Jumping impulse physics homework

[keemosabi]

Homework Statement

When jumping straight down, you can be seriously injured if you land stiff-legged. One way to avoid injury is to bend your knees upon landing to reduce the force of the impact. A 75 kg man just before contact with the ground has a speed of 6.0 m/s.

(a) In a stiff-legged landing he comes to a halt in 1.5 ms. Find the average net force that acts on him during this time.
N
(b) When he bends his knees, he comes to a halt in 0.13 s. Find the average force now.
N
(c) During the landing, the force of the ground on the man points upward, while the force due to gravity points downward. The average net force acting on the man includes both of these forces. Taking into account the directions of these forces, find the force of the ground on the man in parts (a) and (b).
stiff legged landing
N

bent legged landing
N

Homework Equations

The Attempt at a Solution

I got part a, b and the first part of c. How is the second part of c different from the answer in b?

 

[PhanthomJay]

Homework Statement

When jumping straight down, you can be seriously injured if you land stiff-legged. One way to avoid injury is to bend your knees upon landing to reduce the force of the impact. A 75 kg man just before contact with the ground has a speed of 6.0 m/s.

(a) In a stiff-legged landing he comes to a halt in 1.5 ms. Find the average net force that acts on him during this time.
N
(b) When he bends his knees, he comes to a halt in 0.13 s. Find the average force now.
N
(c) During the landing, the force of the ground on the man points upward, while the force due to gravity points downward. The average net force acting on the man includes both of these forces. Taking into account the directions of these forces, find the force of the ground on the man in parts (a) and (b).
stiff legged landing
N

bent legged landing
N

Homework Equations

The Attempt at a Solution

I got part a, b and the first part of c. How is the second part of c different from the answer in b?

The anser you got in a and b should be the net force, which includes both gravity and the normal force of the ground acting on the man. To get the normal force of the ground only, both for in parts a and b, draw the FBD.

 

[keemosabi]

The anser you got in a and b should be the net force, which includes both gravity and the normal force of the ground acting on the man. To get the normal force of the ground only, both for in parts a and b, draw the FBD.

I drew it so it would be just the normal force, 75 * 9.8? But it said that was wrong.

Edit: Or is it the net force plus the gravitational force?

 

[PhanthomJay]

I drew it so it would be just the normal force, 75 * 9.8? But it said that was wrong.

Edit: Or is it the net force plus the gravitational force?

75*9.8 is the gravitational force acting on him, that is, his weight, acting down. The normal force is the force of the ground acting up on him. Since the net force is up, and the gravitational force is down, what's the normal force?

 

[keemosabi]

75*9.8 is the gravitational force acting on him, that is, his weight, acting down. The normal force is the force of the ground acting up on him. Since the net force is up, and the gravitational force is down, what's the normal force?

So 3461.53846154 is the net force, and 9.8 * 75 is the gravitational force, then the force exerted by the ground is just the sum of those two? 4196.5385?