Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Demonstrating weightlessness

Demonstration

To show that in free fall it is not gravity which disappears but the reaction force with a surface. This is how we usually 'feel' our weight.

Apparatus and materials

1 kg masses joined by a spring, 2

Board, with high-friction surface, such as a rubber mat, attached to it

Sandbags or expanded polystrene packaging

Health & Safety and Technical notes


Dropping heavy masses - ensure they fall onto the sandbags or other suitable cushion and that all watchers are well clear of the landing site.

The masses should be able to sit on the high-friction surface with the spring stretched between them without moving. Once lifted off the surface the masses should move quickly together.

Procedure


a Place both masses on the board, with the spring between them stretched. 

b Explain that the only reason the masses do not move together is because of the friction. Demonstrate this by moving them to a lower-friction surface such as a bench top. 
 
c Hold the board horizontally above the sandbags and ask what will happen when the board is dropped. Demonstrate. 
 
d Now the follow-up. Repeat the experiment but say that you are going to throw the board upwards. Again ask what will happen.

Teaching notes


1 While the board is held at rest there is a force between it and the masses. This leads to friction (proportional to the normal force and the coefficient of friction). Once the board is dropped (step c) it is apparent that the friction disappears, although the masses may remain in contact with the board. This is an indication that two objects in free fall together will have no reaction force on each other. As we normally feel our weight as a result of the reaction of the ground pushing us up (when we push the ground down) this absence is manifested as a feeling of weightlessness. 

Is it truly weightless? That depends on your definition. If weight is the force of gravity then no - clearly gravity is pulling the board and masses down. However, the term weightless has come to mean this odd situation, which astronauts find themselves in and so we have to accept that terminology. 
 
It also links in with general relativity and the idea that an accelerating rest frame and a uniform gravitational field can be treated identically. 
 
2 Throwing the board up (step d) will cause confusion - students will expect the masses to start moving once the board peaks. In fact they will start to move immediately. Why? Because they are in free fall, regardless of which way they are moving. An astronaut may be in free fall, accelerating towards the centre of Earth, but moving always at right angles to that direction. This is a very important point: that acceleration and velocity can be in different directions. 

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.