momentum

[Johnboy63]

A car transporter travels at 70mph along a motorway. A car beaks free and starts to roll down the ramp towards the rear of the transporter. By the time it reaches the end of the ramp it is travelling at 20mph in relation to the transporter.

Thats the easy bit. Now the bit I cant agree with anyone: - In what direction (in relation to the road) will the car be travelling when it comes off the ramp? (20mph in the opposite direction to the transporter, 50mph in the same direction????????????) Help!!

[bernardo]

It will (initially) be going at 50mph forwards in relation to the road because it was going at 20 mph backwards in relation to the transporter. That's easy because 70-50=20. But it will soon slow down and stop, because of friction. In particular, the wheels of the car will be rolling backwards, and therefore will create a lot of friction when they first hit the road.

Imagine a car, with its wheels spinning, being lowered by a crane onto a convyor belt (the direction of movement of the conveyor belt is in the oppostite direction to that of the spinning wheels). As soon as the car touches the belt, the crane lets go. The car will very soon acquire the same speed as the conveyor belt (this is equivalent to the car coming to a stop on the road) but initially it will get a jolt because of the friction on the wheels.

[Skids]

Relate both to something stationary, say the road. The transporter has a forward speed of 70mph. When the car is stationary on the transporter it also has a forward speed of 70mph. The car rolls backward at 20mph in relation to the transporter, but is still travelling forward with a speed of 70mph minus 20mph, ie 50mph, in relation to the road.