Momentum and push starting vehicles

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1. Feb 25, 2016

MBBphys

1. The problem statement, all variables and given/known data
This was a question I came across in my A level physics course:

A garage attendant was surprised to be asked to give a lorry a push as the battery was flat. He was even more surprised when he leant on the back of the lorry and the slight movement of the lorry was enough to start the engine. Explain, in terms of momentum, how the garage attendant was able to get the lorry started.

2. Relevant equations
p=mv

3. The attempt at a solution
I didn't understand how this linked to momentum. Pushing a vehicle makes the motor turn, but how does it actually get the engine to start???

Last edited by a moderator: Feb 25, 2016
2. Feb 25, 2016

haruspex

Seems a bit open-ended.
Was the lorry on the level, or facing downhill enough that it didn't need much of a push?
Consider what gear the lorry was in - high or low?
Is it Diesel or petrol?

To answer your specific question about how turning the motor makes it start, what do you think the starter motor does? Maybe you think you have to have a charged battery for the ignition, but the ignition requires very little compared to the starter motor, and as soon as the engine is turning it will generate enough for the ignition. (For petrol engines anyway, not sure about Diesel... More info here https://en.m.wikipedia.org/wiki/Push_start.)

So how does momentum come in? What the question does not explain is that the vehicle should be set rolling with the clutch depressed. When it reaches some speed, the clutch is let in. Think about the momentum of the vehicle and the torque produced suddenly.

By the way, US readers may be flummoxed by "flat battery". To them, it's a dead one.