# What's the speed of ball when travelling over the fence?

## Homework Statement

Two girls are playing baseball in a park. The batter hits the 400-g ball with a speed of 20.0 m/s. It travels over the park's fence that is 4.00 m high. Assuming that the ball loses half of its energy to air resistance, at what speed does the ball travel over the fence?

Ek = 0.5*m*v^2
Ep = m*p*h

## The Attempt at a Solution

0.5*(0.4Kg)*(20 m/s)^2 = 0.5 [(0.4Kg)(9.81 m/s^2)((4m) + 0.5(0.4Kg) Vf^2]
My answer was wrong.

## Answers and Replies

haruspex
Science Advisor
Homework Helper
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2020 Award
Have another think about the first 0.5 on the right of the =.

0.5*(0.4Kg)*(20 m/s)^2 = 0.5 [(0.4Kg)(9.81 m/s^2)((4m) + 0.5(0.4Kg) Vf^2]
My answer was wrong.

You're line of thinking is correct. Just stress on which energy needs to be reduced by half.

I tried reduce the Kinetic energy by half, but the answer is incorrect. Thanks.

I tried reduce the Kinetic energy by half, but the answer is incorrect. Thanks.
That's not what you've done in your equation. Check again.

haruspex
Science Advisor
Homework Helper
Gold Member
2020 Award
I tried reduce the Kinetic energy by half, but the answer is incorrect. Thanks.
Which term in your equation represents the initial KE? Where is that multiplied by one half?