Chapter 10-2 Friction and Gravity

Friction Force of Friction – caused by two surfaces rubbing Strength: depends on 1) how hard the surfaces are pushed together 2) types of surfaces Direction: Opposite to direction of motion

Types of Friction Static Friction – friction that acts when an object is not moving. Need to overcome to START moving Sliding Friction – friction that acts when an object is moving Less than Static Friction Sliding Sliding Friction

Types of Friction Rolling Friction – friction that acts when an object rolls Less than Static Friction or Sliding Friction Fluid Friction – friction that acts between a solid and fluid(remember a gas is a fluid) Less than Static or Sliding

Review Which is the type of friction acting when things are not moving? Name a type of friction is less than sliding friction? In what direction does friction act? What are the two things that the force of friction depends on?

Gravity Isaac Newton was the first person to seriously study gravity Gravity is a force that attracts all objects toward each other. The Law of Universal Gravitation The force of gravity is measured in SI units called Newtons (N).

Gravity The strength of gravity between two objects depends on two factors: 1. masses of the objects 2. distance between the objects

Review: Mass vs. Weight Mass- is the amount of matter in an object Weight- is the force of gravity on an object The greater the mass the greater the force (weight)

Free Fall Galileo found that all things fall at the same rate in the absence of air resistance, regardless of their mass The acceleration of gravity (g) for objects in free fall at the earth's surface is 9.8 m/s 2. Free fall- when gravity is the only force acting on an object

Galileo did experiments to convince others that the acceleration caused by gravity would be the same for all freely falling objects if there was no air to slow down their motion. He dropped two heavy metal balls together from the leaning tower. Although one weighed much more than the other, they reached the ground almost at the same time. 1. Experiment repeated MANY times 2. Led by a thought experiment Important for Scientific Method: 10

Free Fall The rate of falling increases by 9.8 m/s every second. Height = ½ gt 2 For example: ½ (9.8 )1 2 = 4.9 m ½(9.8)2 2 = 19.6 m ½ (9.8)3 2 = 44.1 m ½ (9.8)4 2 = 78.4 m

Air Resistance In air… –A stone falls faster than a feather Air resistance affects stone less In a vacuum –A stone and a feather will fall at the same speed. A type of fluid friction In what direction does it act ?

Air Resistance Terminal Velocity stops accelerating –A person in free fall reaches a terminal velocity of around 54 m/s which means the person stops accelerating At terminal velocity forces are balanced Force of air resistance = gravitational force –With a parachute, terminal velocity is only 6.3 m/s Allows a safe landing

Here two heavy balls begin “free fall” at the same time. The red one is dropped, so it moves straight downward. The yellow ball is a projectile. It given some speed in the horizontal direction as it is released. 14 Projectile An object that is thrown is called a projectile

The horizontal lines show that they keep pace with each other in the vertical direction. Why? They have the same acceleration, g, downward, and they both started with zero speed in the downward direction. 15

The yellow ball’s horizontal speed is not affected by gravity, which acts only in the vertical direction. 16 Repeat Galileo’s Inclined Plane

Cannonballs shot horizontally with different speeds from the ship travel different distances. But each cannonball drops the same distance in the same amount of time, since the vertical acceleration is the same for each. 17