# Homework Help: Why does this kinematics problem seem impossible.HELP

1. Jun 4, 2012

### charmedbeauty

why does this kinematics problem seem impossible.HELP!!!

1. The problem statement, all variables and given/known data

If the man's mass is 70 kg, if L is 0.4 m h is 0.6 m, what is the downwards force (assumed constant) exerted by his feet during the phase A to B? State any physical law or principle you use in obtaining your answer.

vi) For a man whose parameters are given in part (v), would it be possible to jump over a barrier 0.8 m high starting the jump as shown from A to B? Explain your answer.

heres a link to the problem http://www.phys.unsw.edu.au/firstyear/pdf%20files/1131T12008.pdf [Broken]

2. Relevant equations

3. The attempt at a solution

I am really struggling with part vi)

the answer for part v) is 1715N

now i know the highest point reached is when vf=0

but I am having a hard time figuring out the time for this.

All I know is that 1716=ma

so a = 24.5 m/s

and I need to see if this acceleration is large enough for a man of mass 70kg can reach a height h=0.8

are there to many unknowns in this problem or is it just me ??

also part vii)During which (if any) phases of the jump described in part (vi) is the momentum of the man conserved? Explain your answer briefly.

would it be none; since gravity is always acting on the system.??

Last edited by a moderator: May 6, 2017
2. Jun 4, 2012

### SammyS

Staff Emeritus
Re: why does this kinematics problem seem impossible.HELP!!!

What are your answers to parts ii & iii, including how you arrived at them?

Last edited by a moderator: May 6, 2017
3. Jun 4, 2012

### johnconnor

Re: why does this kinematics problem seem impossible.HELP!!!

Ditto SammyS' post.

4. Jun 5, 2012

### charmedbeauty

Re: why does this kinematics problem seem impossible.HELP!!!

ohhh now I see I can use this for vo...

by the way for vicm I did

Ui+Ki=Ui+Ki

and solved for vcm=√(2gh)

Thanks so much I never thought to look at my previous answers and had been stuck for ages. Thanks.

5. Jun 5, 2012

### charmedbeauty

Re: why does this kinematics problem seem impossible.HELP!!!

Does this seem right now...

vf=$\sqrt{2gh}+24.5t$
vf=0

t=$\sqrt{2gh}/24.5$

t=0.14

xy= $\sqrt{2gh}(0.14)+\frac{1}{2}(24.5)(0.14)^{2}$

xy= 0.72

Therefore no -----> 0.72<0.80

Does this seem right?

Last edited: Jun 5, 2012
6. Jun 5, 2012

### SammyS

Staff Emeritus
Re: why does this kinematics problem seem impossible.HELP!!!

The acceleration you have for part iii is wrong.

The force looks right, but that's not the net force exerted upon the CoM .

7. Jun 5, 2012

### charmedbeauty

Re: why does this kinematics problem seem impossible.HELP!!!

ok I have to subtract the acceleration due to gravity so the right acceleration should be 14.7m/s2??