Forces and Newton’s Laws of Motion

1. Forces and Newton’s Laws of Motion

2. Force… A push or a pull. Force is not a thing in itself, but rather an interaction between two objects.

3. Examples • A Flag being blown by the wind • A jet engine propelling an airplane

4. Fact or Fiction? • Motion at a constant velocity requires a force. • If you apply a constant force to an object, it will move with a constant velocity. • If no forces act on it, a moving object will eventually stop. • Inertia is a force.

5. Forces • Push or a pull • Vector quantity • Contact forces- arise from physical contact with each other • Field (Noncontact) forces- force exerted even when not in direct contact (ex. Gravity, electrical force, magnetic forces)

6. Consider the tow truck at the right. If the tensional force in the cable is 1000 N and if the cable makes a 60-degree angle with the horizontal, then what is the vertical component of force which lifts the car off the ground? 866 N. and coming soon.... Forces are Vectors too!

7. Weight VS Mass http://www.pioneernet.net/curtis/wile_e/inline/wile_gravity.gif

8. Mass • the amount of matter in an object • a constant property • a measure of the inertia • measured in kilograms (kg)

9. Weight • the force upon an object due to gravity • weight = mass  accel. due to gravity Fw = mg • measured in Newtons (N)

10. The weight of a 10 kg brick is... • A) 98 N • B) 10 kg • C) 9.8 kg • D) 10 N • E) 98 kg

11. Mass and Weight • On the Moon, the force of gravity is only 1/6 as strong as on the Earth. • In space you are practically weightless but your mass remains unchanged. • Your mass does not depend on where your are. • e.g. Earth, Moon, or space

12. 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

13. Balanced Forces (zero net)

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

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

16. Equilibrium- net force is equal to zero

17. 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

18. 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

19. 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 actually 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.

20. 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: when two surfaces slide one over the other • Rolling Friction: when two surfaces slide one over the other

21. Science Friction • Friction (Ff)- contact force between two surfaces that always opposes motion • 2 types • Static friction (fs) = keeps an object from moving (must be overcome to move an object at rest • Kinetic friction (fk) =acts when an object is moving • Kinetic friction is weaker than static friction Ffk< Ffs

22. Friction is not always a bad thing! • Walking! Your foot pushes backwards and friction opposes it by pushing it forward

23. Static (starting) Friction • The force that opposes the start of the motion. • Static means stationary ( not moving).

24. Kinetic (sliding) Friction • The force between surfaces in relative motion • For the same object, why is the force of kinetic friction less than the force of starting friction?

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. 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. .

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

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

29. 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 higher normal force.

30. A book is at rest on a table

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

32. An egg is free-falling from a nest in a tree. Neglect air resistance.

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

34. 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.

35. Real Life Application

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

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

38. Fish in an Elevator

39. Upward Acceleration • Fnet = FT – FW or • Fnet = FT – mg • Fnet = FT - mg = ma • FT = ma+ mg • FT = m (g + a)

40. Downward Acceleration • Fnet = FT - Fw or • Fnet = FT – mg • Fnet = FT - mg = m ( - a ) • FT = mg - ma • FT = m (g - a)

41. Roller Coaster http://science.howstuffworks.com/roller-coaster-safety-harness-broke.htm

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

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

44. 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.

45. Objects at Rest Remain at Rest http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html

46. Objects in Motion Remain Motion, unless… http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html

47. Acted Upon by a NET Force http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html

48. Newton’s 1st Law of Motion http://quest.nasa.gov/space/teachers/rockets/images/hand_and_ball.gif

49. Newtons’ Second Law • The accel. of an object is directly proportional to the net force acting on the object, and inversely proportional to the mass of the object.

50. Newton’s Second law of Motion …mathematically Net Force = (mass)(accel) Fnet = ma