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*Faith* Offline
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Math Help - February 13th 2009, 11:35 PM

I got set these 2 questions for homework and I just don't understand how to do them, they're mechanics questions so here goes...

A motorcycle of mass 300kg, has a maximum power output of 30kW and a top speed of 45m/s on the horizontal. Assume that the resistance forces that act on the motorcycle are proportional to its speed.

a) Find an expression for the magnitude of the resistance force that acts on the motorcycle at a speed of vm/s.

b) Find the maximum possible speed of the motorcycle as it travels up a slope inclined at 5 degrees to the horizontal.

c) What would be the maximum speed of the motorcycle down the same slope?



A car of mass 1200kg moves on a stright horizontal road. A resistive force acts on the car and its magnitude is 40v N, where is the speed of the car in metres per second.

a) The car travels 200m at a constant speed of 30m/s. Find the work done in overcoming the resistance force.

b)i) If the car accelerates at 2m/s while moving at 20m/s, show that the power of the car is 64,000W.

ii) Given that the maximum power of the car is 64,000W, find the maximum speed of the car.


I know it's a lot to ask but it's half term so I can't ask the teachers and I've searched around but haven't found anything useful so...this is my last resort.


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Re: Math Help - February 14th 2009, 11:30 AM

1. a) Power = Force * velocity and Force = mass * acceleration

|F| = 30kJ/s / (300kg * 9.81ms^-2) = v

|F| = 10.2v


b) Resolving along the plane upwards:

Upward force (F) = P(max)/V(max) = 30,000/45 = 2000/3

2000/3 = mgsin(5) + 10.2v
v = [2000/3 - mgsin(5)]/10.2 = 40.2m/s

c) Resolve again downwards (the engine force and drag force will now be in the opposite directions):

2000/3 + mgsin(5) =10.2v

v = 90.5m/s


not sure about 2 yet, I know W = Fd and F = ma but where to go from there...

Last edited by Hyper Sonic; February 14th 2009 at 11:49 AM.
   
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Re: Math Help - February 14th 2009, 02:48 PM

is this one of the mechanics modules in a level maths? looks familiar, if it is i'l go hunt my notes out for number 2 if you want
   
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Re: Math Help - February 15th 2009, 12:28 PM

Quote:
Originally Posted by ~sal~ View Post
is this one of the mechanics modules in a level maths? looks familiar, if it is i'l go hunt my notes out for number 2 if you want
Please, that would help me so much.


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