1. Dr. Joseph W. Howard ©Spring 2008 Motion So Far…. Distance vs. Displacement Speed vs. Velocity Average Speed Average Velocity Acceleration –“Speeding Up” –“Slowing Down” Distance vs. Displacement Speed vs. Velocity Average Speed Average Velocity Acceleration –“Speeding Up” –“Slowing Down”

2. Dr. Joseph W. Howard ©Spring 2008 Why do objects move?

3. Dr. Joseph W. Howard ©Spring 2008 Let’s start off by observing some various situations

4. Dr. Joseph W. Howard ©Spring 2008 What can we say? Net Force to START motion Net Force to STOP motion Net Force to CHANGE motion, not to continue the motion

5. Dr. Joseph W. Howard ©Spring 2008 Forces and Newton’s Laws of Motion But what ARE FORCES?

6. Dr. Joseph W. Howard ©Spring 2008 Newton’s First Law Objects at rest stay at rest unless a “net force” is applied. Objects in uniform straight line motion stay in straight line motion unless a “net force” is applied. Note: Newton’s First Law Does NOT say… This law does not say that every moving object has a “force” acting on it! This law does not say an object at rest has “no force(s)” acting on it!

7. Dr. Joseph W. Howard ©Spring 2008 What does a “net Force” cause? What happens when you “force” a change on an object? “change in velocity!” Acceleration!!! ? ? But what is it proportional too?

8. Dr. Joseph W. Howard ©Spring 2008 Forces cause Acceleration Is it always the same? Same force? Same Acceleration? Objects “resist” a change…….why? Inertia How do we measure interia? Mass

9. Dr. Joseph W. Howard ©Spring 2008 Mass Mass – The quantity of matter ( n an o s c a l e stuff!) in an object is a measure of an object’s inertia.

10. Dr. Joseph W. Howard ©Spring 2008 Putting it all together Net FORCE to cause a “change” in motion. (stop, start, change direction, etc…. Any change!) Net Force causes an ACCELERATION How much acceleration depends on the object’s MASS

11. Dr. Joseph W. Howard ©Spring 2008 Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. a = F net /m OR F net = ma Units of Force? Newtons Units of Force? Newtons

12. Dr. Joseph W. Howard ©Spring 2008 The More Mass Something Has, the Harder it is to Get A Significant Change in Motion. Applied Force Resulting Velocity Resulting Velocity Applied Force Resulting Velocity Resulting Velocity

13. Dr. Joseph W. Howard ©Spring 200815N15N 6N6N 6N6N More on “net Forces” 2kg 6N6N a = F/m = (6N)/(2kg) = 3 m/s 2 to the right 2kg a = F/m = (6N – 6N)/(2kg) = 0 m/s 2 7N 2kg a = F/m = (15N – 7N)/(2kg) = 4 m/s 2 to the right

14. Dr. Joseph W. Howard ©Spring 2008 If There are More Than Two Forces, They are Added Together as Vectors Force 1 Force 2 Net Force of Force 1 and Force 2 The resultant force is the net force acting on the object. The resultant force is the net force acting on the object. Mass

15. Dr. Joseph W. Howard ©Spring 2008 Friction Friction is a force! It “compels” objects to “change” their motion. In fact, FRICTION always opposes motion.

16. Dr. Joseph W. Howard ©Spring 2008 10 kg 1 m/s A 10 kg box is being pushed across the floor at a constant velocity of 1 m/s. What is the net force acting on the box? A 10 kg box is being pushed across the floor at a constant velocity of 1 m/s. What is the net force acting on the box? Conceptual Pitfall Net Force = ZERO

17. Dr. Joseph W. Howard ©Spring 2008 Conceptual Pitfall A bus accelerates by the engine applying a force of 2000N East to the tires for 1 minute. Suddenly the bus encounters a strong wind with a frictional air resistance force of 2000N West. Which of the following happens? A bus accelerates by the engine applying a force of 2000N East to the tires for 1 minute. Suddenly the bus encounters a strong wind with a frictional air resistance force of 2000N West. Which of the following happens? The bus will continue to gain speed The bus has no net force, therefore it stops because of the wind The bus will continue to move with a constant speed The bus will slow down because there is no acceleration The bus will continue to gain speed The bus has no net force, therefore it stops because of the wind The bus will continue to move with a constant speed The bus will slow down because there is no acceleration The bus will continue to move with a constant speed The bus will continue to move with a constant speed

18. Dr. Joseph W. Howard ©Spring 2008 Newton’s First Law Objects at rest stay at rest unless a “net force” is applied. Objects in uniform straight line motion stay in straight line motion unless a “net force” is applied.

19. Dr. Joseph W. Howard ©Spring 2008 Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. a = F net / m OR F net = ma

20. Dr. Joseph W. Howard ©Spring 2008 Newton’s Third Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. “To every action there is an equal and opposite re- action.”

21. Dr. Joseph W. Howard ©Spring 2008 Newton’s 3 rd in Action The force the person exerts on the heavier boulder is equal in magnitude but opposite in direction to the force the boulder exerts on the person. EVEN IF THE BOULDER IS BEING PUSHED UPHILL !! ! The force the person exerts on the heavier boulder is equal in magnitude but opposite in direction to the force the boulder exerts on the person. EVEN IF THE BOULDER IS BEING PUSHED UPHILL !! !

22. Dr. Joseph W. Howard ©Spring 2008 Wait just a moment... I said Weight! What is weight ? A measure of the Force of gravity acting on an object. Weight = (mass)  (acceleration of gravity) Weight = (m)  (9.8 m/s 2 ) Weight = (m)  (g)

23. Dr. Joseph W. Howard ©Spring 2008 Wait just a moment... Weight = (mass)  (acceleration of gravity) Weight = (m)  (g) Heavy Object? More force of gravity Lite Object? less force of gravity BUT!!! Each has same acceleration of gravity (9.8m/s 2 down)