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**Problem Statement:**4*10^5 Joules of work is required to reduce a car's velocity from 30m/s to 15m/s. What is the car's mass?

**Relevant equations:**Ke=.5*m*v^2

The attempt at a solution:

The attempt at a solution:

V1 = 30m/s

V2 = 15m/s

M = mass of car

.5*M*V1^2 - 4*10^5J = .5*M*V2^2

450M - 4*10^5J = 112.5M

-4*10^2 = -337.5M

1185kg = M

**My Question:**My algebra checks out I think, my only question is if I approached the problem correctly. The question is phrased in terms of work, so do I need to find force and displacement? Or is using the equation for kinetic energy acceptable?

Thanks guys