The Laws of MotionChapter 4

GravitySection 2

What is Gravity?

At this moment you are exerting an attractive force on everything around you

This force is called gravityAnything with mass is attracted by gravityOne of the four basic forces

◦Electromagnetic force◦Strong nuclear force◦Weak nuclear force◦Gravity

What is Gravity?

GRAVITY is an attractive force between two objects that depends on the masses of objects and the distance between them. ◦Increases with mass◦Increases as objects move closer

Only Earth is close enough and large enough that you can feel its attraction

The Law of Universal Gravitation

Scientists wanted to be able to calculate the gravitational force between two objects

Isaac Newton published the Law of Universal Gravitation in 1687.

F = G m1 m2 F = gravitational force

d2G = Universal gravitation constant

m1 = mass of object 1

m2= mass of object 2

d = distance between the two objects.

The Range of Gravity

Attraction rapidly decreases as two masses move away from each other.

How rapidly? Let’s look at the equation…

F = G m1 m2 F = gravitational force d2 G = Universal gravitation

constantm1 = mass of object 1m2= mass of object 2

d2 = distance between the two objects.

Earth’s Gravitational Acceleration

If you dropped a bowling ball and a marble at the same time, which would hit the ground first?

What if there were no air resistance?Acceleration due to gravity (g) is 9.8 m/s2

Earth’s gravitational force can be calculated:F = mg F= force (N)

m= mass (kg)g = gravitational

acceleration

Weight

Gravitational force is acting on you whether you are falling or standing on the ground.

Gravitational force acting on an object is weight.

W = mg W = weight (N) m = mass (kg) g = gravitational

acceleration

Weight and Mass

Weight and mass are not the same◦Weight is a force◦Mass is the amount of matter in an object

Weight can change with gravitational force

Mass will not change

Weightlessness and Freefall

Notice that object in space shuttles always seem to be floating◦They aren’t floating ◦They are falling◦They are all falling at the same acceleration

Let’s consider you on a scale in an elevator:◦What happens when you go up?◦What happens when you go down?

Projectile Motion

Thrown objects do not always travel in straight lines

Anything thrown or shot through the air are called projectiles.

Gravity causes projectiles to follow a curved path

Horizontal and Vertical Motions

A thrown object has both a horizontal and vertical velocity

The horizontal velocity (resulting from the applied force by the thrower) will be constant in the forward direction

But an object thrown is being pulled by gravity.◦The object will have a vertical velocity resulting from

earth’s gravitational pullThe ball appears to curveThe two motions are independent of each other

Horizontal and Vertical Distance

If I throw a ball and then drop a ball of the same mass at the same time, which will hit the ground first?

They will hit at the same time!!

Centripetal Force

When an object goes around a curve, it accelerates toward the center of the curve.

Acceleration toward the center of a curve is called centripetal acceleration

The net force exerted toward the center of the curve is the centripetal force

Centripetal Force and Traction

When a car rounds a corner, a force must act to keep the car moving in a curved path

The centripetal force is the friction force (traction) between the tires and the road

The road is slippery and the force is small, the force may not be large enough to keep the car following the curve