Please refer to: Acceleration Down an Inclined Plane (Kinematics) [1].
The universal gravitational constant G can be measured in class with the Cavendish balance; however, the demonstration is time consuming and delicate. A video tape of the demonstration has been prepared by Prof. C. Buchanan and Jim Abbott using time lapse photography of the optical lever readout. The tape is about seven minutes in duration and presents the students with the data of the experiment so the value of G can be calculated.
Drop a wooden ball simultaneously with a much heavier steel ball to show that they fall together. To show that the steel ball is definitely heavier, place the wooden ball in a short cup on one side of a double pan balance, and then put the steel ball in a short cup on the other side so the balance clunks down.
You might think that by this time everyone knows that a heavy object falls no faster than a light one - at least, everyone who had a high school physics course! But try asking in your Physics 10 class, "Will the heavy ball fall faster than the light ball, or the same?" You will be surprised at the variety of answers and justifications.
Then ask them about the effect of air resistance (which will probably already have come up in the discussion). To illustrate air resistance, take two sheets of paper, crumple one up into a ball, and drop them together. They have the same weight, but the flat sheet has more area "fronting the wind".
Lester Hirsch suggests a drama for jazzing up this last demonstration. Borrow the two sheets of paper from two different students. Tell them not to tear the pages out of their notebooks; you are going to return the pages. After the demonstration hand the flat sheet back to the first student, and carefully smooth out the crumpled sheet and hand it back to the second student. When he makes a face, tell the class that conversation has to start somewhere, and that he has to be one this time!
To make this demonstration somewhat more quantitative, you may wish project a slide of a wooden ball, a steel ball, and a ping-pong ball falling, photographed at 1/20 sec. intervals. (See Three Balls Falling [2])
As a final first-day demonstration, drop the "happy" and "unhappy" balls to show that objects that look identical may have very different physical properties.
A coin and feather (or rubber cork and styrofoam chip) are inside a lucite tube 1.5 m long. When the tube is switched end for end, the rate of falling of the two objects can be compared. The tube is then evacuated to show that they fall at the same rate in a vacuum.
This experiment was repeated on the moon by Apollo 15 astronauts using a feather and a hammer. You can see the NASA video here [3]. We also have this clip on video disk which can be played in class.
A coordinate frame in free fall in a gravitational field is truly inertial; that is, Newton's lst law is obeyed. In this demonstration a coordinate frame similar to that shown in the figure is held by an electromagnet. If the guns are fired while the frame is held, the projectiles will follow parabolic trajectories in the earth's frame and bounce off the intermediate plexiglass sheet. But if the frame is dropped, the guns firing automatically during the falling motion, the projectiles will follow straight lines in the falling frame and reach their target pockets.
Note that this demonstration is equivalent to the Monkey and Hunter [4] demonstration.
Simple Measurement of g: Heights of 6, 9, and 12 feet are marked up on lecture hall wall. Time intervals are announced by a metronome set to 80 beats/min. (Dt = 0.75 sec.) The demonstrator stands on a ladder and drops a ball from various heights on the beat of the metronome. Nine feet will be found to be the height for which the next beat is simultaneous with the ball hitting the floor. Then, g = 2h (Dt)2. This demonstration requires moderately good reaction time. A student volunteer may help. AVAILABLE ONLY IN KNUDSEN 1200
Measuring g with Sonic Basketball: In this experiment the kinematics of a basketball under the influence of gravity is studied quantitatively. The ball is thrown upward above a motion sensor to plot its position, velocity and acceleration as a function of time. (See Sonic Basketball [5])
Three balls (steel, wood, and ping-pong) are suspended from a platform. As they are released, they are illuminated with a repeating strobe of known frequency. The result can be video taped with Video Point Capture. This software can be used to make measurements and analyze the motion of each ball as a function of time. A sample is shown to the right.
Two simple demonstration of weightlessness with minimal equipment are described below. (The Local Inertial Frame [6] is also a demonstration of weightlessness and the fact that Newton's First Law is obeyed in a freely falling frame.)
Clip two clothespins on the sides of a rubber band. Hold one clothespin and let the other hang down by the rubber band. The weighing of the second clothespin is represented by the stretching of the rubber band. Now release the upper clothespin. The rubber band goes slack, and the two clothespins and the rubber band fall together. The clothespin(s) are weightless when falling.
Suggested by Mr. Wizard.
1. Punch a small hole in the side of a styrofoam cup or a 2 liter bottle near its bottom.
2. Hold your thumb over the hole as you fill the cup with water. Ask the students what will happen if you remove your thumb.
3. Remove your thumb and let the water stream out into a catch basin (a pail) on the floor.
4. Again seal the hole with your thumb and refill the cup. Ask the students if the water will stream out if you drop the cup as you remove your thumb.
5. Holding the cup as high as possible, drop the filled cup into the catch basin. The water does not stream out; the cup and water are weightless.
Suggested by Dale Bremmer.
Links:
[1] https://demoweb.physics.ucla.edu/node/407
[2] https://demoweb.physics.ucla.edu/node/402
[3] https://demoweb.physics.ucla.edu/sites/default/files/demomanual/mechanics/gravitational_acceleration/astronaut.mov
[4] https://demoweb.physics.ucla.edu/node/387
[5] https://demoweb.physics.ucla.edu/node/404
[6] https://demoweb.physics.ucla.edu/node/411