How to Calculate Velocity and Force in a Jump from Height?

[seanbugler]

Homework Statement

A person jumps from the roof of a house 3.4 meters high. When he strikes the ground below, he bends his knees so that his torso decelerates over an approximate distance of 0.70 meters. If the mass of his torso (excluding legs) is 41 kg.

A. Find his velocity just before his feet strike the ground.

B. Find the average force exerted on his torso by his legs during deceleration.

Homework Equations

I can't even seem to figure that part out. Help please?

The Attempt at a Solution

I don't know how to start this at all.

 

[sylas]

Homework Statement

A person jumps from the roof of a house 3.4 meters high. When he strikes the ground below, he bends his knees so that his torso decelerates over an approximate distance of 0.70 meters. If the mass of his torso (excluding legs) is 41 kg.

A. Find his velocity just before his feet strike the ground.

B. Find the average force exerted on his torso by his legs during deceleration.

Homework Equations

I can't even seem to figure that part out. Help please?

The Attempt at a Solution

I don't know how to start this at all.

Start with step 1. What his velocity when he reaches the ground?

 

[drizzle]

hint: U=mgh=... [use the law of conservation of energy to fill the gap]

 

[seanbugler]

his velocity would be 8.14 m/s. i think

 

[Anden]

Remember mv^2 * 0,5 = E, that should help you solve the first. For the other remember that Energy is force times distance.

 

[seanbugler]

the force would be gravity in this case right?

 

[sylas]

the force would be gravity in this case right?

Right. Your answer in #4 is correct. (I get 8.16 m/s, using g=9.8 ms^-2)

Now... how do you plan to approach part B?

 

[Anden]

Not exactly, the force comes from the legs, the muscles have to work. (I'm a little unsure though.) In the first question (A) potential energy is transformed to kinetic energy, in the second the same energy is "soaked up" by the legs.

 

[sylas]

Not exactly, the force comes from the legs, the muscles have to work. (I'm a little unsure though.) In the first question (A) potential energy is transformed to kinetic energy, in the second the same energy is "soaked up" by the legs.

Yes, the force for part A is gravity. The force for part B is applied to the torso through the legs to decelerate.