Calculating Vertical Velocity and Force: Jumping Physics Problems"

[kooh]

Homework Statement

A boy has a mass of 82kg, and a vertical leap of 1.2m ,
Find the vertical velocity he leaves the ground to reach a max of 1.2m
Find the force he pushes during the 30 cm lift if he squats 30 cm before jumping

Homework Equations

The Attempt at a Solution

I used v2^2=v1^2+2a(d) to find what the v1 was, assuming v2 was 0 at the max.

 

[PhanthomJay]

Yes, that's right, what did you get for the initial velocity?

 

[kooh]

I got sqrt of 23.52 for the initial velocity when he is standing up straight

As for the force he applied when lifting the 30 cm after crouching I got,
3214.4N

for the 3214.4N , I found the acceleration he had during the 30 cm or .3m then multiplied it by his weight. I had v1 as 0 for this part.

Not sure if its 100% right tho

 

[kooh]

I used -9.8 m/s^2 (up) for acceleration for the first part of the question to find sqrt 23.52 as my initial velocity

 

[PhanthomJay]

I got sqrt of 23.52 for the initial velocity when he is standing up straight

good!

As for the force he applied when lifting the 30 cm after crouching I got,
3214.4N

for the 3214.4N , I found the acceleration he had during the 30 cm or .3m then multiplied it by his weight. I had v1 as 0 for this part.

Not sure if its 100% right tho

you multiplied it by his mass, not his weight, and what you ended up with was the net force acting on him during the 30 cm 'squat' motion. But the net force includes both the force of the ground pushing up on him and his weight acting down on him. You need to separate out the 2 to find the pushing force of the ground on him.