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# Right method for a power and acceleration question watch

1. so there's this question:

A girl on her cycle, with a total mass of 64 kg, peddling down a slope angled at Beta, where sin(beta) = 1/14.

The resistance is now modelled as having magnitude proportional to the speed of the cyclist. Her initial speed is again 5 m s-1 when the resistance to motion has magnitude 20 N.

The cyclist works at a constant rate of 200 W.

Find the magnitude of her acceleration when her speed is 8ms-1.(4)

so this is wwhat I did

since power = force x velocity

force = power / velocity
force = 200 / 8
force = 25N

Then by newtown's 2nd law:

Force = mass x acceleration

Since there's resistance:

25 - 20 = 64 x acceleration

Therefore Acceleration = 0.0781 ms-2 (3 s.f)

Would this be the right answer?

Thanks
2. how can i change title

this is to do with mechanics and power, not work and energy
3. (Original post by RickHendricks)
x
Can you please post the whole question (a picture of it would be good if you have one)? I know you've probably posted the relevant information but it's easier to help if we can see the whole question.

Also, please post maths questions in the maths forum. I've moved this thread.
4. (Original post by RickHendricks)
how can i change title

this is to do with mechanics and power, not work and energy
Edit your post then "preview post" - there you'll have an option to change the thread title.
5. refer to what the question says.You used the wrong value for the resistance force as that's the resistance when the speed is 5ms.
To find the resistance when the speed is 8, you must first find the relationship between the resistance and the speed.
According to the question:
F=kv
20=5k
k=4, use this to find resistance when speed is 8

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