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M1 F=ma question help? watch

1. I don't quite understand whats its saying is happening to the car or what it is asking me to work out.

on a particular journey, the resistance to the motion of a car mass 1000 kg is proportional to its speed. The car is travelling at a constant speed of 15 ms-1 with a driving force of 870N. The driving force is instantaneously increased to 1200N.
a) Find the instantaneous acceleration produced
b) Find the resistance to the motion and the acceleration when the car is moving at 20 ms-1

I got a) right as 0.33 ms-2, but I don't understand what an earth they are asking me to do in part b), I thought the acceleration was constant

btw the correct answers are 1160N, and 0.04ms-2
2. (Original post by JustHoping)
I don't quite understand whats its saying is happening to the car or what it is asking me to work out.

on a particular journey, the resistance to the motion of a car mass 1000 kg is proportional to its speed. The car is travelling at a constant speed of 15 ms-1 with a driving force of 870N. The driving force is instantaneously increased to 1200N.
a) Find the instantaneous acceleration produced
b) Find the resistance to the motion and the acceleration when the car is moving at 20 ms-1

I got a) right as 0.33 ms-2, but I don't understand what an earth they are asking me to do in part b), I thought the acceleration was constant

btw the correct answers are 1160N, and 0.04ms-2
The acceleration was constant but then the driving force is increased ... Hence the acceleration change
3. (Original post by JustHoping)
I don't quite understand whats its saying is happening to the car or what it is asking me to work out.

on a particular journey, the resistance to the motion of a car mass 1000 kg is proportional to its speed. The car is travelling at a constant speed of 15 ms-1 with a driving force of 870N. The driving force is instantaneously increased to 1200N.
a) Find the instantaneous acceleration produced
b) Find the resistance to the motion and the acceleration when the car is moving at 20 ms-1

I got a) right as 0.33 ms-2, but I don't understand what an earth they are asking me to do in part b), I thought the acceleration was constant

btw the correct answers are 1160N, and 0.04ms-2
First you need to find out what R, resistance to motion, is. You're told that in the first instance, R of a car mass is proportional to speed. i.e. R = mass/speed

You want to find the value of R with respect to your new speed value. So multiply the first R value you get by the speed value given in b).

Once you've solved that, you can find the acceleration by doing an F = ma calc and rearranging for a.

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Updated: January 24, 2015
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