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Homework Help: Force made by a mass falling on the floor

  1. Apr 3, 2012 #1


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    1. The problem statement, all variables and given/known data
    Hello, I need help with a simple task.
    I want to calculate the force which appears when a mass falls on the floor without bouncing.
    I have modeled a bicycle crankarm, and I want to calculate the stress in it. In order to do that I need a force.
    So I want to know the force made by a man (100kg) falling from 0,5 meter directly on the pedals.

    2. Relevant equations
    I figured it could be calculated this way:
    I=m∙Δv/t; Δv=√(2gh);
    But I do not know what time should I use.

    3. The attempt at a solution
    I was thinking of using 0,1 second for the time, but I am really not sure if that would be correct. I know this is a simplification of a complex model (it probably vibrates and the rider probably bounces a bit).

    Do you think this is a good way of calculating the force? Do you have any other suggestions?
    Please help me.
    Thank you, Vid
  2. jcsd
  3. Apr 3, 2012 #2
    Can you determine the spring constant of the tires?
  4. Apr 5, 2012 #3


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    thanks for the answer. It really got me thinking. Actually the whole bicycle is a spring and a damper.
    I guess it is going to be hard to estimate its spring and damping coefficient. I will try to google it.
    Well I am making a simulation of the crankarm in Solidworks and that is why I need a maximum force... Do you have a suggestion how I could estimate that in a different way?

    Thank you guys,

  5. Apr 5, 2012 #4
    You problem is complcated by the fact that the riders knees will bend somewhat. That also lessens the impact.

    Is there any way you can measure the spring constant of the bicycle by applying loads and measurements from the pedal to the ground?
  6. Apr 6, 2012 #5
    The man has become an engine, and you make the same error often made by people talking about how much torque or power the engine produces. It produces none at all. The load produces power/torque and the drive train transmits it back to the engine shaft. Look at this problem in the opposite way. How much power can the load take? This for me was at a near stall condition, on a steep grade, lowest gear, and a hundred pound pack on the bike. I decided to approximate this as a full stall with the front wheel against a wall. So now all I had to do was to find the stall torque of my engine, which was me plus the crank. A little testing in the gym showed that I could apply as much as 1500 lb in a sudden burst. I rounded that up to 2000 for safety and multiplied it by crank length. That gave me a max load that the rest of the system had to be designed for. After checking that against all the components, I found that the parts manufactures had all assumed a customer much stronger than me.
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