Finding Momentum w/ Mass and Height

[duke1]

Okay sry for the first thread it wasn't really detailed.

A 62.0 kg male dancer leaps 0.31 m high.
(a) With what momentum does he reach the ground?
kgm/s
(b) What impulse is needed to stop the dancer?
Ns
(c) As the dancer lands, his knees bend, lengthening the stopping time to 0.050 s. Find the average force exerted on the dancer's body.
N
(d) Compare the stopping force with his weight.
(stopping force/dancer's weight)

So the main problem is I can't find any worked out examples of these problems in our book. I may be wrong but I just don't see how to find momentum without another bit of information besides hieght and mass. So I'm stuck and don't even really know where to start trying to work it out. There is a time given in C but it doesn't apply yet to A.

Any ideas?? Help is really appreciated.

bp

 

[mbrmbrg]

Oh! I thought that the dancer's height was given! (As in, a 5' 11", 195 lb dancer...)

At the top of the dancer's leap (given) what is the y-component of his velocity?
With what velocity will he hit the ground? (think free fall)

 

[PhanthomJay]

Okay sry for the first thread it wasn't really detailed.

A 62.0 kg male dancer leaps 0.31 m high.
(a) With what momentum does he reach the ground?
kgm/s
(b) What impulse is needed to stop the dancer?
Ns
(c) As the dancer lands, his knees bend, lengthening the stopping time to 0.050 s. Find the average force exerted on the dancer's body.
N
(d) Compare the stopping force with his weight.
(stopping force/dancer's weight)

So the main problem is I can't find any worked out examples of these problems in our book. I may be wrong but I just don't see how to find momentum without another bit of information besides hieght and mass. So I'm stuck and don't even really know where to start trying to work it out. There is a time given in C but it doesn't apply yet to A.

Any ideas?? Help is really appreciated.

bp

The key is to find the dancer's velocity just before the dancer hits the ground. There are a couple of ways to do this, using motion equations or conservation of energy, that relate height, velocity, and the acceleration of gravity. Can you pick one?

 

[duke1]

ok...so 9.8 is free fall but how can i use that without another time or something? I am sure I am missing something simple

 

[mbrmbrg]

I am sure I am missing something simple

yup!
Do you remember your basic equations for motion with constant acceleration?

 

[duke1]

ok thanks a lot...the hint really helped so i picked out one of the kinematics we got in class...thx for the help

 

[americanforest]

(b) What impulse is needed to stop the dancer?
Ns

How do you find this? Isn't Impulse equal to Force multiplied by time and you don't know the time?

 

[verty]

Duke1, please show us how much of it you can work out now.

 

[duke1]

How do you find this? Isn't Impulse equal to Force multiplied by time and you don't know the time?

I used the kinematic equations...Final Velocity Squared=Distance x acceleration (.31 x 9.8 =V) then multiplied that b/c it was the change in V times the mass...got impulse

 

[duke1]

Duke1, please show us how much of it you can work out now.

I've worked out and finished this one now.

 

[duke1]

Duke1, please show us how much of it you can work out now.

I've worked out and finished this one now.

 

[duke1]

Duke1, please show us how much of it you can work out now.

I've worked out and finished this one now.

 

[duke1]

Duke1, please show us how much of it you can work out now.

I've worked out and finished this one now.