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# m2 jan 2005 question watch

1. a car of mass 1000kg is towing a trailer of mass 1500kg along a straight horz road. The tow bar joining the car to the trailer is modelled as a light rod parallel to the road. The totl resistance to motion of the car is modelled as having magnitude 750 N and is constant. The total resistance to motion of the trailer is modelled as having magnitude R where R is a constant. When the engine of the car is working at a rate of 50kw the car and the trailer travel at a constant speed of 25m/s.

a) show that R=1250 I could do tis bit but....

When travelling at 25m/s the driver disengages the engine and applies the brakes. The brakes provide a constant braking force of magnitude 1500N to the car. The resisting forces of magnitude 750N and 1250 N are assumed to remain unchanged.

b) calculate the deceleration of the car while braking
this is where i got stuck the answer is 1.4

2. force from engine = 0N
F=ma
0 - 1500 - 750 - 1250 = 2500a
-3500 = 2500a
-3500/2500 = a = -1.4ms-2
= 1.4ms-2 deceleration
3. duhhh!!! i thought there was still a force from the engine. there really is no hope for me lol cheers xxx

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