Is Pushing Against a Car's Motion Positive Work?

[Thefox14]

Homework Statement

A car is rolling to the right and you try to slow it down by pushing on it to the left. You are doing

A) positive work
B) negative work
C) no work until the car comes to a stop
D) You can’t tell because I didn’t tell you which way was positive.

Homework Equations

Work = constant force * distance

The Attempt at a Solution

I think its might be A because you are transferring energy to the car thus positive work but I'm not quite sure. I was tempted to pick D, but after doing a little simulation in my head I chose otherwise.

Thanks!

 

[PhanthomJay]

Oh, you're 0 for 2...one more strike and you're out. Your equation for work is wrong...it's work = force times displacemnt times the cos of the angle between the 2. Which way does the force act on the car...and what is the direction of the displacement of that force?

 

[Thefox14]

Well if I said the left was negative, and the right was positive then I'd be applying force in the negative direction while the displacement would be positive. Oh so it would be B then correct? Didn't think about working it out like that

 

[PhanthomJay]

Well if I said the left was negative, and the right was positive then I'd be applying force in the negative direction while the displacement would be positive. Oh so it would be B then correct? Didn't think about working it out like that

Yes, correct. When the force and displacement are opposite, work is negative; when they are in the same direction, work is positive.

 

[rwisz]

Your reasoning is correct, the correct answer is A) positive work. When you push on the car to the left, you are exerting a force in the opposite direction of the car's motion, causing it to slow down. This means that you are doing work on the car, as you are transferring energy to it. The equation for work, as you mentioned, is force times distance, and in this case, the force (pushing to the left) and the distance (the distance the car travels as it slows down) are both in the same direction, resulting in a positive work value. If you had pushed on the car in the same direction as its motion (to the right), then the work would have been negative, as you would have been working against the car's motion and transferring energy away from it. So, in this scenario, you are indeed doing positive work on the car.