Session 2: Experiments on gravity concept 2

• 

Objective 1: to show that when an object is falling, it is subject to two forces: gravity and friction

Objective 2: to experiment the effect of gravity, the influence of the friction force has to be minimised

 

Objective 3:  to show that the acceleration due to the gravitation force does not depend on the weight of the object

Explanation

If you are releasing an object two forces are acting on the object:

1) the gravity force and 2) the friction force. The objective here is to make experiments that will reduce as much as possible the friction force. When the friction force is negligible, the objective is to study the properties of falling objects subject to the gravitation force.  For younger students, the objective is to experiment with them that the fall time for a given height is the same for a heavy object as for a light object.

For older students, at secondary school, they will experiment that acceleration is constant. The consequence is that velocity increases linearly with time.

The concept of friction will also be studed in the program of experiments on friction forces.

Material

• Sheet of paper
• Tennis balls
• Syringe
• Newspaper
• Chronometer

Maximum duration

45 minutes

Main question to be asked

Step one: why does the sheet of paper fall more slowly than the paper ball?

Step two: do heavy objects fall faster or slower than light objects?

Introduction/Starters

You can show a small video to your students on falling objects on the moon, and in non gravity atmosphere in order to discuss with them that the attraction forces can vary depending on the place you are in the universe.

http://www.youtube.com/watch?v=isVO9AAAhxM&feature=related

More funny!

http://www.youtube.com/watch?v=SN77b9DqEbc


Main activities

A. Gravity and friction in action

• Take a sheet of paper let it fall and measure the time before it reaches the floor.
• Change the position of the sheet of paper when you release it, does it change anything?
• Then make a ball of paper and let it fall. Compare the fall time.
• What is the difference between these experiments?

B. Gravity and reduced friction

• Take two tennis balls, fill one of the tennis ball with water
• Take sheets of paper and make a paper ball of the same diameter
• Then let the balls fall two by two or three at exactly the same time. Put a sheet of newspaper on the floor so you can hear the ball reaching the floor.

Conclude on this experiment:

• What are the differences between these three balls ?
• What are the similarities in term of shape ?
• What is the constant in terms of physics ?
• Is the velocity constant ?

Conclusion/Plenary

Objects with a reduced friction force with different weights will take the same time to reach the floor.

During their fall their velocity varies.

During their fall their acceleration is constant and equal to the gravitational acceleration(g) (for secondary schools).