A blow tube is aimed at a monkey attached to a palm tree by an electromagnet. At the same moment the ball is shot from the tube, the monkey is dropped from the tree. The ball will hit the monkey regardless of the initial speed of the ball.
A. Blow tube and rubber ball
B. 9V battery for laser pointer
C. Laser pointer
D. Wooden block with light source and photocell
E. Junction box
F. Fisher Scientific DC power supply (6V, 4A)
G. Toy “Monkey” with an iron washer attached to its head
H. “Palm tree” with an electromagnet
- Set up the demonstration according to the pictures, with the blow tube and the palm tree 10-15 feet apart.
- Connect the solenoid wires from the palm tree to the junction box. Push the “reset” ` button to allow current through the solenoid.
- Attach the monkey to the solenoid.
- Sight the monkey through the blow tube (the laser pointer does not work to aim the blow tube).
- Blow the ball through the tube. You will need to blow hard so that the ball reaches the monkey before they both hit the ground.
Note: This demonstration is humorous and well accepted by the student audience.
This demonstration shows the independence of velocities in the x and y directions. It asks the question: “if you want to hit the monkey with a rubber ball, should you aim the ball above, below, or at the monkey before it falls? While your first instinct may be to aim the ball below the monkey, so that the monkey falls into the path of the ball, the correct answer is to aim the ball directly at the monkey.
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 monkey would stay suspended in the air and the ball would travel along a flat path and collide with the monkey. When gravity is added, the monkey falls towards the earth and the ball follows a parabolic path.
When the ball leaves the end of the tube and the monkey drops from its magnet, they are both starting with an initial velocity in the y direction, Vy = 0. This is because the monkey is at rest suspended above the ground, and the ball is released with a velocity in the horizontal direction only. Not only are the monkey and the ball started with the Vy, they also start with the same height y above the ground. Because the monkey 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 monkey 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 monkey, 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 monkey 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 monkey 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 monkey have no Vy, and when the ball is fired it will travel along a straight path to the monkey. Looking at this example we can see that the only way to hit the monkey 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 monkey..
*As long as the ball is going fast enough in the x direction to hit the monkey 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 monkey before it travels the y distance from where it is released to the ground, the ball will not hit the monkey. Even in this scenario, however, the monkey and the ball will hit the ground at the exact same time.
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