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Gravitation: Infinite Force?!
A common misconception
Date : 12/06/2020
Author Information
Uploaded by : Paul
Uploaded on : 12/06/2020
Subject : Physics
When introduced to gravitational fields, students will encounter the equation F=Gm1m2/r2, Newton`s Law of Gravitation. It is commonly taught that the r in this equation is the distance between the centres of two masses of masses m1 and m2. Students then find that this works well to model, e.g. the Earth-Sun interaction, where the Earth and Sun are (roughly) symmetric objects. However, there are situations where we must be careful with the meaning of r.
The centre of mass of a regular shape or solid which has a uniform density is its geometric centre. In simpler terms, the centre of mass of a uniform sphere would be at the centre of a sphere.So consider the case of a ping pong ball inside a tennis ball (both hollow spheres). If you gave the tennis ball a shake, then it might be possible for the ping pong ball to pass over the centre of the tennis ball. If you naively use the equation above, then r=0, i.e. the distance between the masses is zero. Does that mean there is somehow an infinite force between them?What`s the problem?
The centre of mass of a regular shape or solid which has a uniform density is its geometric centre. In simpler terms, the centre of mass of a uniform sphere would be at the centre of a sphere.So consider the case of a ping pong ball inside a tennis ball (both hollow spheres). If you gave the tennis ball a shake, then it might be possible for the ping pong ball to pass over the centre of the tennis ball. If you naively use the equation above, then r=0, i.e. the distance between the masses is zero. Does that mean there is somehow an infinite force between them?What`s the problem?
This resource was uploaded by: Paul