What Is the Average Force on a Parachutist's Feet When Landing Stiff-Legged?

In summary, the impact force on the jumper's feet when landing stiff-legged with a mass of 80 kg and an impulse duration of 0.019 s is 4.03 x 104 N upwards. This is due to the combination of the normal force from the ground and the force of gravity acting on the jumper.
  • #1
Killic
2
0

Homework Statement


When a person parachutes, the impact velocity is equal to that attained in free fall from a height of 4.5 m. After contacting the ground, the jumper’s momentum is quickly brought to zero by the earth. Assuming that the person has a mass of 80 kg, what is the average force on the jumper’s feet if he lands stiff-legged and the impulse lasts only 0.019 s?

The Attempt at a Solution


v22 = v12 + 2aΔd
v22 = (2)(1.8)(4.5)
v2 = 9.4 m/s [down]

p = mv
p = (90)(9.4)
p = 752 N

F = Δp/Δt
F = 750/0.019
F = 3.9 × 104 N [up]

However, the correct answer is 4.03 x 104 N [up]. My teacher hinted towards the normal force to the person's weight is added to the answer. This confuses me as I thought that the impulse takes that into consideration already. Someone please help clarify this for me. Thanks in advance.
 
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  • #2
Hi Killic... http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

The answer you obtained (through omitting gravity) is the answer that applies if the problem was reworded for horizontal motion, viz., a speed skater comes off the rink at 9.4 m/sec and collides with a plasterboard-encased concrete wall, coming to a stop in 0.019 secs.

But it takes additional force to halt a body moving vertically because gravity is trying to accelerate the body (pulling downwards) all the time that friction (pushing upwards) is trying to slow the body.
 
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  • #3
Killic said:

Homework Statement


When a person parachutes, the impact velocity is equal to that attained in free fall from a height of 4.5 m. After contacting the ground, the jumper’s momentum is quickly brought to zero by the earth. Assuming that the person has a mass of 80 kg, what is the average force on the jumper’s feet if he lands stiff-legged and the impulse lasts only 0.019 s?


The Attempt at a Solution


v22 = v12 + 2aΔd
v22 = (2)(1.8)(4.5)
v2 = 9.4 m/s [down]

p = mv
p = (90)(9.4)
p = 752 N

F = Δp/Δt
F = 750/0.019
F = 3.9 × 104 N [up]

However, the correct answer is 4.03 x 104 N [up]. My teacher hinted towards the normal force to the person's weight is added to the answer. This confuses me as I thought that the impulse takes that into consideration already. Someone please help clarify this for me. Thanks in advance.

Your teacher gave a good lead.
You have calculated the net Force on the jumper.
That is; the result after adding the upward force from the ground to the downward force due to gravity.
 

FAQ: What Is the Average Force on a Parachutist's Feet When Landing Stiff-Legged?

What is force and impulse when landing?

Force and impulse when landing refer to the amount of energy or momentum that is exerted on an object when it comes into contact with the ground. This force and impulse can be measured by the amount of pressure or impact that is applied to the object upon landing.

How does force and impulse affect the body during landing?

Force and impulse can have a significant impact on the body during landing. The amount of force and impulse can determine the level of injury or discomfort that a person may experience. Higher force and impulse can lead to more severe injuries, while lower force and impulse may result in minimal discomfort.

What factors can affect the force and impulse when landing?

There are several factors that can affect the force and impulse when landing. These include the mass and speed of the object, the surface and angle of impact, and the stiffness of the object. Other factors may include air resistance and the presence of other external forces.

How can we reduce the force and impulse when landing?

There are a few ways to reduce the force and impulse when landing. One way is by increasing the surface area of the object, which can help distribute the force over a larger area. Another way is by decreasing the speed or mass of the object, which can also help reduce the impact. Additionally, using proper techniques and protective gear can also help reduce force and impulse during landing.

What are some examples of force and impulse during landing in everyday life?

Force and impulse during landing can be seen in everyday activities such as jumping off a ledge, landing after a jump in sports, or even walking and running. Other examples include the impact felt when stepping off a curb or when jumping down from a higher surface. Force and impulse are also important to consider when participating in activities such as skiing, skateboarding, or other extreme sports.

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