# Work and mass problem

1. Aug 3, 2016

### Erenjaeger

1. The problem statement,
all variables and given/known dataIt takes 190 kJ of work to accelerate a car from 20.8 m/s to 29.3 m/s. What is the car's mass?

2. Relevant equations
w=fd
f=ma
1/2mvf2-1/2mvo2=maΔx=FΔx

3. The attempt at a solution
not sure how to start this problem, because they dont give you time or displacement but you can probably work it out from the velocities im assuming? just not 100% sure on how to

2. Aug 3, 2016

### Orodruin

Staff Emeritus
Why do you think you need time or displacement?

3. Aug 3, 2016

### Erenjaeger

Could you solve by going m=f/a ??

4. Aug 3, 2016

### Orodruin

Staff Emeritus
No. Stop and think a bit about what your relevant equations are telling you about how the input parameters are related to what you want to know.

5. Aug 3, 2016

### Erenjaeger

in the third equation, using energy. Can somehow use that to solve for mass?

6. Aug 3, 2016

### Orodruin

Staff Emeritus
You tell me. In order to solve for the mass, you need to know all other variables of the equation.

7. Aug 3, 2016

### Erenjaeger

the only other variables you need to know are the velocities right? which we are given

8. Aug 3, 2016

### Orodruin

Staff Emeritus
So what is the solution?

9. Aug 3, 2016

### Erenjaeger

1/2mv2-1/2mvo2=190000J
1/2m(20.8m/s)2-1/2m(29.3m/s)2 =190000J
1/2m432.64-1/2m858.49=190000J
m⋅216.64-m⋅429.245=190000J
is this correct so far?? not fully sure how to go any further if it is ?

10. Aug 3, 2016

### Orodruin

Staff Emeritus
You are aware that multiplication is distributive, right?
x(y+z) = xy + xz

11. Aug 3, 2016

### Erenjaeger

yeah

12. Aug 3, 2016

### haruspex

The left hand side will clearly be negative, assuming mass is positive.