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

[Hoyin]

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?

Homework Equations

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.

 

[haruspex]

Have another think about the first 0.5 on the right of the =.

 

[siddharth23]

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.

 

[Hoyin]

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

 

[siddharth23]

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]

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?