Calculating Take-Off Velocity for High Jumper Kwaku Boateng

[x2017]

Homework Statement

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?

Homework Equations

ΣFΔt=mΔv

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
V_f-V_i=(ΣFΔt)/m
V_i=-[(ΣFΔt)/m]+V_f
V_i=(1959.6)(0.20)/79.1
V_i=-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?

Thank you in advance for your help!

 

[RUber]

Do you need to include the force of gravity?

 

[x2017]

Do you need to include the force of gravity?

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!