# Gravity & Projectile Motion: Physics Lab

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• 0:01 Projectile Motion
• 0:58 Physics Lab Steps
• 2:35 Lab Solution
• 5:07 Lesson Summary

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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to define projectile motion and use the equations of projectile motion to make predictions about motion in a real-life scenario. A short quiz will follow.

## Projectile Motion

Projectile motion is motion in two or more dimensions where the only force is gravity. A classic example would be a cannon being fired at an angle. Such a cannonball has a velocity both up and sideways. And the cannonball is, in this case, the projectile.

In other lessons, we introduced the equations that govern 2D projectile motion. Here they are again.

Equations in the Y-Direction:

vf = vi + a * t

y = vi * t + ½ a * t^2

y = vf * t + ½ a * t^2

vf^2 = vi^2 + 2a * y

y = (vf + vi) t / 2

Equations in the X-Direction:

vx = x / t

In the y-direction, we have an acceleration, so we can use the kinematics equations of constant acceleration. This acceleration is caused by gravity, so a will be equal to -9.8 m/s/s on Earth. And in the x-direction, once the object is launched, there are no forces, so the cannonball (or whatever it is) continues at a constant velocity. For this reason, we only have this one equation in the x-direction. But now it's time to use these equations in a real-life scenario. It's time to investigate projectile motion!

## Physics Lab Steps

For this physics lab, you will need:

• A marble
• A smooth table
• Materials to create a sturdy ramp (for example, a plank of wood and some books)
• A target printed from the Internet, with several rings
• A stopwatch
• A tape-measure or ruler

Once you have your materials, set up a sturdy ramp on top of the table. Leave at least 30 centimeters (ideally longer) between the end of the ramp and the edge of the table. The more smooth the transition from ramp to table, the easier the lab will be.

Place your printed target on the floor, and observe your magnificent set up. Your goal is to calculate where on the floor to position your target, so that your marble will roll down the ramp, across the table and hit the bulls-eye on the floor. But you are not allowed to do test runs. The marble cannot leave the table until you've done all your calculations and are ready to go. If you let it do so, you lose the game. You are allowed to take whatever data you want on the tabletop. You can roll the marble down the ramp and across the table, so long as it never leaves the table. You can also take measurements with the ruler and stopwatch.

Using the equations for projectile motion and whatever data you are able to take, calculate where to place the target on the floor. The center of the target represents an A grade for the lab, the next ring is a B and so on. Can you get an A the first time?

If you fail, you must change the height of the table by putting a book underneath each table leg, and start again. If you fail again, you must change the slope of your ramp. If you fail a third time, it's game over.

So now, it's time to pause the video and get started. Good luck!

## Lab Solution

If you're listening to this part of the video, you've already tried the lab yourself. If not, go back and try it. There's no fun in cheating. But if you need a hint, I'll give you one: The first step is to figure out how fast the marble is going as it moves across the table. That will give you a velocity in the x-direction.

Okay, so if you've already figured that out and successfully completed the lab, here's the solution.

Step 1: Measure the speed of the marble moving across the horizontal part of the table. Measure the distance using a ruler or tape measure, and measure the time using the stopwatch, average your numbers, and then divide the distance by time to get the speed. So, here you're figuring out how fast it's moving across the table. In the equations, that number is your Vx, your velocity in the x-direction.

Step 2: Measure the height of the table surface above the floor. That's your y-variable.

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