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.

Attached Files:

• Dynamics.jpg
File size:
26 KB
Views:
81
Last edited: Jul 21, 2012
2. Jul 21, 2012

Pranav-Arora

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

Pranav-Arora

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!