Newton's Law of Gravitation
Earth and Space

Distinguishing between gravity force and mass

Physics Narrative for 11-14 Supporting Physics Teaching

The distinction between the gravity force on an object and the mass of an object is one which many pupils find confusing

Teacher: In everyday speech the weight of something is the force to support it against the pull of gravity. If there is no gravitational field then there is no need to support it. The unit of force is the newton, whether it is the supporting force or the pull of gravity.

Teacher: Now, who can tell me about mass?

Jim: The mass of an object is a measure of the amount of stuff or matter in the object. The unit of mass is the kilogram.

Teacher: That's along the right lines, but we can be more exact. The mass is a measure of how difficult it is to accelerate an object – that is to change its motion. It is connected to the amount of matter, but that's measured in moles. But you're right about the unit of mass.

Newton's Law of Gravitation
is expressed by the relation F=G(m_1)(m_2)/r^2
can be used to derive Kepler's First Law

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