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Video Encyclopedia of Physics Demonstrations
Video Encyclopedia of Physics Demonstrations
Here is a list of those video demonstrations I have found particularly interesting and instructive.
Textbooks are: CJ = Cutnell and Johnson fourth edition; HRK = Halliday, Resnick, and Krane
extended second edition; RC = Roberson and Crowe sixth edition. End-of-chapter questions are
denoted by a Q, end-of-chapter problems by a P, and in-text examples by an E followed
by the chapter number.
Demo 01-14 Guinea and Feather
A similarly shaped paper disk and metal disk are dropped side by side both before and after
pumping the air out of the tube to show that air resistance is what keeps them from hitting the
bottom simultaneously.
Suggested method of use: Freeze the video after the vacuum pump is turned on and ask the
students to predict the outcome when the tube is now inverted.
where we teach this: 1D Kinematics in Physics I
textbook references: CJ Fig. 2.18; HRK Sec. 2-8
Demo 01-18 Dropped Slinky
A slinky is held at one end with the other end freely hanging. If it is released from rest, will the
free end initially accelerate upward or downward? Slow-motion footage reveals that the free end
initially remains motionless, and the slinky collapses down to its rest length.
Suggested method of use: Play the video and then ask the students to explain why the lower end
initially remains motionless. The answer is related to the fact that at each point along the spring
the tension supports the weight below that point.
where we teach this: speed of waves on a string in Physics I
textbook references: CJ Q16.6; HRK P19.22
Demo 01-19 Candle in Dropped Jar
A candle is placed inside a sealed jar containing enough air to burn for a while. If the jar is
dropped, what happens to the candle? Slow-motion footage reveals that the candle immediately
goes out because there are no longer any convection currents to bring fresh oxygen to the flame.
Suggested method of use: Play the video and then ask the students to explain why there are no
longer any convection currents. The answer is that in the frame of the jar, apparent
weightlessness occurs so that g = 0, and hence the buoyant force is zero: warm gases no longer
rise.
where we teach this: natural convection in Physics I
textbook reference: CJ Sec. 13.1 Demo 02-02 Monkey Gun
The classic demonstration of the monkey and the hunter.
Suggested method of use: Play the video and then ask the students as a group or homework
problem to prove that the bullet will strike the monkey. To make this tractable, I suggest using
real numbers. Here is an actual problem I assigned on a recent Physics I exam broken down into
easy steps.
A hunter in a forest spots a monkey hanging from a tree 11 m above his position. The hunter is
56 m from the base of the tree. He quickly aims his rifle and shoots a bullet with a muzzle
velocity of 110 m/s. At the instant he fires, the monkey lets go and drops toward the ground.
(i) At what angle must his gun barrel be aimed above the horizontal to hit the monkey? _______
In the next three parts, we will prove that at this angle the monkey does get shot. Show all work
in each step below.
(ii) How long after being fired does the bullet cross the monkeys path?
(iii) How far above the hunters position is the monkey at that time?
(iv) Now show that the bullet has risen that same distance vertically into the air and therefore hits
the monkey.
where we teach this: 2D kinematics in Physics I
textbook references: CJ P3.47; HRK p. 59
Demo 02-04 Vertical Gun on Accelerated Car
A free-rolling cart has a spring-loaded gun which launches a ball perpendicular to the cart. In the
first experiment, the cart rolls down an inclined plane. Will the ball land ahead of or behind the
gun? The answer is it lands back in the gun. In the second experiment, the cart is accelerated
along a horizontal surface using a string attached to a weight hanging over the edge of the table.
This time where will the ball land? It lands behind the cart.
Suggested method of use: Play the video and ask the students to guess what will happen at the
two stopping points in the movie, after the appropriate questions are asked.
where we teach this: independence of x and y motion in 2D kinematics in Physics I
textbook references: CJ E3.4; HRK Fig. 4.7
Demo 02-11 Local Vertical with Acceleration
A liquid accelerometer is equivalent to a carpenters level. What will it read if attached to a cart
gliding down a frictionless inclined plane?
Suggested method of use: Play the video and ask the students to guess what will happen at the
stopping point in the movie, after the above question is asked. The cart experiences apparent
weightlessness along the direction of the incline and hence the accelerometer reads level. where we teach this: weightlessness in freefall in Physics I
textbook references: CJ p. 102; HRK p. 87
Demo 02-13 Inertia Ball
The classic demonstration of a heavy mass connected by a rope to a rigid ceiling. A second,
identical rope hangs freely from the bottom end of the mass. What happens if we pull gently on
the free hanging rope? Same question if we pull with a quick jerk?
Suggested method of use: Play the video and then ask the students to explain the results. A free-
body diagram helps.
where we teach this: inertia in Physics I
textbook references: CJ Q4.3; HRK Q5.2
Demo 02-23 Water Rocket
The usual kids toy. Air is pumped into a small rocket and released. The rocket is found to reach
great heights if half filled with water but barely rises if full of air alone, even if pumped twice as
much to keep the pressure increase the same in both experiments.
Suggested method of use: Play the video and ask the students to explain the results. The key is to
note that the water escapes with the extra air, carrying significant momentum with it.
where we teach this: completely inelastic collisions in Physics I
textbook references: CJ E7.7; HRK p. 213
Demo 03-06 Stability of Rolling Car
Either the front or the rear axle on a hot wheels car can be locked. In which case will the car
stably descend an inclined plane? The answer is only if the front axle is locked.
Suggested method of use: Play the video and ask the students to explain the results. The key is to
note that the coefficient of static friction (applicable to the rear wheels since they are rolling
without slipping in the stable case) is larger than the coefficient of kinetic friction (applicable to
the front wheels since they are locked) and hence the rear wheels lag behind. This means cars
must be designed to provide most of the braking at the front wheels.
where we teach this: rolling without slipping in Physics I
textbook references: CJ E8.9; HRK p. 259
Demo 03-24 Double Cone on Incline
A double cone appears to roll uphill on rails which spread apart. This spread means that the
center of gravity actually lowers, as physics demands.
Suggested method of use: Stop the video before it explains the result and ask the class to provide
it. where we teach this: center of gravity in Physics I
textbook reference: HRK Sec. 14-4
Demo 03-26 Toppling Cylinders
A symmetric right cylinder topples when its cap is removed. The secret is that there are two balls
inside it.
Suggested method of use: Play the video and ask the students to draw a free-body diagram which
explains the results. I already gave you a handout explaining this in great detail.
where we teach this: second condition for static equilibrium in Physics I
textbook references: CJ Sec. 9.2; HRK P14.18
Demo 04-05 Pulley and Scales
A pulley is attached to the bottom of a spring scale mounted in a frame that in turn hangs from an
upper spring scale. The weight of the frame and its contents is 5 N as read on the upper scale.
When the free end of the rope is pulled until the lower scale reads 10 N, what will the upper
scale read? The counterintuitive answer is 10 N.
Suggested method of use: Play the video and ask the students to guess the result. After viewing
the answer, explain it by drawing a clear free-body diagram.
where we teach this: mechanical advantage of pulleys in Physics I
textbook references: CJ E4.20; HRK P5.67
Demo 04-15 Meter Stick on Fingers
Balance a meter stick horizontally on two fingers and then move your fingers toward each other.
The stick will remain balanced and your fingers will meet under its center of mass even if:
(i) your t