LEARNING GOALS Describe gravitational force. Distinguish between mass and weight.
GRAVITY Gravity: an attractive force between any two objects Depends on the masses of the objects and the distance between them
GRAVITY You Your book is close enough to exert a force you can feel but it is too small. Jupiter is large enough to exert a noticeable force, but its too far away. The Earth is large enough and close enough to exert a noticeable force.
THE LAW OF UNIVERSAL GRAVITATION Newton’s equation enables the force of gravity to be calculated between any two objects if their masses and the distance between them is known. G = universal gravitation constant m 1 = mass of the 1 st object m 2 = mass of the 2 nd object d = distance between the two masses
THE LAW OF UNIVERSAL GRAVITATION As the distance between two objects increases, the gravitational force between them decreases. No matter how far apart two objects are, the gravitational force never completely goes to zero!
THE EARTH’S GRAVITATIONAL ACCELERATION Close to the Earth’s surface, the acceleration of an object in free fall is 9.8 m/s 2. Free fall ignores all forces except gravity! This acceleration constant is sometimes labeled g.
THE EARTH’S GRAVITATIONAL ACCELERATION Using Newton’s 2 nd Law of Motion, the force of Earth’s gravity on a falling object is F = force of gravity on a falling object (N) m = mass of the object (kg) g = gravitational acceleration constant (9.8 m/s 2 ) F = mg
EXAMPLE What is the gravitational force on a sky diver with a mass of 600 kg?
WEIGHT Even if you aren’t falling, the force of Earth’s gravity is still pulling you downward. Weight: the gravitational force exerted on an object Weight can be calculated by the following equation: W = mg
WEIGHT AND MASS Weight and mass ARE NOT THE SAME THING! Weight is a measure of force, mass is the measure of the amount of matter in an object. Weight changes as gravity changes; mass does not.
EXAMPLE A person on Earth has a mass of 54 kg, what is their weight?