Exploratorium

Projectiles

Key Ideas: - A projectile is anything that is "flying" in the air. - Projectiles consist of two vectors; one horizontal, one vertical. - Only force acting on a projectile is gravity which acts downwards. - Faster the initial speed, the farther it will travel. - Closer the angle is to 45 degrees, farther it will travel.

Demos: 1. The kids and I will drop random things to prove that acceleration due to gravity acts equally on all objects. 2. To show how objects that are initially traveling horizontally are in the air for the same amount of time, we will push toy cars off a table. They will go at differing speeds to prove that the faster ones go farther. 3. With beanbags, students will try to make baskets by shooting it over a string and having it go in. This will show there are many different trajectories a projectile can take and still go the same distance. 4. Students will make catapults and shoot cotton balls.

Script: Hello everybody I am Ross and I would like to know if any of you have ever seen a projectile. None of you have? You (point to kid that is obviously the most scared) are you lying to me? (Kid shakes his head then looks at his mom asking if he can please leave). Who here has hit a baseball, shot a basketball, or played Angry Birds? Well, that baseball, basketball, and Angry Bird on its way to hitting that alien pig thing are all projectiles. Really, anything that is thrown the air has a trajectory and is therefore a projectile. I would like you to look at this picture, now a trajectory is a path in the air. So if you (point to a kid) were to throw me across this room, I would be a projectile, just like a baseball, football, soccer ball, or anything that you throw. Now, who here has seen a projectile (everyone raises hand). Nice.

Now, we will look at the first characteristic of every projectile. (Pull baseball from table, throw it in the air and nonchalantly catch it with one hand instantly gaining the respect of the group for your athleticism) Why didn't the ball, which was also a projectile, not go flying off to the ceiling? (Kid calls out) Exactly, gravity. Now gravity makes all things fall at the same rate if you were to drop them. Obviously, if you throw something upwards, it will be in the air longer, and if you throw something downwards (bounce ball) it will be in the air less. However, if you drop two things from the same height they will fall at the same rate. Watch: (drop two things) it doesn't matter how much they weigh (keep dropping things, invite kids up and drop a bunch of things at once). Gravity pulls things to the ground, so that is why projectiles eventually fall to the ground. Well, that is not always true. If you have something super light like this tissue, the air pushing it up will make it float down slower. Think of it as the tissue being so light it is almost as light of air, so gravity pushing it down is not that much greater than air pushing it up (Using hands mime gravitational force and air resistance on the tissue. But for now we will only look at objects that are sufficiently large enough.

Time to move on to... racecars. Let's see what happens when I push this racecar off the table. It goes flying a pretty sweet distance, eh? Now let's see what happens when I push it so it goes a slower speed. Doesn't go all that far. I am going to let you guys push the cars as fast or as slow as you want, now what happens the faster you push it (kid raises hand) correct, it flies further. When the car is in the air it moves two different ways. Horizontally, which is this way (mime parallel to the ground), it goes the same speed through the air, so technically, if we had an invisible bridge, it would keep on going the same speed forever. Independently, it moves vertically, where it has to deal with a force of...wait for it, gravity. So the moment it leaves the table, this car is getting towards the ground faster and faster, just as it did before. Now one last thing. When I count to three you will push the racecar and as soon as it leaves the table i will drop this racecar at the height of the table, I want you to watch (Listening works better, actually) which one hits the ground first. Why did they hit the ground at the same rate? Because only gravity is acting on each racecar, they fall at the same rate.

For the time being, I want you to think about throwing a shot put, which for all of you that don't know is a track and field event, where I want to push-throw this ball as far as I can. Obviously, I will make it leave my hand at the greatest speed possible. Now what happens if I push it directly upward (Do it). It doesn't go anywhere. What happens when I push it directly in front of me (Do it). It goes farther but not all that far, it is barely in the air. So, how should I release this, directly up, directly out, or somewhere in between? (Call on kid) Correct, you want to push it so it is in the air enough to have time to travel, but outward enough that it can actually travel a horizontal distance. This is good!

We will now play a carnival game I just invented. You see the string and the basket? Now you have to get this ball in that basket, any way that you want as long as it goes above the string. So you can throw it really high over the string, or have it go barely under (Let each kid try a bunch of times). So we see that we can change the angle we throw the ball and the speed we throw the ball, and it can still go the same distance. Think of it like a basketball shot, you can shoot it directly at the rim not shooting it all that hard, or you can shoot it just a little bit harder, yet higher, and although they both go the same distance, it will have a better chance of going in.

Okay, now time to build some very simple, but still pretty cool, catapults. Now you all have your open tubes, duct tape, and plastic spoons. This will be very simple. You take the spoon, place it on top of the tube and tape it so the spoon faces upward. Everybody got it? Good. Now you put the cotton ball on the spoon, hit the end, and you will all have your cotton ball projectiles.Test different ways, which sets it off at the best angle, which sends the cotton ball off at the greatest speed? (Have a competition of whose cotton ball will go the farthest.) Now, for those that flung it the farthest, did it go fast off the spoon, or slow compared to others? Which way did it go off the spoon? Why do you think yours went the farthest?