# F= ma on a car .?

1. Sep 20, 2011

### khurram usman

F= ma on a car.....?

suppose i am pushing a toy car or a real size car with a constant velocity...say 10m/s. now as there is no change in velocity so a=0 . then according to F=ma , F=0.
but the situation seems otherwise ..that is i am applying force

2. Sep 20, 2011

### DaveC426913

Re: F= ma on a car.....?

Friction with the ground is an existing force you must overcome with a counterforce in order to keep the vehicle moving at constant velocity. Once you do so, the net force on the vehicle is zero.

3. Sep 20, 2011

### Staff: Mentor

Re: F= ma on a car.....?

Newton's 2nd law says that the net force must equal 'ma': ΣF = ma. If the car moves with constant velocity, there must be another force counteracting your push to give a net force of zero.

4. Sep 20, 2011

### olivermsun

Re: F= ma on a car.....?

The "net" force (sum of all forces) on the car is 0, so the car does not accelerate.

There is some resistance to movement of the car, perhaps due to friction; call it $-F_\text{friction}$. Left to its own, the car would slow down because of this resisting force.

You are pushing the car forward with a force as well, so call this force $+F_\text{pushing}$.

If $F_\text{pushing}-F_\text{friction} = 0$, then the net force is 0 and the car continues along its merry way without a change in velocity.

Edit: Look, 2 other correct answers in the time it took me to type this!

5. Sep 20, 2011

### khurram usman

Re: F= ma on a car.....?

so the net force should be 0 in my case
that is the force which i am applying + resistance or friction = 0?

6. Sep 20, 2011

### DaveC426913

Re: F= ma on a car.....?

Yup.

7. Sep 20, 2011

### khurram usman

Re: F= ma on a car.....?

ok thanks