Tie a string to a bagel and whirl it overhead to illustrate that tension from the string caused by the pull of your hand creates a force that causes the bagel to move in a constant circular path. Now let go of the string to show that once the tension of the string is gone the bagel will fly off in a straight line from the point that it was released from the tension. This is consistent with Newton’s law that absent another force, a body in motion will remain in motion.
For this demonstration, you will need three types of paper – smooth, glossy and rough. You will also need objects, such as different sized books, to place on the papers. Place one of the pieces of paper on a desk and put a book on it, then in one smooth motion pull the paper from under the book. Try this with different sizes of objects and different textures of papers and make notes of the differences. This shows students that different textures produce different amounts of friction. With some textures, there is very little friction and very little movement of the body or object at rest.
Use a tennis ball to show that a body in motion will continue in motion unless it is acted upon by another force. Mark a target on the floor with masking tape and have students attempt to drop the ball on the target while running. Because there is no force acting against the horizontal motion of the runner, the ball’s horizontal motion of the ball remains. To hit the target the runner must release the ball ahead of time to offset the force of the horizontal motion. Filming the action of the runner and the ball and playing it back in slow motion will help the class see the ball’s trajectory.
Make a simple pendulum using a string and object hanging from it. A pendulum is powered by the amount of gravitational potential energy it possesses, which is measured by gravity and by the length of the string and the mass of the object. Hold the object at a marked position and let it swing. Count how many times it swings past its vertical position. Change to a heavier object and then a lighter object. Try other alterations such as making the string longer or shorter.