Universal Law of Gravity All objects in the universe attract each other by the force of gravity. Universal Law of
Gravity Gravity varies depending on two factors: 1) the mass of the object doing the pulling, and 2) the distance from the center of that object On Earth gravity = 9.8 m/s/
s = Acceleration For every second that an object falls its speed increases by
9.8 m/s Weight= Mass (m) X gravity (g) Unit of mass = kg Unit of acceleration = m/s/ s Unit of weight = Newton
1 Newton= about pound The Apple & the Moon Isaac Newton realized that the motion of a falling apple and the motion of the Moon were both actually the same motion, caused by the same force the gravitational force.
Universal Gravitation Newtons idea was that gravity was a universal force acting between any two objects. At the Earths Surface
Newton knew that the gravitational force on the apple equals the apples weight, mg, where g = 9.8 m/s2. W = mg Universal Gravitation From this, Newton reasoned that the
strength of the gravitational force is not constant, in fact, the magnitude of the force is inversely proportional to the square of the distance between the objects. Universal Gravitation Newton concluded that the
gravitational force is: Directly proportional to the masses of both objects. Inversely proportional to the distance between the objects. Inverse Square Law
Newtons Law of Universal Gravitation is often called an inverse square law, since the force is inversely proportional to the square of the distance. Experimental Evidence
The Law of Universal Gravitation allowed extremely accurate predictions of planetary orbits. Action at a Distance In Newtons time, there was much discussion about HOW gravity worked - how does the Sun, for instance, reach across empty space, with no actual contact at all, to exert a force
on the Earth? This spooky notion was called action at a distance. The Gravitational Field During the 19th century, the notion of the field entered physics (via Michael Faraday). Objects with mass create an invisible
disturbance in the space around them that is felt by other massive objects - this is a gravitational field. The Gravitational Field So, since the Sun is very massive, it creates an intense gravitational field
around it, and the Earth responds to the field. No more action at a distance. Gravitational Field Strength To measure the strength of the gravitational field at any point, measure the gravitational force, F, exerted on any test mass, m. Gravitational Field Strength, g = F/
m Gravitational Field Strength Near the surface of the Earth, g = F/m = 9.8 N/kg = 9.8 m/s2. In general, g = GM/r2, where M is the mass of the object creating the field, r is the distance from the objects center, and G = 6.67 x10-11 Nm2/kg2.