IGCSE Physics - Momentum and Impulse

I am really stuck on an Isaac Physics question 19.8
Your mass is 60.6kg and you are about to land on your feet after a jump, falling at 0.85m/s. Calculate the average force acting on each foot if: you bend your knees and stop in 0.75s.

This is what I have done:momentum = mass x velocity
momentum = 60.6 x 0.85 = 51.51 kg m/s
change in momentum = impulse = 51.51 (as it has now stopped so 51.51-0=51.51)
impulse = Force x time
Force = impulse / time
Force = 51.51 / 0.75 (time taken to stop)
Force = 68.68
because there are 2 feet = 68.68/2 = 34.34 N

I have followed the hints and watched the videos. When I enter my answer of 34.34N it says that F=ma is the resultant force and not the force on the feet and that I need to take into consideration the weight.

I am not sure what I am doing wrong and would greatly appreciate some guidance.
Thank you

Reply 1

skittlessh

12

use f=(mv-mu)/t *i think*

Reply 2

PandagirlRLC

OP

5

Original post by skittlessh

use f=(mv-mu)/t *i think*

I have technically used that as impulse = change in momentum (mv-mu)

Reply 3

Stonebridge

13

What they mean is that there are 2 forces acting on you, your weight downwards, mg, and the force through your feet, P.
It looks like they have used F=ma to find the resultant force, F, on the body.
They would have found acceleration from v = u + at
t = 0.75s
v = 0
u = 8.5m/s
Then F = ma
to get F
This is the resultant force that stops the motion of the body.
The forces acting on the body are mg downwards and P on the feet upwards.
It is the resultant of these 2 that equals F from F = ma

The same logic applies to using momentum to find the force, as you did.

Reply 4

PandagirlRLC

OP

5

Original post by Stonebridge

What they mean is that there are 2 forces acting on you, your weight downwards, mg, and the force through your feet, P.
It looks like they have used F=ma to find the resultant force, F, on the body.
They would have found acceleration from v = u + at
t = 0.75s
v = 0
u = 8.5m/s
Then F = ma
to get F
This is the resultant force that stops the motion of the body.
The forces acting on the body are mg downwards and P on the feet upwards.
It is the resultant of these 2 that equals F from F = ma

The same logic applies to using momentum to find the force, as you did.