SECTION 2 (PART 1) – Gravity

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

FORMULA SHEET F = mg m = F/g g = F/m

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

FORMULA SHEET W = mg m = W/g g = W/m

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?