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4-Participant

Finding the force of a punch (when hitting a punching bag)

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!

Here are some visuals on the problem:

3 REPLIES 3
24-Ruby V
(To:hgpt)
No pictures are visible to me
4-Participant
(To:Werner_E)

Quite strange, I've edited the post and attached them.

23-Emerald I
(To:hgpt)

Collisions (hitting a punching bag) usually requires dealing with "impulse" rather than force.  (Impulse is the integral of force over time.)  What you are describing sounds amazingly like a Charpy Impact test--dropping a known weight on a known arm to hit a specimen.  Charpy testing looks at the position of the "hammer" before and after the test to determine the energy transferred to the specimen.

The apparatus consists of a pendulum of known mass and length that is dropped from a known height to impact a notched specimen of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after the fracture (energy absorbed by the fracture event). (Courtesy Wikipedia)

Sussing out the "acceleration" of the punch would require definition of what exactly that was.  "Speed of the punches," would be the frequency?  Frequency is the inverse of period.

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