Work done by air resistance and gravity

[Maiia]

Homework Statement

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?

 

[LowlyPion]

Homework Statement

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?

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.

 

[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)?

 

[LowlyPion]

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)?

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.