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Two momentum questions

  • Thread starter rootX
  • Start date
  • #1
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Homework Statement


A 72 kg man jumps
a) If the jump results in an upward speed of 2.1 m/s, what additional upward force does the floor exert, if the person pushes the ground for 0.36 s?


Homework Equations


F= mv/t


The Attempt at a Solution


F_net = Impule/t

F_byFloor - W = F_net

f_byFloor = F_net + W
= (mv_f / t) + mg
= m ((v_f / t)+g)
= 1126.32 N

But, the answer in the book is 420 N, and it just ignored Weight of the person.
If my Force equation (F_byFloor - W = F_net) is right?




Homework Statement



A bee lands on one end of a floating 4.75 g Popsicle stick. After sitting at rest for a moment, it runs towards the other end with a speed of 3.80 cm/s relative to still water. The stick moves in the opposite direction at 0.12 cm/s relative to the still water. What is the mass of bee?

Homework Equations



mv_b + Mv_popsicle = 0

The Attempt at a Solution



I am confused about the velocities. Like, How the results would be changed if I use bee velocity relative to Popsicle. And, do I need to have velocities of both objects relative to some stationary thing in order to solve this problem?
 

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
26,258
618
Question 1) asks for 'additional upward force', not 'total upward force' implying you should ignore the weight. For 2) a consequence of the conservation of momentum is that the center of mass of SYSTEM doesn't accelerate unless acted upon by a force from OUTSIDE of the system. Consider the bee and the popsicle stick as a system.
 

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