# Velocity and acceleration

1. Sep 30, 2008

### swede5670

1. The problem statement, all variables and given/known data
Flash Maddie is running at a whopping velocity of 30 mph. She then sees Mr. T driving down the road straight towards her so she accellerates -5 mph/s for 10 seconds. what Is Maddie's velocity now. (In mph)

2. Relevant equations
V = Change in distance / change in time
a = change in velocity / change in time

3. The attempt at a solution
I'm just not sure how to approach the problem in general and I'm not sure how to approach the negative acceleration.

2. Sep 30, 2008

### Hootenanny

Staff Emeritus
Whilst you could in principle use those equations, this question is best solved using kinematics equations (of uniform acceleration).

3. Sep 30, 2008

### Redbelly98

Staff Emeritus
Use this equation:
Plug in the numbers you know, including any negative signs, and solve the equation. That will give you a key piece of information for solving the problem.

4. Sep 30, 2008

### swede5670

Hootenanny: What are these equations and how would I use them?
Red Belly: Should I multiply -5 MPH/s by 10 and then subtract it from 30 mph?

Last edited: Sep 30, 2008
5. Sep 30, 2008

### Hootenanny

Staff Emeritus
Yes, that's correct (with one minor correction, you should add the -50 to 30). In actual fact, both methods are identical:

$$a=\frac{\Delta v}{\Delta t} = \frac{v_f-v_i}{\Delta t}$$

$$\Rightarrow v_f = v_i + a\Delta t$$

Which is one of the kinematic equations I was refering to.

6. Sep 30, 2008

### Redbelly98

Staff Emeritus
It may be that swede's class has not quite gotten to the full set of kinematic equations ... at any rate, Hootenanny is entirely right.