2. DIFFERENT FORMS OF FRICTION  Even though different forms of friction exist, they have one thing in common: If two objects are in contact, frictional forces always try to prevent one object from sliding on the other object.  EXAMPLE: Rub your hand against the table top. You can feel the friction push against the motion of your hand. Now rub it the other way. You should feel the direction of the friction change.  Friction will always slow an object down.  If friction could be removed an object in motion would continue to move in a straight line with constant speed.

3. TYPES OF FRICTION Have you ever tried pushing something heavy, like the blocking sled in football or maybe you helped your parents move something heavy like a refrigerator? You might have discovered that nothing happened at first. Then as you pushed harder and harder, the object suddenly started to move.

4. When you first started to push, friction between the heavy sled and the ground or the refrigerator and the floor opposes the force you are exerting and the Net Force is zero. This type of friction is called static friction. Static Friction is the type of friction that prevents an object from moving when a force is applied. Static Friction is caused by the attraction between the atoms on the two surfaces that are in contact.

5. Static Friction keeps an object at rest, Sliding Friction slows down an object that slides. EXAMPLE: When you push an object across a room, you notice the sliding friction between the bottom of the object and the floor. You have to keep pushing to overcome the force of the sliding friction. Sliding Friction is due to the microscopic roughness of the two surfaces.

6. EXAMPLE: The brake pads in a car use sliding friction against the wheels to slow the car. Bike brakes work the same way. ASSESSMENT: What is the difference between Static Friction and Sliding Friction?

7. Static Frication prevents two objects at rest from moving past one another and Sliding Friction slows down two objects moving against one another.

8. MORE FRICTION!!! Another type of friction is needed to make a wheel or tire turn, it is called rolling friction. Rolling Friction occurs between the ground and the part of the tire touching the ground. Rolling Friction keeps the tire from slipping on the ground.

9. EXAMPLE It’s usually easier to pull a load on a wagon or cart that has wheels rather than drag it across the ground, right? This is because Rolling Friction between the wheels and the ground is less than Sliding Friction between the load and the ground.

10. Force is Required to Change Motion The first law says there can be no change in motion without a net force. That includes slowing down! It takes a net force 9often friction) to make things slow down. If forces are truly balanced, a moving object will keep moving forever with the same speed, in the same direction.

11. Inertia The first law is often called the law of inertia because INERTIA is the property of an object to resist changes in motion. Inertia comes from mass. Objects with more mass have more inertia.

12. EXAMPLE When you first enter the grocery store pushing the cart it was fairly easy wasn’t it? But once Mom began putting the groceries in it, it becomes harder and harder as she keeps putting in more and more items! Why? Because the mass of the cart is increasing!!

13. Think Back… Remember… MASS is the amount of matter an object contains.

14. Newton’s Second Law Newton’s first law says that a force is needed to change an object’s motion. But what kind of change happens? Acceleration According to Newton’s second law, the amount of acceleration depends on both the force and the mass.

15. Force and Motion Connect Through Acceleration The second law is the connection between force, mass, and motion. The connection occurs through acceleration, which results in changes in speed and/or direction. ACCELERATION is the change of velocity.

16. Acceleration is Proportional to Force The second law says that acceleration is proportional to force. What does that mean? Well, it means that all other things being equal, if the force doubles, the acceleration also doubles. If the force is reduced by half, the acceleration is also reduced by half.

17. EXAMPLE