Measure the velocity of a speeding bullet using a totally inelastic collision. (See Ballistic Pendulum (Ballistics) [1])
Various balls are dropped in a transparent tube to show nearly elastic, partially inelastic, and totally inelastic collisions. The height of the bounce, marked off qualitatively on the tube, is a measure of the elasticity. A lead ball which does not bounce at all is startling.
With another piece of apparatus a small steel ball is dropped on a heavy, polished, slightly concave steel plate. The collisions are so elastic that the ball bounces dozens of times before its energy is exhausted.
With the collision ball apparatus one can show that if a moving ball collides elastically with an equal mass ball at rest, the entire kinetic energy and momentum of the first ball will be transferred to the second.
If one ball collides with a row of equal mass balls, all the kinetic energy and momentum will be transferred to the last ball.
One larger mass ball is provided to show the dependence of these results on the mass of the colliding ball.
An interesting experiment is to pull back two balls on one side and one on the other and release them simultaneously.
A. Understanding car crashes - It's Basic Physics, put out by the Insurance Institute for Highway Safefy and narrated by a high school physics teacher. This video is a more modern version of the Crash Test Dummies video to illustrate Newton's second law in the context of car collisions. The video uses scenes in which an egg is thrown at a brick wall and sheet to demonstrate momentum transfer and impulse. The typical running time of the appropriate segment of video is less than five minutes and the entire video is twenty-two minutes.
B. "Physics and Automobile Collisions" by Dean Zollman - This laser disc video illustrates the concepts of Newton's Laws, impulse and momentum, conversion of kinetic energy to other forms, and conservation of momentum in 2D collisions with physics narration. You may wish to watch the disk through and pick out parts appropriate for your class, or I recommend the following parts as a short general survey:
Chapter 1 | Introduction | 38 sec |
Chapter 2 | 1st Law, impulse and air bags | 2-1/2 min |
Chapter 4 | Price of damage to car with and without shock absorbing bumpers | 1-1/2 min |
Elastic and inelastic collisions between carts can be demonstrated as one end of the carts are equipped with magnets and the other end with Velcro. A moving cart collides elastically with a stationary cart of equal mass using the magnetic ends. The originally stationary cart moves away with all the velocity. Elastic collisions between carts of different masses can be tried qualitatively or quantitatively with Data Studio.
Completely inelastic collisions result by colliding the Velcro ends of the carts. A carts velocity is measured before and after it has collided inelastically with another cart of equal mass. It is demonstrated that the velocity of the two carts after the collision is half the initial value.
Explosions are demonstrated by touching the ends of two carts together and releasing an internal plunger from one of the carts.
All of the above demos may also be demonstrated with the air track at the instructor's request. The air track has an advantage over the dynamics track in that there is less friction associated with it. However, the air track has a draw-back in that it is much nosier than the dynamics track and in a lecture setting it is difficult for the instructor to be heard.
Two identical looking balls are suspended as pendula. One at a time they are lifted up to swing against a standing block. One ball easily knocks the standing block over, but the other does not. There is another pair of balls so you can show that the "happy" ball is elastic and bounces high, whereas the "unhappy" ball is totally inelastic and does not bounce. (F. Bucheit, Physics Teacher 23, 28, 1994).
This demonstration displays the impulsive force in a collision as a function of time using the Pasco dynamics track. The track is elevated at one end and the cart is allowed to accelerate with the force sensor connected. Data Studio [2] is used to measure and plot the force as a function of time. The momentum is just the area under this curve.
Choose two students, one heavy and one light, and stand them on the large reaction carts. When they push on each other's hands, the light student acquires a proportionally larger velocity than the heavier student.
Small reaction carts illustrate the same principle on the lecture table, as do gliders on an air track connected by a compressed spring.
Ask your students to answer from:
A. heavier person
B. lighter person
C. both the same
Which person feels the greater force? Which person gets the greatest impulse? Which person undergoes the greater change in momentum? Which person undergoes the largest acceleration?
This pretty demonstration illustrates two principles: one, the period of a conical pendulum is the same as a linear pendulum; and two, all the momentum is transferred in an equal mass elastic collision.
Two identical ivory balls hang side by side from the ceiling of the lecture hall. You can illustrate the equal mass collision by pulling one out and letting it collide with the other, as in the Collision Balls [3] demonstration. Now pull both balls back, away from the class, hold one lightly with your fingers, and collide the other into it from the side. The colliding ball will stop "dead" and swing toward the class in linear pendulum motion, and the struck ball will swing around in conical pendulum motion. Since the periods of the motions are the same, the balls will again collide at the end of the swing and exchange motions. The situation will continue for some time with the balls exchanging conical and linear pendulum motion at successive collisions.
Available only in the Knudsen Lecture Halls and PAB 1425
Does the weight of an hourglass change when the sand is falling? This demo shows the truth! A funnel with the sand held back by a cork and string arrangement is perched to drop sand into a glass beaker on a double pan balance. A laser bounces a beam off a small mirror attached to the pointer of the balance. When you burn the string, the sand starts falling, and the motion of the laser spot on the blackboard indicates the result. A set of transparencies to use with this demo is shown below.
The sand in the falling column does not contribute to the weight reading. But you can easily show from Newton's Second Law in the form F = dp/dt that the extra impact force of the falling sand exactly equals the missing weight of the total falling column of sand. Thus, while the sand is falling and impacting, the weight of the hourglass is equal to its weight when no sand is falling.
But initially, as the sand starts falling, there is "missing weight" in the column before the sand hits bottom, so the hourglass grows momentarily lighter. Similarly, at the end there a few moments while the impact force remains constant as the falling column decreases to zero, so the hourglass grows momentarily heavier. The movements of the laser spot on the blackboard faithfully trace out the graph of the weight of the hourglass as a function of time. |
A movie clip of the typical setup is shown on the right. Note the position of the laser spot as the sand begins to drop and as its level in the funnel changes. The graphs below were produced with data from a setup using a PASCO force sensor in place of the scale. |
Links:
[1] https://demoweb.physics.ucla.edu/node/383
[2] https://demoweb.physics.ucla.edu/node/416
[3] https://demoweb.physics.ucla.edu/node/276