Why does this kinematics problem seem impossible.HELP

[charmedbeauty]

why does this kinematics problem seem impossible.HELP!

Homework Statement

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

Homework Equations

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 v_f=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 ??

I have tried so many different ways but I keep getting stuck. please help!

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.??

 

[SammyS]

Homework Statement

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

Homework Equations

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 v_f=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 ??

I have tried so many different ways but I keep getting stuck. please help!

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.??

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

How did you arrive at your answer to part iv ?

 

[johnconnor]

Ditto SammyS' post.

 

[charmedbeauty]

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

How did you arrive at your answer to part iv ?

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

by the way for v_icm I did

U_i+K_i=U_i+K_i

and solved for v_cm=√(2gh)

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

 

[charmedbeauty]

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

How did you arrive at your answer to part iv ?

Does this seem right now...

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

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

t=0.14

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

x_y= 0.72

Therefore no -----> 0.72<0.80

Does this seem right?

 

[SammyS]

The acceleration you have for part iii is wrong.

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

 

[charmedbeauty]

The acceleration you have for part iii is wrong.

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

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