Forces and Newton’s Laws of Motion

1. Forces and Newton’s Laws of Motion

2. Tension • Tension (T)- force that acts through a solid object such as a rope or chain; directed along the rope and pulls equally on the objects on either end. .

3. Normal Force • Normal Force (Fn)- contact force that always acts perpendicular to the surfaces that are touching

4. Free-Body Diagram (FBD) • Free-body diagrams are drawings used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

5. Basic Force Diagrams A Car on a Level Surface All forces on the car are vertical, so no horizontal force can be generated. A Car on a Banked Turn The normal force on the car due to the road is no longer vertical, so a component of the normal force acts in the horizontal direction. • Normal force (N) - contact pressure that pushes surfaces together. • It is easier to drag a light chair across a room than a heavy table. • This is because the weight of the table exerts a greater normal force.

6. FBD: A book is at rest on a table

7. A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.

8. An egg is free-falling towards the ground. Neglect air resistance.

9. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance.

10. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance.

11. Real Life Application

12. More real life… http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html

13. Another Force Diagram… The chandelier is separated from the chain for interpretation purposes only! http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html

14. Bring in the angles-a bit more complex  http://www.ux1.eiu.edu/~cfadd/1150/04Nwtn/appl.html

15. Box on an Incline http://img.sparknotes.com/figures/1/174f9bfb5307940e1bb1c2ddac88fd8d/problem1_5.gif

16. Net Force Balanced = No Net Force = Fnet = 0 N • Determined by combining ALL forces acting on an object. • Zero net force = zero acceleration • If there is a net force, there will be an acceleration. Unbalanced = Net Force = Fnet≠ 0 N http://4.bp.blogspot.com/_fDHmeCjB-bk/SS4hRNbLwaI/AAAAAAAAAHw/gx0p9HIDS1o/s1600-h/12.gif

17. Balanced Forces (zero net) www.physicsclassroom.com

18. Unbalanced Forces (non-zero net) Imagine pushing your text book so that it would slide across the desk… www.physicsclassroom.com

19. Equilibrium • Balance • In regards to motion an object has constant velocity if it is in equilibrium. • NO acceleration. • Types: • Static Equilibrium • Dynamic Equilibrium

20. Static Equilibrium Scales pushing up Velocity is zero Examples: Weighing yourself on a set of scales Weight down Car parked on an incline Normal Friction Weight down

21. Dynamic Equilibrium Velocity is nonzero and constant Examples: Driving at constant velocity Normal up Friction Force from road Air resistance Weight down Terminal velocity when parachuting Weight down

22. Friction • The force that opposes the motion between two surfaces that are in contact. • Friction is the "evil monster" of all motion. Regardless of which direction something moves in, friction pulls it the other way. • Move something left, friction pulls right. Move something up, friction pulls down. • It appears as if nature has given us friction to stop us from moving anything.  • Friction is a force that “appears” when there is relative motion between two objects. • Although two objects might look smooth, microscopically, they are very rough and jagged.

23. Friction • A force that opposes motion. Friction acts in a direction opposite to the object's direction in motion. Without friction, the object would continue to move at a constant speed forever • Static Friction: when object is at rest • Kinetic Friction: when object is moving • Sliding Friction: • Rolling Friction:

24. Friction is not always a bad thing! • Walking! Your foot pushes backwards on the ground and friction opposes the push of your foot by pushing your foot forward

25. Coefficient of Friction • A constant that depends on the two surfaces in contact • Ff = FN • (mu)….coefficient of friction Coefficient of Friction - Friction on an Incline

26. Newton’s First Law (1642-1727) • “The Law of Inertia” • A body remains at rest or moves in a straight line at a constant speed unless acted upon by a net force. • Objects do not accelerate unless a net force is applied.

27. Objects at rest remain at rest and objects in motion remain in motion, unless acted upon by an outside force. Newton’s 1st Law of Motion http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html

28. Newton’s 2nd law of Motion …mathematically Net Force = (mass)(accel) Fnet = ma

29. m m m m m M M M m NEWTON'S 2nd LAW OF MOTION a F F a F a F a F a F a

30. When the acceleration is g we have Free Fall m 2m F 2F

31. Terminal Velocity Net Force Acceleration = g Velocity = 0 but motion is about to begin mg F Acceleration < g v increasing downward mg F Acceleration << g v still increasing downward just not as rapidly as before mg F Acceleration = 0 Terminal velocity mg

32. Falling with Air Resistance & Terminal Velocity • When falling the force of air resistance becomes large enough to balance the force of gravity. • At this instant in time, there is no net force — the object stops accelerating (see D below); terminal velocity has been reached.

33. Newton’s Third Law • Action-Reaction • When one object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction on the first object. Example of Newton’s 3rd Law: http://bp3.blogger.com/_vQA3QRUnk3M/RrgyfrRniPI/AAAAAAAAAMs/DYlhM7pDeI8/s1600-h/newton

34. Newton’s Third Law of Motion For every action, there is always a reaction of equal (magnitude) and opposite (direction) reaction. “action” or “reaction” refers to force. Action/Reaction forces do NOT act on the SAME object!

35. Reaction: road pushes on tire Action: tire pushes on road

36. Reaction: gases push on rocket Action: rocket pushes on gases

37. Action- Reaction Forces • Do Action-Reaction forces cancel each other? • No, they are acting on different objects. Forces can only be added together when they are acting on the same object. http://quest.nasa.gov/space/teachers/rockets/principles.html

38. Newton’s 3rd Law of Motion Action Force: A guy is pushing a ball leftwards Reaction Force: the ball is pushing the guy rightwards

39. Tug-a-war • If Fido and Rover play tug-a war, how do the “pulls” of the dogs compare? • If each dog pulls with 50 N of force, what is the tension force in the middle of the rope (between the dogs)?

40. While driving, Mrs. Ingle observed a bug striking the windshield of her car. The bug hits the windshield and the windshield hits the bug. Which of the two forces is greater: the force on the bug or the force on the windshield? EXPLAIN!

41. Putting Newton's Laws of Motion Together • An unbalanced force must be exerted for a rocket to lift off from a launch pad or for a craft in space to change speed or direction (First Law). • The amount of thrust (force) produced by a rocket engine will be determined by the rate at which the mass of the rocket fuel burns and the speed of the gas escaping the rocket (Second Law). • The reaction, or motion, of the rocket is equal to and in the opposite direction of the action, or thrust, from the engine (Third Law). http://www.spacetoday.org/images/Rockets/ArianeRockets/Ariane5LaunchArianespace.jpg http://quest.nasa.gov/space/teachers/rockets/principles.html