# Force and change in momentum with time

1. Jan 10, 2006

### chowyo123

Hey, It's been about 2 weeks since I've had a physics class and I just can't seem to put things together again.
I have two problems that seem like they should be easy, and I'm sure they are, but am unsure on how to do them.

A constant force of 6.00N acts on a 3.00-kg object for 10.0 s. What are the changes in the objects momentum and velocity?

I have the equation momentum p = mv. p=(F)(delta t). Could I just do 6.00-N x 10=(3.00-kg)v and solve for v to find the velocity?

The second problem is
The velocity of a 625-kg car is changed from 10.0 m/s to 44.0 m/s in 68.0 s by an external constant force. What is the resulting change in momentum of the car? What is the magnitude of the force?
I tried to do p=(625)(44-10.0) to find the momentum, but that does not seem right to me. What do i do?

2. Jan 10, 2006

### Hootenanny

Staff Emeritus
For question one I don't think your looking for a velocity, your're looking for a change in velocity.

3. Jan 10, 2006

### lightgrav

(delta p) = F (delta t) ... there has to be a change on both sides !

or only one side, with a ratio on the other side : F = (delta p) / (delta t).

or nowhere, as in : p = m v .

now, in your (625)(44 - 10) , does the other side have a delta, or not?

and whatever happened to UNITS ?!!

4. Jan 11, 2006

### andrevdh

The impulse that an object experiences is equal to the change in it's momentum!

5. Jan 15, 2006

### chowyo123

Alright, I ended up remembering how to do these problems.

6. Jan 15, 2006

### Mindscrape

Yeah, just remember that $$F=ma$$ only works at low velocities, but $$F=\frac{dp}{dt}$$ is true always.