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Velocity from a force time graph

  • #1

Homework Statement



The figure is a graph of the force exerted by the floor on a woman making a vertical jump.
At what speed does she leave the ground? Hint: The force of the floor is not the only force acting on the woman.

Homework Equations





The Attempt at a Solution



I am completely lost with this question. First I derived her mass from the 600N from the graph. Which was about 61.22kg

Then I calculated the Impusle from the area underthe graph, but where from? All the area or minus the forces of gravity? then I used:

Impulse= m(delta v)

But Im not getting anywhere :(
 

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Answers and Replies

  • #2
Astronuc
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Starting with this: Impulse= m(delta v)

Then delta v = Impulse/m, and delta v = v - vo, and vo = ?
 
  • #3
Im guessing Vo is zero becasue she is jumping straight up?

Do I calculate Impulse to be the entire area under the graph or just a certain segment under the graph?
 
  • #4
Astronuc
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Im guessing Vo is zero becasue she is jumping straight up?
Correct. Standing she starts are rest.

Do I calculate Impulse to be the entire area under the graph or just a certain segment under the graph?
Weight which is a force, does not provide for acceleration. One has to consider the force in excess of the weight.
 
  • #5
Correct. Standing she starts are rest.



Weight which is a force, does not provide for acceleration. One has to consider the force in excess of the weight.
Thanks for your replies.
So since F=mg, and I derived mass from 600/9.8. does that mean that the impusle is the area of the graph from anything that is >600n?
 
  • #6
Astronuc
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Thanks for your replies.
So since F=mg, and I derived mass from 600/9.8. does that mean that the impusle is the area of the graph from anything that is >600n?
The 600 N is her weight, when she is not moving, so the force in excess of 600 N goes toward accelerating her mass.
 
  • #7
Astronuc
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Another way of looking at this problem is to convert F(t) to a(t), since a(t) is just F(t)/m, however, one has to look at Fnet(t), which is the difference between Ftotal(t) and mg.
 

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