# Determining force to crush an object using a pendulum

• RobStoll
In summary, three individuals are trying to determine the force needed to crush a part using a pendulum with a mass of 4.25 pounds and a length of 1 foot. They use the formula F=(mgh)/s and consider factors such as potential and kinetic energy, velocity, acceleration, and gravity. The final calculated force is 4080 pounds, but one individual believes the element of time may be missing in the approach. After further discussion, they determine that the impact force is likely to be 2040 pounds.
RobStoll
Afternoon all,

It has been WAY too many years since I have had to apply my schooling in physics to the real world. There are 3 of us arguing over how to solve the following. Let me lay this out:

We have a 4.25 pounds (.1321 slugs or 1.9278 kg) pendulum that is 1 foot (.3048m) long
We are starting it 90° from impact and releasing it to impact the face of our part. We are trying to determine the amount of force that was applied to crush the part .025 inches (.002083 feet or .000635m)

The formula they both eventually agreed on was:
Force = (2*m*g*h)/s and was developed from:

m = mass
h = drop height
s = crush distance
PE = Potential Energt
KE = Kinetic Energy
v = velocity
a = acceleration
g = gravity constant

PE = KE or mgh = 1/2mv^2
v^2 = 2gh
They say we can get the acceleration (decel in this case) by taking the v^2 at the moment of impact from the previous line and dviding by the crush distance so
a=v^2/s

From there they applied the good old fashioned:
F=ma so
F = m * v^2/s or m2gh/s

This results in the following numbers then:
F = (.1321slugs*2*32.17ft/sec^2*1foot)/.002083feet
F = 4080 pounds.

We all agree that it seems unlike that a 4.25 pound weight is impacting the part with 4000+ pounds of impact force when being swung on a 1 foot pendulum.

Is this the correct approach therefore the correct answer? I seem to think not. I think we are missing the element of time in this. I approached it from the Energy to Momentum To force which has a time component.

Any help would be greatly appreciated.

Thank you,
Rob Stoll
Design Release Engineer
Jiffy-tite Co. Inc.

2 things:

1. I accidently left in the 2 so the final formula should have been F=(mgh)/s (therefore 2040 pounds)

2. We determined that this is likely to be the impact force. It is much higher then our original instincts, but it appears to be correct. I still wouldn't mind you thoughts.

Rob Stoll

## 1. What is a pendulum?

A pendulum is a weight suspended from a pivot point that can swing back and forth due to the force of gravity. It is often used in physics experiments to study the principles of motion and energy.

## 2. How can a pendulum be used to determine the force required to crush an object?

A pendulum can be used to determine the force required to crush an object by measuring the height and velocity of the pendulum before and after it collides with the object. Using this data, the force of impact can be calculated using the principles of conservation of energy.

## 3. What factors can affect the accuracy of using a pendulum to determine force?

The accuracy of using a pendulum to determine force can be affected by factors such as air resistance, friction, and the elasticity of the object being crushed. These factors can cause variations in the pendulum's motion and affect the accuracy of the calculations.

## 4. Can a pendulum be used to determine the force required to crush any type of object?

Yes, a pendulum can be used to determine the force required to crush any type of object as long as the object can be securely fixed in place and the pendulum's motion is not significantly affected by external factors.

## 5. Are there any safety precautions to consider when using a pendulum to determine force?

Yes, it is important to ensure that the pendulum is set up and operated safely to avoid any accidents. This includes securing the pendulum and object in place, wearing appropriate protective gear, and following proper procedures for data collection and analysis.

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