# Dynamics Question: Finding mass using Vi, Vf, D and acceleration

1. Jul 21, 2012

### Apollinaria

Hi everyone, I'm new to forum and am a 3rd year university student who has never taken physics until now.. I'm currently studying for a dynamics unit test and came upon this question....

1. The problem statement, all variables and given/known data

A car whose brakes are locked skids to a stop in 70m from an initial velocity of 22.22m/s. Find the coefficient of kinetic friction.

Vi: 22.22m/s
Vf: 0m/s
d: 70m
a(deceleration): -3.526m/s^2
g= 9.81m/s^2
μ=?

2. Relevant equations

- Found deceleration using kinematics formula Vf^2=Vi^2 =2ad
- Ff= μFn
- Fg=mg
- Fnet(y)=ma ; a(up/down)=0 ; Fnet(y)=0 ; Fn-Fg=0 ; Fn=Fg

3. The attempt at a solution

In order to find Fn, in order to be able to solve for μ, I need to know the mass of the car... I have no idea where to even begin solving for the mass of the car using the currently known values :(

If anyone has time, please point me in the right direction.

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Last edited: Jul 21, 2012
2. Jul 21, 2012

### Saitama

I suppose it should be Fnet(x)=ma.
What is Fnet(x) here? (Here only one force is acting in the direction of motion of car i.e force of friction)

EDIT: Oh, btw, welcome to PF!

3. Jul 21, 2012

### Apollinaria

Hi there :)
Fnet(x)=ma

We have the acceleration but still not the mass. That's where I lost it :uhh:

EDIT: Thanks very much! I've been circling around this forum for weeks now and finally decided to join since magic tutor couldn't save me and the test is on Monday :P

4. Jul 21, 2012

### Saitama

I haven't checked your calculations but can you write $μmg=ma$? See the m cancels on both the sides, you don't require the mass of car now.

5. Jul 21, 2012

### Muphrid

I'm not sure why you think you need the mass. If you already have the acceleration, then you know that the force of kinetic friction $F_k = ma$ is also equal to $\mu F_g = \mu m g$. The mass cancels out.

6. Jul 21, 2012

### Apollinaria

OMG! I can't believe I missed that! I see what you did there
Grazie mille!:!!)

7. Jul 21, 2012

### Apollinaria

Because logic and the obvious escape me
This is what happens when a mathematically challenged biology student tries to do physics Thanks for your reply!