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hgpt1-Visitor
1-Visitor
July 14, 2017
Question

Computing the punching force (when hitting a punching bag)

  • July 14, 2017
  • 2 replies
  • 9976 views

Hello!
About a week ago I've received the following problem:

 

You have a ground fixed punch bag (resting on a spring that's attached to a counterweight).
What is the minimum sensor setup in order to determine:

  • the force of the punch in the bag
  • acceleration of the punch
  • the speed of the punches (for multiple punches)

Notes:

  • no sensor must be attached to the person punching the bag, only on the bag
  • bag is punched in a limited vertical area

Justify the answer with:

  • sensors
  • calculations

Working as hardware engineer, I must admit I'm not that experienced with physics.

I've been trying to figure a way to solve it for a week now. What I thought of is mounting an IMU in the center of the bag and after that gather experimental data by hitting the bag with a pendulum. What's hard for me to determine is the force with which the bag is hit, how much of that force gets absorbed by the padding and the remaining force that drives the bag afterwards. Finding the acceleration and speed of the much would be pretty easy after finding the force, by applying Newton's second law.


Any help would be highly appreciated!

Check attachments section for some visuals on the problem.
Note: I'm sorry if I'm double posting this. For some reason the previous post auto-closed.

2 replies

R
RichardJ19-Tanzanite
19-Tanzanite
July 14, 2017

This is a Mathcad forum. If you want help with a Mathcad worksheet, please post the worksheet.

 

If you want help with a general engineering or physics problem you might get help here, but there are probably better forums (e.g. http://www.eng-tips.com/).

    V
    vlehner1-Visitor
    1-Visitor
    July 14, 2017

    Hi,

     

    to calculate the forces, accelerations and velocitys, you will have to make some measurements before starting calculations.

    Is there any possibility to measure the contact-time?

    The punch will be not elastic in the real situation, we will loose energy.

    Basically you can start the calculation with this:

     

    Scan.jpg

     

     

      H
      hgpt1-VisitorAuthor
      1-Visitor
      July 14, 2017

      You can assume a contact time of 30ms. 
      Considering the following as known:
      known_values.PNG

      known_values2.PNG
      There are multiple problems to be solved before taking real world measurements as:

      • where are the IMU's going to be placed and why?
      • how can you accurately measure the force applied by the pendulum
      • what happens when multiple punches are thrown?

      I started with the math the same way you did, but got stuck. Here is my progress (after many tries).


      Solution_proposal.PNG

       

      I was also thinking at analising the system as a black box and using Matlab System Identification Tool for finding the model, but then I will also need to know the applied force in order to link it to the acceleration of the bag.

      V
      vlehner1-Visitor
      1-Visitor
      July 14, 2017

      We must search for a Law for the Spring.

      The bag is a pendulum which has its dynamic with the spring.

      I'm just searchig for it.

      Then we can go on in calculations.

       

      Can you give me some information about the spring?

      For example stiffnes, wire-diameter, length (unloaded)

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