# Homework Help: Dynamics physics help please

1. Mar 29, 2012

### shubham1

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

A 1350 kg car narrowly avoids running off of a washed out bridge. The coefficient of friction between the car and the road is 0.55. If the skid marks are 100 meters long, what was the initial velocity of the car?

2. Relevant equations

Fg = mg
Ff = coefficient x Fn

3. The attempt at a solution

Find Fg
Find Ff
Find acceleration
Find final velocity

2. Mar 29, 2012

### Staff: Mentor

Re: Dynamics physics urgent help please!!

shubham1, you'll have to show a bit more of an attempt. What have you tried? Show us what you've tried so we can know how to help you.

3. Mar 29, 2012

### NewtonianAlch

Re: Dynamics physics urgent help please!!

d = 100m
v = 0 m/s (final)
u = ? m/s (initial)

a = Look in your book for an equation relating the coefficient of friction to the mass of the object.

What is one of the kinematic equations you know relating acceleration, displacement, and velocities?

4. Mar 29, 2012

### shubham1

Re: Dynamics physics urgent help please!!

Important info I forgot to mention: Applied braking force is 9500 N.
I am not sure which way is the Fnet.
I found Fg, which is 13230 N downwards.
I found Ff which is 7276.50 backwards.
I found acceleration which is 12.427 N backwards.
However, my teacher told me to make the Fnet positive, but which way is the Fnet? Is it where the friction is? and also, is the Fa opposite to the Ff.

5. Mar 29, 2012

### NewtonianAlch

Re: Dynamics physics urgent help please!!

Fnet is probably the net force, which is going forwards. The braking force is what is slowing down the car, which is why your acceleration is going to be negative, as you said you found acceleration to be "backwards". So if acceleration is going this way <--- and is negative, and Fnet should be positive, it should be going ---> this way.

6. Mar 29, 2012

### shubham1

Re: Dynamics physics urgent help please!!

Fnet=ma
If a is negative (backwards), then shouldn't Fnet also be negative.

7. Mar 29, 2012

### NewtonianAlch

Re: Dynamics physics urgent help please!!

I think you're going to have write out what these symbols mean before it gets confusing.

This isn't as complicated as it looks, have you first considered the equation you will be using to solve for the initial velocity? When you know what equation you will use, it will be a lot clearer.

8. Mar 29, 2012

### shubham1

Re: Dynamics physics urgent help please!!

For sloving initial velocity, I will use Vf^2 = Vi^2 + 2ad

9. Mar 29, 2012

### NewtonianAlch

Re: Dynamics physics urgent help please!!

Acceleration = F/m

Where F = frictional force.

You claimed your acceleration was 12N backwards, how did you get 12 Newtons for acceleration? The unit for acceleration is not Newtons.

10. Mar 29, 2012

### shubham1

Re: Dynamics physics urgent help please!!

Sorry, I meant 12.427 m/s^2 backwards.

I basically did:

Fnet = Fa + Ff (sum of all the forces)
ma = -Fa - Ff
(1350)(a) = -9500-7276.50
a = -12.427 m/s^2 backwards

11. Mar 29, 2012

### azizlwl

Re: Dynamics physics urgent help please!!

If the skid marks are 100 meters long,

You are not using this data