Problem (possibly) involving Springs, Free Fall and Air Resistance

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SUMMARY

The discussion focuses on a physics problem involving a mass m dropped from height H, experiencing air resistance f during its motion. The key equations include the potential energy (PE) and kinetic energy (KE) relationships, specifically PEi = KEf and the force equation F = -kx. The participants explore how to calculate the mass's velocity before impact, the height it reaches after bouncing, and the total distance traveled before coming to rest. The net acceleration during descent is defined as (g - f/m), while the net retardation during ascent is (g + f/m).

PREREQUISITES
  • Understanding of potential energy and kinetic energy concepts
  • Familiarity with forces and motion equations, specifically F = -kx
  • Knowledge of air resistance and its effects on falling objects
  • Basic grasp of free fall dynamics and rebound mechanics
NEXT STEPS
  • Explore the derivation of the spring constant k in relation to air resistance
  • Study the principles of energy conservation in elastic collisions
  • Learn about the effects of air resistance on projectile motion
  • Investigate iterative methods for calculating total distance in multi-bounce scenarios
USEFUL FOR

Students studying physics, particularly those focusing on mechanics, as well as educators looking for examples of energy conservation and motion under air resistance.

Castiel
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Homework Statement


A mass m is dropped from a height H onto a hard, flat surface. When it collides with the surface, it rebounds with the same speed it had before the collision. However, there is a constant force of air resistance f acting on the mass as it undergoes this motion. Answer the following in terms of f, m, H, and g.

a) How fast is the mass moving just before it hits for the first time?
b) How high does the mass go after the first collision?
c) After how many bounces, the ball comes to rest on the surface. While its overall displacement was -H, it traveled a considerably larger distance. Determine the total distance traveled by the ball.


Homework Equations


F=-kx
PEsp=1/2kx^2
KEi+PEi-f(friction)=KEf+PEf+PEsp


The Attempt at a Solution


Should I look at this problem as if the ground were a spring? If so, how do I determine k and x for the equation? It doesn't necessarily travel any distance (for the x variable)

a) Would I simply use PEi=KEf? Should I assume that the mass starts from rest? Or should I put in PEi+KEi=KEf.

b)If I were to assume that it was a spring, how would I translate k and x into other variables to fit with the specifications of the equation (using f, m, H and g) If not, I have no idea where to even begin. Would I simply use the previously found final velocity from a and use that as my new initial, then calculate friction and find the distance?

c) Would I use the final velocity before hitting the ground of the second bounce to calculate the initial velocity for the third bounce, and so on? I have no idea where to even begin to solve this part of the problem. Any hints or suggestions would be greatly appreciated.
 
Physics news on Phys.org
When mass is moving down, the net acceleration is (g - f/m), where f is the air resistance which opposes the free fall.
While coming down, the net retardation is ( g + f/m)
 

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