Feed the Cat Demo

Monkey Hunter Setup pt1

A blow tube is aimed at a cat attached by an electromagnet. At the same moment the ball is shot from the tube, the cat is dropped from the tree. The ball will hit the cat regardless of the initial speed of the ball.

Materials:Monkey Hunter Setup pt2
A. Ball launcher
B. Bracket
C. Safety Key
D. Laser Sight
E. Black/Red Lead
F. Cat (disk)
G. Nylon Washers (4)
H. Wingnuts (3)
I. 2 AA size batteries
J. Projectile Balls (3)
K. Thumbscrew
L. L Bracket
M. Rod Clamp
N. Electromagnet
O. Ring Stand
P. C-Clamp

Demo:

  1. Set up the demonstration according to the Monkey & Hunter Demo Box pictures andMonkey Hunter Control Box instructions, making sure the ball launcher and the ring stand are 10-15 feet apart.
  2. After assembling the electromagnet to the ring stand and hooking it up, attach the cat to the bottom of the electromagnet.
  3. Adjust the launcher such that the laser pointer marks the center of the cat.
  4. Pull back the launcher handle as far back as possible, then remove the safety key and release the handle. This should launch the ball through the tube and reach the cat, demonstrating projectile motion.

Note: This demonstration is humorous and well accepted by the student audience.

Explanation:

This demonstration shows the independence of velocities in the x and y directions. It asks the question: “if you want to hit the cat with a rubber ball, should you aim the ball above, below, or at the cat before it falls? While your first instinct may be to aim the ball below the cat, so that the cat falls into the path of the ball, the correct answer is to aim the ball directly at the cat.

To explain why this is the correct answer, we must recall Galileo’s law: all objects fall towards the earth with the same acceleration, 9.8m/s2, irrespective of their mass. We must also consider the fact that vertical and horizontal velocities are independent of one another. Furthermore, we know that gravity only acts in the y direction, and therefore only affects the y component of the velocity of any object.

Let’s consider what would happen if gravity did not exist for this demo. The cat would stay suspended in the air and the ball would travel along a flat path and collide with the cat. When gravity is added, the cat falls towards the earth and the ball follows a parabolic path.

When the ball leaves the end of the tube and the cat drops from its magnet, they are both starting with an initial velocity in the y direction, Vy = 0. This is because the cat is at rest suspended above the ground, and the ball is released with a velocity in the horizontal direction only. Not only are the cat and the ball started with the Vy, they also start with the same height y above the ground. Because the cat and the ball have the same initial velocity and acceleration, they will fall the same distance in the same amount of time; because they have the same initial height h, at any point in time the cat and the ball will have the same height above the ground.

Given this information, it is easy to see why the ball will always hit the cat, no matter its initial velocity in the x direction*.

The following explanation is paraphrased from the wikipedia article: “The Monkey and Hunter”, which can be found here: https://en.wikipedia.org/wiki/The_Monkey_and_the_Hunter

We can also look at this problem and see the result by changing the inertial reference frame with which we examine the problem. In the explanation above, we use the inertial reference frame of the Earth. We know that on Earth, all objects fall with the same acceleration towards the Earth of -g. If instead of looking at the Earth’s inertial reference frame and saying that the ball and the cat are accelerating downwards at a = -g, we switch to a reference frame that is being accelerated up at a constant acceleration equal to g, we see the ball and the cat staying stationary on the y axis and the earth accelerating upwards to them with an acceleration equal to g.

Here, we see that the ball and the cat have no Vy, and when the ball is fired it will travel along a straight path to the cat. Looking at this example we can see that the only way to hit the cat is to aim straight at it. Angles stay consistent in transformations to non-inertial reference frames, so we can see that even if we transfer back to the inertial reference frame of the earth, we continue to see that the ball should be fired while looking directly at the cat.

*As long as the ball is going fast enough in the x direction to hit the cat before they simultaneously hit the ground. If the ball is fired at a slow enough speed that it cannot travel the x distance from the end of the blow tube to the cat before it travels the y distance from where it is released to the ground, the ball will not hit the cat. Even in this scenario, however, the cat and the ball will hit the ground at the exact same time.

Notes:

The blow tube will need to be aimed manually as the laser pointer does not point to the actual target. If you use the laser pointer to set up the demo you will need to fine tune the aim before you show the students.

Written by Sophia Sholtz