Experiment 1B: Free Fall

PURPOSE: Measure the acceleration due to gravity.

APPARATUS: An object suspended from an electromagnet falls when the magnet is turned off. There is a wire on each side of it as it falls. Every sixtieth of a second, a high voltage pulse is sent to one wire, making a spark jump to a metal ring on the falling object and from there to the other wire. The spark marks heat sensitive paper covering this second wire, periodically recording the object’s position.

CAUTIONS:

Do not use the apparatus without someone else in the room to pull the plug if you are being shocked.

The spark source's red lead should go to the free fall device's high voltage wire. The black (ground) lead goes to the connector at the right. Do not mix them up.

Do not move the apparatus or you may throw it out of level, making the object land improperly.

PROCEDURE.

1. When your group's turn comes, run off a tape:

a. Turn on the electromagnet with the key on its power supply.

b. Suspend the object from the electromagnet and steady it so that it hangs motionless.

c. Turn on the switch on the face of the spark source, which should be set for 60 Hz. Pick up the hand pushbutton and hold it down to turn on the sparks.

d. Switch off the power supply to drop the object. When it lands, take your thumb off the button and turn off the switch on the face of the spark source.

2. Lay your group’s tape on your table. Do not use the large first dot. Number the next fifteen dots after that. Time, in the answer sheet’s first column, is the dot number over 60 because there were 60 sparks per second. Stop when the table is full; there are more dots than you need.

Each interval should be just a little larger than the one before. If a dot is missing, ask what to do.

3. Put a meter stick on its edge so that its markings are right next to the dots. Record where the dots are. (Their position on an x axis along the tape, not the distance from the previous dot.) Magnifying glasses are available. Do not throw out the tape until you are told nothing needs rechecking.

4. In the third column, leave the first line blank because there is no line above it to subtract from. Also leave the last line blank because it has no line below it. On the rest, calculate Δx for the 2/60 second centered on that line’s dot. That is, Δx from the dot on the line above to the dot on the line below. Imitate this example:

Time, t (sec)	Position, x (cm)	Δx from dot before this one to dot after it (Δt = 1/30 s) Δx (cm)	Velocity, v = Δx/Δt (cm/s)
0	0.00	–	–
1/60	0.95	2.55	76.5
2/60	2.55	3.85	115.5
3/60	4.80	–	–

5. Calculate the speed at each dot, except the first and last, using the Δx and Δt centered on that dot. (The average speed over a time interval equals the instantaneous speed at the midpoint of that interval.) Don't round off excessively, or your answer’s accuracy will suffer. Keep in mind that dividing by 1/30 is the same as multiplying by 30. Try punching up the numbers in the example to be sure you see what to do.

6. Plot a graph of velocity versus time. Observe these rules, as you should with any graph:

- Time goes on the horizontal axis.

- Pick a scale which makes the graph pretty much fill the page (without going off the edge.) A larger scale is more accurate.

- Label both axes with the variable plotted along each and the unit each is measured in.

6. Draw what appears to be a best fit line through your data: One that passes as close as possible to as many points as possible.

7. Find the slope of your graph to obtain the object’s acceleration. Observe the following rules which, again, also apply to all future labs:

- Find it from the best fit line, as in the picture on the left, not from points which lie off the line as in the middle. (The line averages out some of the random errors in individual points.)

- Find it from points near opposite ends of the line, not points near each other as on the right. (If small numbers are off a little, it makes more difference than if big numbers are off a little.)

Conclusion: The acceleration has an uncertainty of about 3%. Within this uncertainty, does your value for g agree with the accepted one?

PHY 121 Experiment 1B – Free Fall

DATA TABLE:

Time, t (sec)	Position, x (cm)	Δx from dot before this one to dot after it (Δt = 1/30 s) Δx (cm)	Velocity, vx =Δx/Δt (cm/s)
		–	–
		–	–

Attach graph, calculate g below or on back. Accepted value = ______________.