# Finding Take-Off Velocity

Tags:
1. Feb 17, 2016

### x2017

1. The problem statement, all variables and given/known data
Kwaku Boateng is a former Canadian Olympic high jumper. If an average vertical ground reaction force of 1959.6 N acted on Kwaku's foot for 0.20 s during the upward phase of his jump, and Boateng's mass is 79.1 kg, what would his take-off velocity be?

2. Relevant equations
ΣFΔt=mΔv

3. The attempt at a solution
I tried it twice...

Attempt 1:
ΣFΔt=mΔv
Δv=(ΣFΔt)/m
Δv=(1959.6)(0.20)/79.1
Δv=4.95m/s

The online assignment says that this answer is incorrect.

Attempt 2:
ΣFΔt=mΔv
Vf-Vi=(ΣFΔt)/m
Vi=-[(ΣFΔt)/m]+Vf
Vi=(1959.6)(0.20)/79.1
Vi=-4.95m/s

The online assignment says that this answer is incorrect.

Should I be using another equation? Or have I made a simple mistake?

2. Feb 17, 2016

### RUber

Do you need to include the force of gravity?

3. Feb 17, 2016

### x2017

Ahhhhh, yes that's it!

ΣFΔt=mΔv
Δv=(ΣFΔt)/m
Δv=[1959.6-(79.1)(9.81)](0.20)/79.1
Δv=2.99m/s

The computer is telling me 2.99m/s is the correct answer, thank you for your help RUber!