# Finding velocity before impact

• seanbugler
In summary: To find the average force, you can use the formula F=ma and solve for F. In summary, the problem involves finding the velocity of a person jumping from a 3.4 meter high roof and bending their knees to decelerate over a distance of 0.70 meters. The velocity can be found using the law of conservation of energy, and the average force exerted on the torso by the legs can be found using the formula F=ma.
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.

seanbugler said:

## 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?

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

his velocity would be 8.14 m/s. i think

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

the force would be gravity in this case right?

seanbugler said:
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?

Last edited:
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.

Anden said:
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.

## 1. What is meant by "velocity before impact"?

Velocity before impact refers to the speed at which an object is moving just before it collides or makes contact with another object or surface.

## 2. How is velocity before impact calculated?

Velocity before impact can be calculated using the formula v = u + at, where v represents the final velocity, u represents the initial velocity, a represents acceleration, and t represents time.

## 3. What factors affect the velocity before impact?

The velocity before impact is affected by several factors, including the initial velocity of the object, the acceleration due to gravity, and any external forces acting on the object.

## 4. Why is it important to know the velocity before impact?

Knowing the velocity before impact is important in understanding and predicting the outcome of a collision or impact, and can also help in designing safety measures to reduce the impact force.

## 5. Can the velocity before impact be negative?

Yes, the velocity before impact can be negative if the direction of the object's movement is opposite to the direction of its final velocity. This is often referred to as a deceleration or a negative acceleration.

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