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1. You are riding a 450-kg horse at 14.1 km/h east along a desert road. You have inertia equal to 60.0 kg. A police officer driving past (whom you know and who knows your inertia and the horse's inertia) measures your speed relative to the police car and calculates total kinetic energy of you and your horse to be 16.32 kJ .
What possible speed(s) could the police car have been driving at the instant the officer measured your speed?

So first I calculated velocity of (horse+man) relative to the car to be 8m/s.

Then in order to calculate the velocity of the car relative to the earth,

Vec= Vem+Vmc where e= earth c=car m=man
= 47/12 - 8

giving -4.8 m/s. So I presume this means the man is traveling the opposite direction to the car?

But I'm not sure where to get the other possible speed.

2. Without looking at it any more closely, is it because the kinetic energy includes v² which means there are two possible values for v, the positive and negative?

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Updated: February 10, 2018
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