physics next step question

[eric george delgado]

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Don't understand how they got 100m

 

[Dirtybird]

I didn't use the method they gave in the solution, but still came up with the same answer.

It might be easier to look at this question from a work/energy perspective. Friction is a nonconservative force, which is equal to total change in potential + kinetic energy (equation circled in red).

Yes, I got a negative answer but it still makes sense. What matters is the total distance the force of 2N had to act on the box in order to bring it to a stop (Kf = 0). The work is negative because the movement is in the opposite direction of the frictional force.

If that last concept sounds confusing, Khan academy has a decent explanation about negative work.

 

[itsalwayssunny96]

So I didn't look at the way they solved this question but did it the same way as they did. First, you have to keep in mind that forces are vectors so pick a positive for one direction and a negative for the other.... this is arbitrary and all that matters is that you are consistent. In this case, I made the forward direction positive (which is the way the box is moving) and the opposing direction negative. Solve for acceleration and plug into Vf^2 equation and you should get a distance of 100 m. The 100 m is positive since the box is moving in the direction to the right which I designated as positive. You can also do it Dirtybird's way! Whichever way makes more sense. Oh, and the way I knew Vf was 0m/s is that you can assume that the box is going to stop based on how they asked the question. It will stop sliding and reach a total maximum distance once there is 0 velocity.

 

[eric george delgado]

thank you both! very much appreciated.

 

[itsalwayssunny96]

thank you both! very much appreciated.

No problem man.