Posts Tagged ‘projectile motion’
I kind of made this one up, kind of adapted it from the electronic timer manual. The idea is that we use a ramp to accelerate a steel marble, have it pass through timer gates, measure the distance between the gates and calculate the velocity. Do that a couple of times for accuracy.
Now measure the height of the table using a meter stick. Use a fishing weight on a string to find the point directly under the edge of the table. We now have the horizontal velocity, the height of the table, we can calculate how long it will take to fall. Next we do the math and place a penny where the steel marble should land.
In the beginning, it would works sometimes, but not always. I determined that our heavy epoxy table tops caused the steel ball to bounce, losing some of its horizontal velocity. The bounce was easily dampened by placing two or three sheets of paper under the end of the ramp. We also had some issues with hitting the side of the photogate. Lining up the gates with the ramp was a minor issue, but an important one. After the speed was determine, we moved the photogate away used the already calculated speed.
Here is the Lab as I wrote it up, it needs some updates like the paper under the ramp. I realized later that I should have had the kids measure the starting height of the ramp. Then we could go back to it later when we do energy and analyze the results.
Next year I think I will have them hit the penny first time, then a dime, then a small washer, so each time it must be more accurate.
As always, comments and ideas are welcome.
I’ve been searching for a projectile motion activity/lab for my physical science class. I completely forgot about this one, and I’ve done it for years. We are going to make catapults from popsicle sticks, hot glue, rubber bands, and a spoon.
I like this lab because the hot glue allows for rapid build and repair, and there is a definite need for repair and modification in this project. The kids typically build the frame from the popsicle sticks and then try to attach the spoon with rubber bands. The spoon is the launcher for the catapult. Usually, they find there is no way to connect the rubber bands, so they start adding little posts. The next problem they encounter is the rubber bands aren’t tight enough so the object doesn’t launch very far. They then tighten the rubber bands and the frame starts to collapse. So they need to go back and reinforce the frame.
At this point one of three things happens.
- The catapult is improved and they start launching marshmallows.
- They realize they can rebuild the catapult rather quickly and so start all over and improve their design.
- The catapult falls apart, they attempt improvements, but basically give up.
The picture isn’t a great example, but it is a starting point.
Students get a Nerf gun, a meter stick, a level, and a long metric tape measure. They launch several darts horizontally from 1 meter in height and mark where the darts first hit the ground. They measure this distance and average them together. Then using this distance and the time it takes for an object to fall 1 meter, they can calculate the muzzle velocity of their Nerf gun. (My Nerf gun in the picture has a muzzle velocity of around 12 m/s.)
Part two, they are to use what we did in class to calculate how far the dart will go if they are launched from the ground at 10º, 15º, and 20º. (Any higher angle and they hit the ceiling.)
Part three, they go back to the range and using a large cardboard protractor that I made, they launch the darts from the ground at those three angle and see how they did. The lab is attached below along with another one I do at the same time. Enjoy.
For some reason, I find the books totally inadequate on this section. I also don’t like their method of teaching it. Our book has one small section and a few problems, but not enough for the kids to see the pattern and understand what’s happening. I break projectiles into three main parts.
Part 1 – Horizontally Launched Projectiles
The idea here is that a ball is kicked out horizontally with no vertical velocity up or down. An important concept here is that a ball dropped off a cliff or kicked out horizontally, it will take the same time to fall. I break these problems into three variations:
- I give the students the Vx and the height of the cliff and ask them to find the distance horizontally that the ball hits the ground.
- I give the students the Vx and the distance and ask the students to find the height of the cliff.
- I give the students the height of the cliff and the distance horizontally that the ball hits the ground and ask them to find the initial velocity.
One the second day we play “Survivor Physics.” We pick a vocal member of the class and vote him off the airplane. I draw a picture of him falling and hitting the ground below. I usually give them the height of the airplane and the speed of the airplane, they have to tell me how long it takes him to fall and how far horizontally he travels before hitting the ground. Actually I say, “How long does he scream before he goes splat?” (Problem 1 all over again.)
Next problem, I name a student who drops a water balloon off a building and on to another student. They get the height of the building and the distance the other student is standing from the bottom of the building. They need to calculate the horizontal launch velocity. (Problem 3)
Last, I draw a watch tower and tell them that a student pushes another student off the tower with a certain horizontal velocity. He has to push him onto a waiting cactus below that is a certain number of meters out. They need to determine the height of the tower.
Now before you get up in arms over student abuse, understand they are all asking for it to be them that is pushed and thrown and dropped on. It is all in good fun and they hopefully see a connection to the work. Hopefully.
Part 2 – Upwardly Launched Projectiles
I teach my students to break these launched projectile problems into two halves; the up half and the down half. We can solve every problem this way. The steps are basically the same for every problem.
- Get Vx and Vy from the velocity and angle.
- Get the time_up using Vy/g
- Get the height of the projectile using using 0.5gt^2.
- Get the total_time by doubling time_up if the projectile lands at the same height it was launched from.
- Get the distance using d=(Vx)*total_time
Using this method, they can also move the cannon or whatever we launch with up on a hill. They can then calculate the up time and the down time and see they are different for a different height. They can also do problems where they determine the height some distance down range by finding out how far an object falls after reaching the maximum height. These are some of the more tedious projectile problems, but I don’t ease up, they work through them until they get them right.
Part 3 – Unknown Velocity
I was worried my kids wouldn’t be able to solve these problems because you end up with two unknowns and two different equations. But because the problems are done exactly the same each time, they figured them out fairly quickly. To solve these, you get the total time like above, just keep a variable for velocity. Then solve for total time horizontally using Vx and the distance traveled. Setting them equal to each other lets you get the launch velocity of the projectile.
I know this was a lot of words. I spend almost two weeks on these problems. Along the way I take out specifications on military weapons and use those to find the distance they travel. We use the specs from the Barrett .50 caliber sniper rifle for the horizontal only and upward launch. The numbers are mind blowing, I find it really gets their interest, especially if any of them are hunters.
The test will be attached below, I need to scan it.
When I teach projectile motion, I like to do exercises where the students have to predict the results and then perform the experiment. One of my favorite toys is something called Stomp Rockets. They aren’t as easy to find as they were 10 years ago. They are plastic rockets that are launched from a tube and plastic bladder that you stomp on. If you set the angle to around 45° and jump up and come down hard with your heals, you can launch a rocket the length of a football field. If you launch them straight up, you can use a stopwatch to determine the maximum altitude. This is obviously an outside activity, but there are also foam rockets that we have launched in the gym. Outside is more fun.
I built a supersized cardboard protractor to set the launch angle and I had to rebuild the launch stand because it just wasn’t sturdy enough. The challenge is that you can’t get the same launch pressure twice, so the lesson has to be generic enough to get the point across without getting accurate results. We tend to experiment with the relationship between the launch angle and the distance.
I’ll put some pictures here some day, I haven’t done the activity in quite a while.