looking at part b for over 30 minutes and I just don’t know what to do. I’m just looking for equations that I can rearrange and plug into another but I’m struggling so much with this topic
I usually start with a basic equation with this type of question:

Energy before = energy after + work done.

In this case energy is just K.E. And work done is done against resistance to motion and by the brakes, so our equation becomes:

KE before = KE after + work done against resistance to motion + work down by brakes.

And plug in the values.
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#3
(Original post by ghostwalker)
I usually start with a basic equation with these type of questions:

Energy before = energy after + work done.

In this case energy is just K.E. And work done is done against resistance to motion and by the brakes, so our equation becomes:

KE before = KE after + work done against resistance to motion + work down by brakes.

And plug in the values.
Don’t think I’ve done this correctly.
KE before = 1/2 * 1150*20^2 =975. Did the same but replaced 20 with 12 and get:

975=719+50,000+Work Done by brakes. I still don’t get it sorry
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2 years ago
#4
(Original post by Y12_FurtherMaths)
KE before = 1/2 * 1150*20^2 =975
You sure?? Doesn't look like a very small number or anything?
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#5
(Original post by RDKGames)
You sure?? Doesn't look like a very small number or anything?
I thought the formula was 1/2 mv^2 and that’s what I did?
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2 years ago
#6
is this fm or normal maths?
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#7
(Original post by Gent2324)
is this fm or normal maths?
Fm
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2 years ago
#8
(Original post by Y12_FurtherMaths)
I thought the formula was 1/2 mv^2 and that’s what I did?
It is... but I'm just suggesting you put that into the calculator correctly.
1
2 years ago
#9
(Original post by Y12_FurtherMaths)
I thought the formula was 1/2 mv^2 and that’s what I did?
Think he means that you've made an error in your calculation. Two large numbers being multiplied together...
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#10
(Original post by RDKGames)
It is... but I'm just suggesting you put that into the calculator correctly.
Oh right yes. No idea what I must’ve typed in! But I end up with 97.2KJ and that’s the answer in the text book.
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2 years ago
#11
(Original post by Y12_FurtherMaths)
I end up with 97.2KJ and that’s the answer in the text book.
Well done

No idea what I must’ve typed in!
Time for bed by the sound of it!
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#12
(Original post by RDKGames)
Time for bed by the sound of it!
Definitely I’m so tired! Mechanics seems to be an area I’m struggling with quite a lot due to all the physics in it since I could hardly answer a physics maths question at GCSSE due to all the different equations I didn’t understand. Everyone else in my fm class does physics and is very strong in it. Is there anything you recommend I do to help me catch up/improve?
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2 years ago
#13
(Original post by Y12_FurtherMaths)
Definitely I’m so tired! Mechanics seems to be an area I’m struggling with quite a lot due to all the physics in it since I could hardly answer a physics maths question at GCSSE due to all the different equations I didn’t understand. Everyone else in my fm class does physics and is very strong in it. Is there anything you recommend I do to help me catch up/improve?
I was in the same position. Everyone in my class was doing physics so they were fine with most of mechanics, whereas I didn't take it past GCSE.

Nothing I could really recommend other than spending more time on the principle topics of mechanics and practice them until they stick with you - which is what I did. TBH, most topics started sticking with me around March time (though I was slow), so don't worry too much about them if you're not getting it.
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#14
(Original post by RDKGames)
I was in the same position. Everyone in my class was doing physics so they were fine with most of mechanics, whereas I didn't take it past GCSE.

Nothing I could really recommend other than spending more time on the principle topics of mechanics and practice them until they stick with you - which is what I did.
Yeah I think that’s a good idea. It worked for me at maths GCSE so I’ll do the same with mechanics. Thanks for your support
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