2. Friction and Gravity

3. Friction Friction Friction is the force that one surface exerts on another when one surface rubs against the other surface. The strength of the force of friction depends on two factors; -the type of surfaces involved -how hard the surfaces push each other Which surface produces greater friction, the ice or the grass?

4. Is Friction Useful or Not? Friction can be useful. Examples: An automobile moves because of friction between its tires and the road.An automobile moves because of friction between its tires and the road. The rubber soles on the lady’s shoes keep her from slipping in the rain.The rubber soles on the lady’s shoes keep her from slipping in the rain.

5. Three Kinds of Friction 1. Sliding Friction: solid surfaces slide over each other ice skater on ice 2. Rolling Friction: object rolls over a surface ball bearings in wheels(skateboards, bikes) 3. Fluid Friction: objects move through a liquid or a gas moving parts in machines bathed in oil

6. Sliding, Rolling and Fluid Friction Sliding Rolling Fluid Liquid/oil and gas/air What are two ways to reduce friction?

7. Gravity Newton realized that a force acts to pull objects toward the center of the Earth. This force is gravity.Newton realized that a force acts to pull objects toward the center of the Earth. This force is gravity. Gravity is the force that pulls objects towards each other.Gravity is the force that pulls objects towards each other.

8. Free Fall This is when the only force acting on the object is gravity.This is when the only force acting on the object is gravity. An object in freefall accelerates as it falls at 9.8 m/s.An object in freefall accelerates as it falls at 9.8 m/s.

9. Projectile Motion An object that is thrown is called a projectile. A projectile will land at the same time that an object that is dropped straight down will land. Both objects are in free fall.

10. Air Resistance Objects falling through the air experience a type of fluid friction called air resistance. The greater the surface area, the greater the resistance. Which would fall slower, an acorn or an oak tree leaf?

11. Weight Weight is a measure of the force of gravity on a person /object.Weight is a measure of the force of gravity on a person /object. Remember this is different than mass which is a measure of the amount of matter in that object.Remember this is different than mass which is a measure of the amount of matter in that object. Since weight is force, Newton’s 2 nd Law is written:Since weight is force, Newton’s 2 nd Law is written: Weight = mass X acceleration due to gravity

12. Universal Gravitation This law states that the force of gravity acts between all objects in the universe. Any two objects in the universe are attracted to each other. You are attracted to the Earth and other objects around you.

13. The force of attraction between two objects varies with mass and distance If the mass doubles, the force increases.If the mass doubles, the force increases. If the distance is cut in half, the force increases.If the distance is cut in half, the force increases. If the mass is cut in half, the force decreases.If the mass is cut in half, the force decreases. If the distance doubles, the force decreases.If the distance doubles, the force decreases.