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Teaching Gravity

Rob Phillips's picture

This forum is to discuss how we help students come to grips with the concept of gravity. There are several lecture demonstration videos about this in the Resources section for teachers to use and adapt but is there anything you do that seems to work that you could share with others?



Centripetal Force

A force causes a mass to accelerate. In this video we are dealing with one force that is constant and directed downwards i.e. the gravitational force and a second variable force supplied by either a tether on a ball or by a bucket on water.

Centre Of Mass

If you apply a force through the “centre of mass” of an object that is free to move then the object will not rotate. It may translate but it will not rotate. If you apply a similar force to the object that is not directed through the “centre of mass” then the object, which before merely translated, will now rotate as well as translate.

Acceleration Due To Gravity

This video really has two parts. The first demonstrates that dropping two different masses (balls) from the same height results in them reaching the ground at the same time. There has to have been some acceleration because the balls were not moving at the moment they were released and we know that if air resistance is not a significant force during the motion of the balls then they will accelerate with a constant acceleration towards the surface of the Earth.

Gravity - all things fall with the same acceleration?

Here we have a video highlighting the importance of considering all aspects of the situation before saying rote-learned statements. Many students may say “all things fall with the same acceleration” and feel somewhat pleased with themselves that they remembered to say acceleration and not velocity. This video demonstrates that it is the centre of mass of a freely falling object that has the value of the local acceleration due to gravity and sometimes this causes other parts of the object to accelerate faster than “g”. 

Force Fields

A net force will cause a change in motion of an object. This is obvious in the case of contact forces; a push or a pull can make an object speed up, slow down or turn through an angle. Sometimes though the motion of an object can change and there is no obvious contact with anything else to cause this change.  This is when the concept of a force field becomes useful. Objects that have mass are attracted to each other, e.g. the ball and the planet Earth. Objects that have charge are either attracted to or repelled by each other e.g. plastic brush and hair as you brush your hair.

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