Free Falling

Astronomy, Physics:	Gravitational Force & Acceleration
Grade Range:	Elementary School, Middle School
Format:	Stage

This demonstration helps to clear up any misconceptions about the force of gravity, and it is a way for students to see how gravity affects everything equally.

Materials

• Gravity Tube
• Vacuum Pump
• Feather or Similar
• Small Stone or Similar

Safety Precautions

Please read the General Safety section of the Demonstration Safety page before performing this demonstration.

Demonstration

1. Show the feather and small stone to the students. Ask them to point at the one that will fall faster (The small stone). Show that the stone does fall faster, and ask for ideas on why that is.
2. Explain that air resistance is the key factor, and that gravity is a constant acceleration. Pull off the top end of the gravity tube and place the feather in one of the compartments, and the stone in the other. Put the top end back on snugly.
3. Attach the vacuum pump to the gravity tube. Open the valve on the tube and turn on the vacuum pump. You may need to hold the hose on the valve to ensure a good connection, and after 20-30 seconds close the valve. Turn off the vacuum and explain that there should now be only a small amount of air in the gravity tube.
4. Rotate the knob on the top end to have the feather and stone drop. They should fall at the same speed or close to the same speed! Ask the students for ideas on why the feather fell faster this time compared to when you dropped it.
5. If students would like to see it again, you can pick up and flip the tube slowly to have the feather and stone return to the top. Have someone help you by spinning the knob to get the objects back in the compartments.

Why This Works

Gravity is an attractive force between really big masses, measured on the astronomical scale. We experience a gravitation acceleration towards the earth because it has a lot of mass, and likewise the earth experiences gravity from the sun because the sun is so much bigger than it. The force of gravity on earth is:

Force of Gravity: Fg =	mass * gravitational acceleration = m*g
Gravitational Acceleration: g =	9.81 meters/s2 OR 32.1 feet/s2

Everything on the surface of the earth experiences the same amount of gravitational acceleration, but this can be hard to see due to the air that is all around us. Air resistance is the amount of frictional force that a falling or moving object can experience with the air around it, since it needs to move through the air around it. If we have two objects of similar size but different mass, we see that the more massive object falls faster. This is because, although the objects experience the same amount of air resistance, the more massive object will have more gravitational force to it because of the extra mass. This larger gravitational force means that the object is able to overcome more of the air resistance, which allows it to accelerate more.

When we have the objects fall in the tube, we see them fall at the same speed. This is because there is little to no air inside the tube, which means that there is little to no air resistance. Because there is less air to push against, both objects can experience the full acceleration of gravity, and will fall at the same rate!

Additional Information

• A common misconception is that the mass of an object can change how fast it falls due to gravity. This is incorrect, as the mass of an object does not affect how fast it falls due to gravity. The gravitational acceleration is a constant, and does not change. The air resistance an object experiences is what affects the falling speed.
• This demonstration is a part of the Astronomy Show.