Impact Pendulum

Objective:

To construct a pendulum device that will produce a tip velocity above 60 in/s (150 cm/s).

Constraints:

I) The tip of the pendulum needs to be robust enough so that it can withstand impact withvarious objects and materials.

2) Width of the impact head, or tup, should be narrow (1 thin beam width or less).3) Heavy bricks should be close to the impact head.4) Pendulum supports, or uprights, need to be rigid and robust.5) Need to minimize tipping of pendulum.6) Assume that the efficiency of the pendulum is about %80.

Robust is defined as the ability of the assembly to not pop apart, or fail, during operation.

Construction:

The impact pendulum consists of five main parts:

1) Pendulum arm.2) Tup.3) Support uprights.4) Base.5) Specimen mount.

Response Measuring:

On a piece of paper construct angle references from 0 to 180°. It is recommended that every 100is marked with a heavy line and every 0 is referenced

Some Relations:

k Final Potential Energy . .Efficiency. — = ; final potential energy is the maximum height reachedV Initial Potential Energywith no obstruction in path.

Performance Checks:

fl Tip Velocity:

a) Place the pendulum rig close’to the edge of a desk.

b) Place a small object like a nut for a Y4-20 or 5/16- bolt such that the tup will strike it atthe bottom of the swing.

c) Release the pendulum arm from the horizontal position.

d) Record where the object lands, x, and how far it traveled in the vertical, y.

e) Use the data from d) to calculate tip velociy. It will be assumed that the impact wasperfectly elastic and that the velocity of the object is that of the tup.

2) Ener2v Loss

a) Release the pendulum from the horizontal with nothing in the way of the tup.

b) Observe what angle the pendulum arm swings to and record.

c) Repeat 4 more times.

d) Average th results and deiermine the amount of energy loss on each half swing.

e) Estimate the tip velocity.

3) Impact Tests:

Type of materials that can be tested: paper, aluminum foil, pencil lead,

a) Place specimen in specimen holder.b) Place specimen holder in base such that the tup will strike specimen at bottom of

swing.c) Release the pendulum from the horizontal and observe the final angle after striking

the specimen.d) The difference in energy between the energy of 2)b and 3)c will be how much energy

was required to fail the specimen.

Example Pendulum Tester:

tup

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upports

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Required Calculations:

1) Estimating the tip velocity:

From conservation of energy:

where h is the height that the center of mass of the pendulum travels through. Having the twoheavy bricks at the end of t1e pendulum arm will move the CG toward the tup.

The angular velocity, w, of the arm can then be found and used to determine the velocity of thetip, ortup.

— —* = (01orm

Depending on how snug the axle is or how much binding of the shaft occurs, a very conservativeestimate of 80% efficiency could be expected. However, if things are lined up correctly, theamount of energy lost on one swing is nearly negligible.

2) Projectile motion:

By looking at how th tup launches a small solid projectile, it is possible to get a rough idea ofthe tip velocity.

y=y0+v0t+-gt2

yo = voy = 0

—* t =

Then,

x = v0t —> = - and can assume that v is approximately the tip velocity of the tup.

Sample data:

Impact Pendulum Assembly

The following is a brief summary on how to construct an impact pendulum using Legocomponents. The key objective is to have the pendulum swing with minimal resistance andtravel with sufficient velocity so that impact tests can be conducted on materials such asaluminum foil, paper, and small brittle material like pencil leads.

Key Components:

There are three key components for the impact pendulum:

1) Pendulum arm with tup.2) Upright supports3) Specimen holder.

Pendulum

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imen hol

Pendulum Arm and Tup:

Impact pendulum-A Fall 2007 Page 1 of 9

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Locatin f gravity of the pendulum by finding the balance point:

This point can be used to determine the angular velocity of the pendulum. Then the velocity ofthe tup can be determined.

Uprights: 2 are required:

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Specimen holder (one version):

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Page 8 of 9Impact pendulum-A

Notes on Operation:

1) When released from the horizontal, there is enough momentum transfer to cause theassembly to tip over. Be sure to hold the top of the assembly down to prevent tipping.But, don’t press down too hard as it may pop apart.

2) The protractor scale seen in the background can be made using a CAD program.

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It is held in place with tape and two thin long Lego beams. Be sure that the origin of theprotractor aligns with the axle of the pendulum.

3) Align the specimen so that the tap will strike it at the bottom of the swing.

4) Be sure that the pendulum is aligned as close to the center of the two uprights.

5) The closer the weights are to the end of the pendulum, the better.

Impact pendulum-A Fall 2007 Page i) of 9