# Stalled car net force hurry!

1. Oct 10, 2011

### physicsgurl12

1. The problem statement, all variables and given/known data
A stalled car is being pushed up a hill by 3 people and it is moving at a constant speed the net forcce on the car is
a in the opposite of the cars motion
b in the same direction as the cars motion-wrong
c zero
d none of the above

2. Relevant equations

noone

3. The attempt at a solution
is it c zero because theres gravity and the normal force from the hill?

2. Oct 10, 2011

### SammyS

Staff Emeritus
How do you know that these forces cancel each other completely?

3. Oct 10, 2011

### physicsgurl12

i don't?

4. Oct 10, 2011

### SammyS

Staff Emeritus
You should. One (or two) of Newton's Laws tells you why.
...

If they don't cancel, then the net force isn't zero.

5. Oct 10, 2011

### physicsgurl12

because there opposie and equal forces?

6. Oct 10, 2011

### DaveC426913

What do you know about the net force acting upon an object that is moving with constant velocity?

7. Oct 10, 2011

### SammyS

Staff Emeritus
But how do you know they're opposite & equal. There's something in the problem that should tell you.

(One of Newton's Laws)

8. Oct 10, 2011

### physicsgurl12

that its a constant speed?

9. Oct 10, 2011

### DaveC426913

And what does that tell you about the forces acting on it, specifically the net force?

10. Oct 10, 2011

### SammyS

Staff Emeritus
Answer Dave's question by using Newton's Second Law.

11. Oct 10, 2011

### physicsgurl12

f=ma
so the forces =0 because if there is constant speed theres no acceleration

12. Oct 10, 2011

### SammyS

Staff Emeritus
We'll assume it's moving in a straight line, so constant sped means constant velocity.

Yes, that's why you know the net force is zero ! The reason it's zero is that the forces cancel. Kind of a picky point here, but it could be important for understanding what's going on.

Last edited: Oct 10, 2011
13. Oct 10, 2011

okay thanks