1. The Laws of Motion
Chapter 4

2. Gravity
Section 2

3. At this moment you are exerting an attractive force on everything around you
This force is called gravity
Anything with mass is attracted by gravity
One of the four basic forces
Electromagnetic force
Strong nuclear force
Weak nuclear force
Gravity
What is Gravity?

4. 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
What is Gravity?

5. 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
d2 G = Universal gravitation constant
m1 = mass of object 1
m2= mass of object 2
d = distance between the two objects.
The Law of Universal Gravitation

6. The Range of Gravity m2= mass of object 2
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 constant
m1 = mass of object 1
m2= mass of object 2
d2 = distance between the two objects.
The Range of Gravity

7. 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
Earth’s Gravitational Acceleration

8. 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

9. 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
Weight and Mass

10. 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?
Weightlessness and Freefall

11. 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
Projectile Motion

12. 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 pull
The ball appears to curve
The two motions are independent of each other
Horizontal and Vertical Motions

13. 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!!
Horizontal and Vertical Distance

14. 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

15. 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
Centripetal Force and Traction