The Monkey and the Hunter

What happens

There was a young monkey who loved to swing in trees. He was hanging from a branch one day when he saw an evil hunter hiding in a nearby tree taking aim at him with a rifle.

"No problem," thought the monkey, "I will just watch the rifle. When I see the flash from the muzzle, I will immediately let go of the branch and fall out of the bullet's way."

Is the monkey right?

In the lecture demonstration a few substitutions are made:
Monkey's FingersElectromagnet
Tree BranchIron rod
RifleAluminum pipe
BulletAluminum slug
HunterPhysics Instructor
Muzzle flashThin wire cut by bullet
Gun powderPuff of hot air

A metal can is substituted for the monkey for simianitarian reasons. The can is held up by an electromagnet on a high rod. An instructor across the room aims a blowgun (a pipe) at the can by sighting down the hole. He or she also shines a laser pointer down the pipe to show that it is aimed at the can. A metal slug is inserted into the pipe to act as the bullet.

The instructor shoots the bullet by blowing into the pipe. As the bullet leaves the end of the blowgun it breaks a thin wire which was placed across the end of the pipe. The wire was in the electromagnet's circuit and in breaking it stops the current flowing through the electromagnet. Thus the magnetic force ceases and the can starts to drop just as the bullet leaves the blowgun.

Will the can fall out of the bullet's way?

What to notice

Does the bullet hit the can as it falls? In the lecture you could listen for the "clang" as the bullet hits the can. On the next page you can see five frames from a video taken during the lecture demonstration. In these frames the bullet first is seen as a streak coming down from the right. In the next frame the streak can be seen approaching the can as it falls. The bullet enters the can, and, in the third frame, emerges from the can just after it has hit the can's back. The frames are 1/30th second apart.

What to understand

The vertical accelerations of the can and the bullet are the same. Air resistance is slightly different for the two, but this is not important here. Since both the can and the bullet start falling with zero vertical velocity at the same height they also are at the same height when they collide.