1. Newton ’ s First Law Law of Inertia

2. What You Will Learn ► Distinguish between balanced and net forces ► Describe Newton ’ s First Law of motion ► Explain how friction affects motion

3. Why Is It Important ► Forces can cause the motion of an object to change

4. Review Vocabulary ► Motion- a change in position ► Velocity-the speed and direction of a moving object ► Inertia- is the tendency of an object to resist any change in its motion

5. What is force? ► Force -is a push or a pull When you throw a ball you expert a force on the ball. After the ball leaves your hand, gravity exerts a force on the ball bringing it to the ground. When the ball hits the ground, the ground exerts a force on the ball bringing it to a stop.

6. Examples of Forces ► Magnetic Force ► Gravitational Force ► Force of a bat on a baseball Can you give examples of force?

7. Combining Forces ► More than one force can act on an object ► If you hold a magnet near a paper clip, you, the magnet, and gravity all exert forces on the paper clip. ► Net Force- is the combination of all forces acting on an object. It determines how the object moves. ► If motion of an object changes, then its velocity changes.

8. Balanced Forces ► Balanced Forces- two or more forces exerted on an object cancel each other out and they do not change the objects velocity. ► Net Forces is zero.

9. Unbalanced Forces ► Unbalanced Forces- If the effects of the forces do not cancel each other out and the object changes velocity, then the forces are unbalanced. ► Net forces is then the difference between the forces.

10. Newton ’ s First Law ► A force does not need to be continually applied for an object to keep moving, giving it a net force of zero. ► The 1 st Law If the net force acting on an object is zero, then the object remains at rest, or if the object is already moving, it continues to move in a straight line with constant speed.

11. Newton ’ s 1 st Law-Simplified

12. Friction ► Friction - is the force than brings nearly everything to a stop. ► It is the force that acts to resist sliding between two touching surfaces ► Friction also acts on objects that are sliding or moving through substances such as air or water.

13. Examples of Friction

14. Friction (cont.) ► To keep an object moving, a force must be applied to overcome the effects of friction. ► If friction is removed, an object will continue to move in a straight line at a constant speed. ► What effect does the air have on the puck in the game below? ► How would the puck move if there were no layer of air?

15. Types of Friction ► Sliding- the force between two touching surfaces. The bonds between the two surfaces are broken and form again as the surfaces pass each other.

16. Types of Friction (cont.) ► Static- the force that prevents an object from moving when force is applied and is caused by the attraction between the atoms on the two surfaces that are in contact. Example is moving a refrigerator.

17. Types of Friction (cont.) ► Rolling - occurs when an object rolls across a surface. ► The amount of force when rolling across a surface is much less than the force needed when sliding between two surfaces.

18. Test Yourself ► What do the different forms of friction have in common? They all slow an object down!

19. What is the difference between static friction and sliding friction? ► Static friction prevents two objects at rest from moving past one another. ► Sliding friction slows down two objects moving against each other

20. Test Yourself ► What type of friction is it? A wagon carrying a small child Rolling A basketball player slipping of a wet floor Sliding A box at rest on a ramp static

21. Test Yourself Explain whether the forces involved are balanced or unbalanced? You push a box until it moves Unbalanced, the box starts moving You push a box but it does not move Balanced, the box does not move You stop pushing a box and it slows down Unbalanced, the box slows down

22. Test Yourself What two factors cause static friction between two surfaces to increase? The surface is rougher or the object is heavier Can a moving object have a net force of zero? Yes if an object is already moving, it will continue at constant velocity until acted upon by an outside force