丁香园AV

Bucket pull ~40 Minutes

NOTE: Somewhat difficult experiment. Reserve for enthusiastic class.

Description:A bucket full of different amounts of water will be pulled along different surfaces.

Objective: To investigate forces, namely friction

BC Science Content:
-effects of balanced and unbalanced forces in daily physical activities (grade 6)

Materials: meter/yard stick, plastic 4L? ice cream bucket, string ~50cm long, masking tape, "good" rubber band, cup, water

Doing it:

A1. Stretch the rubber band many times. This will 'warm it up' and make it more accurate. Tape the elastic around the end of the ruler. Tie a loop in the string through the elastic. Tie the other end of the string to the bucket's handle. Tape one long piece of masking tape over top the numbers on one side of the ruler. You have made a crude force meter, and you will calibrate it with cups of water, marking your scale on this piece of tape.

B1. Hold the ruler vertical with the rubber band/string hanging down alongside the ruler. mark a '0' where the knot is resting (Fig 1.1). Put one cup of water in the bucket and then mark a '1' where the knot is resting. continue for about 6cups of water.
B2. for each numbered mark on the tape put one between it and the next one, adding 'halfs' to your scale.

C1. add more cups of water until there are a total of 10 cups in the bucket. carry your bucket to a surface (i.e. tile, carpet, gravel, grass).
C2. You will put the bucket down and pull it holding on to the end of the ruler the elastic is taped on. the knot will stretch along the scale you made. you will want to pull the bucket just hard enough so it moves at a constant speed. Make sure you hold the ruler close to the ground when you pull so you aren't pulling up at all. What do you think will happen? How far will the band stretch with 10 cups of water in it?
C3. record how far it stretches on the scale for the initial amount of water (10 cups) and then take out one cup of water and measure again. Repeat this, removing one cup at a time, as long as you can. It may become to hard when there are only one or two cups of water in the bucket.

D1. plot your data on the graph.

Explanation: The amount of friction force(stickiness?) that is felt by an object is related to how heavy and object is. if an object is twice as heavy you should have to pull twice as hard to move it. Remember that when something is moving at a constant speed (not accelerating) the forces are balanced, so when you see how hard you have to pull to move it at a constant speed, you are indirectly finding the frictional force on the object.

Extension: Deeper look at the calibration and forces involved. Running slower/faster when towing the bucket.