# Find the change in kinetic energy of the car

1. Mar 9, 2010

### hcps-chent1

1. The problem statement, all variables and given/known data
A 2436 kg car has a speed of 10.7 m/s when it hits a tree. The tree doesn’t move and the
car comes to rest.
a) Find the change in kinetic energy of the car.
b) Find the amount of work done by the car as its front is pushed in.
c) Find the magnitude of the force that pushed the front of the car in by 54 cm.

2. Relevant equations
Kinetic energy=1/2mass*speed^2

3. The attempt at a solution
1/2*2436(10.7^2)=139449J of kinetic energy now what

2. Mar 9, 2010

### PhanthomJay

Punt. Or since the problem asks for the change in KE, how much KE does it have when it stops, and what is its change? Then think the Work-Energy theorem.

3. Mar 11, 2010

### hcps-chent1

got a and b now what about c

4. Mar 11, 2010

### dacruick

question C seems like its a little bit vague. This is my take on it. The car is going 10.7 m/s, and stops to 0 m/s in a distance of 54 centimetres. If you assume that it stops through a constant deceleration (which it probably wouldn't in a real situation), you can find out that deceleration. Once you've found that, you have the mass of the vehicle, and the deceleration of the vehicle, and you should be able to find the magnitude of the force.

5. Mar 11, 2010

### PhanthomJay

Or you could apply the definition of work. The force calculated is an average force.

6. Mar 11, 2010

### Bassir

Divide the work done (part b) by distance (0.54 m).

W = Fd
F = W/d

7. Mar 17, 2010

### hcps-chent1

thanks

8. Mar 17, 2010

### dacruick

haha yeah thats definitely the right way. for some reason i read that it was a work and energy problem, and then decided to not take that into account during my response