# physics

A 8kg ball is thrown up with an initial speed of 20m/s. Suppose there is a constant air resistence of 30N opposing the motion during its entire up and down path. How long will it take to get to the highest point in its path? how high is the highest point? How much work was done by the air resistence during the entire path up and down? What is the kinectic energy when it gets back to its original point?

posted by drwls
2. I don't how to start it off

posted by dipset
3. When the ball reaches its highest point H, potential energy will be
M g H = (1/2)MVo^2 - f*H
where f is the friction force, 30N.

H = (1/2)MVo^2/(Mg + f)

Solve for H.

The time to get to that high point will be
T = H/Vo, since it will be decelerating at a constant rate.

The KE when it hits the ground will be M g H - f*H. This will equal
(1/2)MVo^2 - 2fH

posted by drwls
4. Thank you. one more question what would be the acceleration on the way up? on the way down?

posted by dipset
5. acceleration due to gravity

posted by Dr Russ
6. The acceleration is
g + (f/M) on the way up, and g - (f/M) on the way down. It is pointed down in both cases.

They are different because both g and friction point down on the way up, but are in opposite directions on the way down. For that reason, it arrives at the ground at a lower velocity than it took off.

posted by drwls

## Similar Questions

1. ### physics

a 0.8kg ball is kicked with the speed 20m/s at an 50 to the horizontal.calculate the kinetic energy of the ball at maximum height. m=0.8kg v=20m/s h=?
2. ### Physics

Ball is thrown horizontally off a cliff with an initial speed of 20m/s. a) how long does it take the ball to hit the ground 20m below. b)how long would it take to hit the ground if it was dropped from the cliff? c) how long would
3. ### physics

A ball is thrown upward. Its initial vertical component of velosity is 30m/s and its initial horizontal component of velosity is 20m/s. What is the ball's speed 2 s later? Give: Viy = 30m/s Vix = 20m/s T = 2 sec a) V = gt
4. ### physics

Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the
5. ### Physics

When baseball outfielders throw the ball, they usually allow it to take one bounce on the theory that the ball arrives sooner this way. Suppose that after the bounce the ball rebounds at the same angle theta as it had when
6. ### physics

A ball is thrown vertically upward with an initial speed of 25 m/s. Then, 1.8 s later, a stone is thrown straight up (from the same initial height as the ball) with an initial speed of 32.1 m/s . How far above the release point
7. ### Physics

A ball is thrown vertically upward with an initial speed of 11 m/s . Then, 0.67 s later, a stone is thrown straight up (from the same initial height as the ball) with an initial speed of 25 m/s . How far above the release point
8. ### AP Physics

A baseball player throwing the ball from the outfield usually allows it to take one bounce on the theory that the ball arrives sooner this way. Suppose that ball hits the ground at an angle θ and then bonuses but loses half