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Slingshot Lab

Lesson Plans > Science > Physics > Projectiles

Slingshot Lab

When I started teaching many years ago there was a saying at the school where I worked. "If it's green and slimy it is biology, if it stinks it's chemistry, and if it doesn't work it's physics!" This is a complicated experiment requiring a considerable amount of time and careful data collection and calculations, but if done carefully, it works!

This experiment does require quite a bit of time. It may not be possible to finish it in one lab period unless you have very long periods. If you have to split it up into two labs, be sure that you have a way to either leave the slingshot undisturbed until the next class or be sure the students can take it down and put it back in exactly the same way.

I used the following materials: surgical rubber tubing, C-clamps and flat blocks of wood to firmly attach the tubing to a solid support like the edges of a lab table, a piece of lead for the projectile, string, meter stick or metric tape, slotted weight sets, and a wastebasket. Depending on your lab set up you may need to make changes in my materials and procedure.

Cut a piece of the tubing long enough to span the aisle between two lab tables. Clamp the ends to the tables using a C-clamp and if necessary a block of wood. The tubing should be stretched tightly enough to have no sag. Stretch a string across by wedging it under the clamps to establish a reference line. Mark the center of the tubing. Hang a weight pan at the center and measure the sag. Add weights to the pan tabulating the total mass and the distance the tubing sags. In my lab we added weights until the tubing had stretched about 3/4 of a meter. WARNING: Warn your students to never lean over the tubing in case something slips and the tubing rebounds. Monitor students carefully to insure safety.

Once students have collected enough data, they should plot a graph of force (not mass) vs. distance. At any point the area under the resulting curve is the work done in stretching the tubing that distance and is therefore the elastic potential energy. If you have a short lab period this will be a good point at which to stop.

The projectile is made from a sheet of lead flashing. Take a piece about 10 cm long, 6 cm wide and fold it twice giving a piece about 10 cm by 2 cm. As I remember it, I had projectiles with a mass of 150g, more or less. Bend one end up to make a J shaped object and bore a hole in the other end. You will need to experiment with the dimensions to find what works best for your situation. Give each group the mass of their projectile and tell them how far they should stretch the tubing and the angle at which they should launch it.

Using the amount of stretch they now calculate the elastic potential energy stored in their tubing by finding the area under the curve in the graph they have plotted. They probably will not have a straight line as this does not follow Hooke's Law. When I was teaching they had to calculate this area manually, possibly you have some sort of graphing application on your computer which will make this part of the lab easier.

When the projectile is launched some of the energy in the tubing will be used to lift the projectile to the height of the table. We will assume that the remainder will be converted into kinetic energy. Once the students have calculated the KE as the projectile breaks the plane of the table, they can calculate its speed. They now have a projectile motion problem and can calculate the horizontal distance from the launch point to the landing point of the projectile. I used a regular wastebasket as a target, so the vertical distance is not the distance to the floor, but the distance from the launch point to the top of the wastebasket. They also need to use the height of the reference string and the angle of launch to calculate a point on the floor which will provide the correct angle. Once they have completed their calculations and have two marks on the floor they are ready for the test.

WARNING: There is some danger in this step so my students are not allowed to do it by themselves. I was always directly supervising the group and probably was the one to actually launch the projectile. Be sure no one is standing in the path.

Everyone in the class will want to watch the launch. Work this up for drama!

A string is threaded through the hole on the end of the projectile. The projectile is hooked over the tubing. The string is moved to the point on the floor that makes the correct angle. Very carefully the projectile is pulled back the desired amount. Once the stretch is correct and the angle is right the reference string stretched between the two tables is removed. Wait until everything is motionless, no vibration, and burn the string close to the projectile. I always burned the string. If done correctly the projectile will land in the wastebasket and someone should shout "Physics Works!!"

I really liked this lab. It combines many concepts in mechanics, and if done carefully gives a dramatic conclusion. I can't give hard and fast rules for lengths of tubing, mass of projectile and desired range. You will have to experiment to see what works for your set up. There are some good points to bring out in the final analysis. We assumed that all the work done stretching the tube ended up as potential and kinetic energy. That probably is not correct. We also assumed that as the projectile was launched it passed through the equilibrium spot for the center of the tube. It probably went somewhat under that point because of the weight of the projectile. Most of the errors will result in a reduction of range. My rule was that once they calculated the landing point and made a mark on the floor, they must place the wastebasket so it covered the mark. Most of them would place the basket as close to the launch point as they could and still cover the mark.

Lesson by Mr. Physics

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