Reaching Orbit - It's a Blast!

Student Activities

Gravity Never Takes a Holiday

Transparency Master	The Set Up A simple and effective demonstration can be performed using a small wooden stand and two ping-pong balls. The stand is constructed using a heavy block of wood for the base, a piece of doweling, (about one metre tall), and short thin "T" bar across the top. Drill both the base and the cross piece at their exact centres with holes to accommodate the doweling. Do not drill all the way through the wooden pieces. Assemble the three pieces (base, cross-bar, and doweling) as shown in the photograph. Carefully balance a ping-pong ball on opposite ends of the cross-bar. It helps to make two very shallow depressions at the end of the cross-bar to prevent the ping-pong balls from rolling off.

The Set Up

A simple and effective demonstration can be performed using a small wooden stand and two ping-pong balls.

The stand is constructed using a heavy block of wood for the base, a piece of doweling, (about one metre tall), and short thin "T" bar across the top. Drill both the base and the cross piece at their exact centres with holes to accommodate the doweling. Do not drill all the way through the wooden pieces.

Assemble the three pieces (base, cross-bar, and doweling) as shown in the photograph.

Carefully balance a ping-pong ball on opposite ends of the cross-bar. It helps to make two very shallow depressions at the end of the cross-bar to prevent the ping-pong balls from rolling off.

The Demonstration Choose a student to "bat" one of the ping-pong ball across the room. The only limitations are that the ping-pong ball must be struck horizontally so that it is launched neither upwards nor downwards. Batting one ball horizontally will simultaneously cause the second ball to drop downwards from its resting place on the opposite end of the cross-bar. The Challenge Using a "secret ballot" method have students report which ball hits the floor first. Collect the ballots and present the results. Repeat the demonstration trying to bat the ping-pong ball slightly further each time. Since the free-fall time for a ping-pong ball from a height of one metre is only a fraction of a second, because launching the ball exactly horizontally takes some practice, and because of each observer's bias, this demonstration will generate a great deal of discussion and debate.
Transparency Master
Discussion and Conclusion As the ball is launched faster and faster, the distance it travels before hitting the ground increases. Nevertheless, the two balls will always reach the floor simultaneously. What if? Imagine that one could throw a ball fast enough so that its curved flight path was exactly parallel to the Earth's curvature. Suppose the path of the ball were high enough to avoid hitting trees, mountains, and even the molecules that make up the Earth's atmosphere. What would you have achieved (besides a world's record for ball throwing)? CONCLUSION: The height (distance from the Earth) of the orbiting object is not important, (provided the object is above the energy absorbing drag of the Earth's atmosphere). It is the speed of the object that allows it to attain orbit, not its height.

The Demonstration

Choose a student to "bat" one of the ping-pong ball across the room.

The only limitations are that the ping-pong ball must be struck horizontally so that it is launched neither upwards nor downwards.

Batting one ball horizontally will simultaneously cause the second ball to drop downwards from its resting place on the opposite end of the cross-bar.

The Challenge

Using a "secret ballot" method have students report which ball hits the floor first.

Collect the ballots and present the results.

Repeat the demonstration trying to bat the ping-pong ball slightly further each time.

Since the free-fall time for a ping-pong ball from a height of one metre is only a fraction of a second, because launching the ball exactly horizontally takes some practice, and because of each observer's bias, this demonstration will generate a great deal of discussion and debate.

Transparency Master

Discussion and Conclusion

As the ball is launched faster and faster, the distance it travels before hitting the ground increases. Nevertheless, the two balls will always reach the floor simultaneously.

What if?

Imagine that one could throw a ball fast enough so that its curved flight path was exactly parallel to the Earth's curvature.

Suppose the path of the ball were high enough to avoid hitting trees, mountains, and even the molecules that make up the Earth's atmosphere. What would you have achieved (besides a world's record for ball throwing)?

CONCLUSION: The height (distance from the Earth) of the orbiting object is not important, (provided the object is above the energy absorbing drag of the Earth's atmosphere). It is the speed of the object that allows it to attain orbit, not its height.

Gravity At Work

The Set Up Place a tennis ball or a golf ball inside a sock. Tie the open end of the sock with a piece of long string to secure the ball inside. The Demonstration In a clear, empty area, begin to whirl the ball around in a circle over your head. Note that the faster you whirl it, the harder you must pull back on the string. What happens if there is suddenly no force on the string? Let it go and observe the result.
Transparency Master
Discussion and Conclusion What keeps planets and satellites travelling in a circular path? Why don't they fly straight off into space? What if? What would happen to the orbital speed of a satellite orbiting the Earth, if by some mysterious process the Earth's mass, (and hence its gravitational force), were to increase? CONCLUSION: Gravity is essential in keeping a planet or satellite in orbit. An orbiting object does not escape the force of gravity. It is, in fact, trapped in a perpetual circular orbit by the gravitational attraction of the Earth.

The Set Up

Place a tennis ball or a golf ball inside a sock.
Tie the open end of the sock with a piece of long string to secure the ball inside.

The Demonstration

In a clear, empty area, begin to whirl the ball around in a circle over your head. Note that the faster you whirl it, the harder you must pull back on the string.
What happens if there is suddenly no force on the string? Let it go and observe the result.

Transparency Master

Discussion and Conclusion

What keeps planets and satellites travelling in a circular path? Why don't they fly straight off into space?

What if?

What would happen to the orbital speed of a satellite orbiting the Earth, if by some mysterious process the Earth's mass, (and hence its gravitational force), were to increase?

CONCLUSION: Gravity is essential in keeping a planet or satellite in orbit. An orbiting object does not escape the force of gravity. It is, in fact, trapped in a perpetual circular orbit by the gravitational attraction of the Earth.

Prepared by YES I Can! Science