# Homework Help: Work done by air resistance and gravity

1. Nov 10, 2008

### Maiia

1. The problem statement, all variables and given/known data
A baseball is hit high into the upper bleachers of left field. After the ball lands in the bleachers, what is the work done by gravity and the work done by air resistance?

Wouldn't they both be zero because they are acting 90 degrees to the path of displacement?

2. Nov 10, 2008

### LowlyPion

Neither are actually. The vertical component of the ball's flight is upwards against gravity and then maybe down before impacting the bleachers. And the ball encounters air resistance in the direction of its motion.

3. Nov 10, 2008

### Maiia

in that case, if you were just considering the work when it hits the bleachers, the work for air resistance and gravity would both be positive, right? Which means the displacement for the ball is both down and in the direction (left/right depending on which way it is hit)?

4. Nov 10, 2008

### LowlyPion

No. The air resistance is negative work. It is against the direction of motion.

The Work done by gravity is also negative as it subtracted from the original kinetic energy of the struck ball.
That work however went into increasing the Potential energy that is given by the height of the ball in the grandstand * m*g.

To the extent that the ball went higher than it landed - it went up and then came down a bit - the downward path would have had positive effect on kinetic energy, but I think the problem wants you to consider the net effect, hence negative net work.