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# M2 Work, Energy and Power watch

1. I can't see where I'm making the mistake. It must be silly but I keep using the same idea (just hoping it's my arithmetic that's wonky) and so it keeps coming out wrong.

With its engines working at a constant rate of 369kW, a train of mass 100 tonnes ascends a hill of 1 in 50 at a constant speed of 54km/hr. Find the magnitude of the resistance to motion experience by the train.

54 km/hr = 54000/360 m/s = 150m/s
Tractive force = P/v = 369000/150 =2460N

Force downhill = R + 100000g/sqrt(1 +50x50)

Tractive force = force downhill

R is NOT 5000N as per answer in the book

Thanks
2. (Original post by maggiehodgson)
I can't see where I'm making the mistake. It must be silly but I keep using the same idea (just hoping it's my arithmetic that's wonky) and so it keeps coming out wrong.

With its engines working at a constant rate of 369kW, a train of mass 100 tonnes ascends a hill of 1 in 50 at a constant speed of 54km/hr. Find the magnitude of the resistance to motion experience by the train.

54 km/hr = 54000/360 m/s = 150m/s
Tractive force = P/v = 369000/150 =2460N

Force downhill = R + 100000g/sqrt(1 +50x50)

Tractive force = force downhill

R is NOT 5000N as per answer in the book

Thanks
First off there are 3600s in an hour - you can reality check your working by seeing that 150m/s would be one really fast train.
Also your calculation for the component of the weight parallel to the plane seems to be wrong.
It should be:
3. (Original post by joostan)
First off there are 3600s in an hour - you can reality check your working by seeing that 150m/s would be one really fast train.
Also your calculation for the component of the weight parallel to the plane seems to be wrong.
It should be:
Oh gosh, yes.

Sin alpha = 1/sqrt(1 + 50x50) isn't it? I seem to get the right answer with that.

Thanks so much - a really silly error.
4. (Original post by maggiehodgson)
Oh gosh, yes.

Sin alpha = 1/sqrt(1 + 50x50) isn't it? I seem to get the right answer with that.

Thanks so much - a really silly error.
Yup, that's right, and no worries.

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