# Finding force that ground has on person

• joseph_kijewski
In summary, a person jumps from the roof of a house and bends their knees when they hit the ground, decelerating their torso over a distance of .70m. Their torso has a mass of 42kg and a velocity of 8.7m/s just before their feet strike the ground. The average force exerted on their torso by their legs during deceleration is 2268 N, which is equal to the net force. This force is necessary to support the weight of the torso and provide the necessary acceleration to stop it from moving.
joseph_kijewski

## Homework Statement

A person jumps from the roof of a house 3.9-m high. When he strikes the ground below, he bends his knees so that his torso decelerates over an approximate distance of .70m. If the mass of his torso (excluding legs) is 42 kg, find a. his velocity just before his feet strike the ground, and b. the average force exerted on his torso by his legs during deceleration

## The Attempt at a Solution

Had no trouble with a, got 8.7m/s as maximum velocity, however I seem to be wrong on b. Here is my work

1/2at^2=0.70
a=1.40/(t^2)
a*t=8.7
(1.40/t^2)*t=8.7
(1.40/t)=8.7
8.7t=1.40
t=1.61
8.7/.161=5.40 m/s^2
42*54.0=m/s^2 upward acceleration
42*54.0=2268 N
My problem here is that everyone on the internet says that the force exerted on his torso by his legs IS 2268, but I feel like that should be only the net force, because gravity is still in play and the ground must have a higher force to counteract that. Why would the answer not be 2268+9.8*42=2679.6--->2700N (sig fig)??

Hi Josepph_kijevski and welcome to PF.

You are correct. The 2268 N is ma which is the same as the net force. The legs are providing the necessary force to support the weight of the torso and to provide the necessary acceleration to stop the torso from moving.

Next time please include the relevant equations so that we know where you start from.

kuruman said:
Hi Josepph_kijevski and welcome to PF.

You are correct. The 2268 N is ma which is the same as the net force. The legs are providing the necessary force to support the weight of the torso and to provide the necessary acceleration to stop the torso from moving.

Next time please include the relevant equations so that we know where you start from.

Thank you!

## 1. What is the force that the ground has on a person?

The force that the ground has on a person is known as the normal force. It is a contact force that acts perpendicularly to the surface of contact between the person and the ground.

## 2. How is the force of the ground on a person calculated?

The force of the ground on a person is calculated using Newton's second law of motion, which states that force is equal to mass multiplied by acceleration (F=ma). In this case, the acceleration is due to gravity, and the mass is the person's weight.

## 3. Does the force of the ground on a person change in different locations?

Yes, the force of the ground on a person can vary in different locations. This is because the force of gravity, and thus the weight of the person, can be affected by factors such as altitude and the density of the Earth's crust.

## 4. How does the force of the ground on a person affect their movement?

The force of the ground on a person is a crucial factor in determining their movement. It provides the necessary support for the person to stand, walk, or run. The magnitude and direction of this force also play a role in determining the person's balance and stability.

## 5. Can the force of the ground on a person be stronger than their weight?

Yes, the force of the ground on a person can be stronger than their weight. This can happen when a person is standing on a surface that is accelerating upwards, such as an elevator. In this case, the normal force exerted by the ground will be greater than the person's weight due to the additional acceleration of the surface.

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