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The car has mass 1250 kg and is moving at a speed of 28.0 m s–1. The lorry has mass 3500 kg and a speed of 25.5 m s–1. The length of the flat back of the lorry allows a braking distance of 5.0 m.

By considering both momentum and energy show that the stunt is possible, provided a minimum force of about 600 N slows the car down. You should support your explanations with calculations.

Treat the situation as one in which two objects join together

**I manage to get the answer of 576 N using the method told to use.**

However, I wanted to know, why doesn’t the answer work out if I do:

1. find velocity using conservation of linear momentum (as would in the normal procedure)

2. use constant acc equations to find the deceleration

3. then do F=ma?

This would give Acc = -9.73 and F = 12kN?

However, I wanted to know, why doesn’t the answer work out if I do:

1. find velocity using conservation of linear momentum (as would in the normal procedure)

2. use constant acc equations to find the deceleration

3. then do F=ma?

This would give Acc = -9.73 and F = 12kN?