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when you jump from an elevated position you usually bend your knees upon reaching the ground. by doing this, you make the time of the impact about 10 times as great as for a stiff legged landing. in this way, the average force your body experiences is...

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about 1/10th as great by bending your legs 

Time extended decreases acceleration value. 

solve using Newton's 2nd and Uniform Acceleration Laws. plug in values for descending. 

Vf = 0.0 m/s 

Vi = 10 m/s 

t = 1 s 

Acceleration is given by 

a = [Vf - Vi] / t 

a = [ (0.0 m/s) - (10 m/s) ] / (1 s) 

a = [ -10 m/s ] / (1 s) 

a = -10 m/s^2 

Solve the same thing, but with time 10x, so t = 10 s 

a = [ (0.0 m/s) - (10 m/s) ] / (10 s) 

a = [ -10 m/s ] / (10 s) 

a = -1 m/s^2 

Solve for first force and second force, using a mass of 100 kg 

F = m * a 

F = (100 kg) * (-10 m/s^2) 

F = -1,000 N 

F = (100 kg) * (-1 m/s^2) 

F = -100 N 

So divide long-force by short-force 

(-100 N) / (-1,000 N) = 0.1 (which is 1/10)

The average force of person with bending legs becomes 1/10 times the force exerted with stiff legs.

Let m be the mass of a person.

Now, the expression for the average force offered by the person at standing condition is,

[tex]F_{1}=\dfrac{mv}{t_{1}}[/tex]  .........................................................(1)

Here, v is the speed of person and [tex]t_{1}[/tex] is the impact time.

Now, consider the bending condition,, where the impact time is about 10 times as great as for a stiff legged landing. Which means,

[tex]t_{2}=10 \times t_{1}[/tex]

Then, average force is,

[tex]F_{2}=\dfrac{mv}{t_{2}} \\\\F_{2}=\dfrac{mv}{10 \times t_{1}} ............................................................(2)[/tex]

Taking ratio of equation (1) and (2) as,

[tex]\dfrac{F_{1}}{F_{2}}=\dfrac{\dfrac{mv}{t_{1}}}{\dfrac{mv}{ 10 \times t_{1}}} \\\\F_{1} = 10 \times F_{2}\\\\F_{2}=\dfrac{F_{1}}{10}[/tex]

Thus, we can conclude that on bending the legs, the average force of person becomes 1/10 times the force exerted with stiff legs.

Learn more about the average force here:

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