Newton's Laws of Motion

1. Newton's Laws of Motion

2. Essential Question • How can I explain the causes of motion on an object? • Learning Objectives: Laws of Motion, Friction, Force

3. Background Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in 1687. Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects on the scale we experience in our everyday lives.

5. Newton’s Laws of Motion 1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2nd Law – Force equals mass times acceleration. 3rd Law – For every action there is an equal and opposite reaction.

6. Newton's Laws of Motion First Law: An object at rest stays at rest or an object in motion, stays in motion (in the same direction/at the same speed) unless acted upon by an unbalanced force • Also called the law of inertia

7. Inertia • A property of matter • The tendency of an object to resist any change in its motion • The greater the mass the greater the inertia • The greater the speed the greater the inertia

8. Examples of Newton’s 1st Law a) car suddenly stops and you strain against the seat belt b) when riding a horse, the horse suddenly stops and you fly over its head c) the magician pulls the tablecloth out from under a table full of dishes d) the difficulty of pushing a dead car e) lawn bowling on a cut and rolled lawn verses an uncut lawn appear f) car turns left and you to slide to the right

9. Examples of Newton’s 1st Law

10. 1st Law Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

11. 1st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

13. Newton’s First Law • Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.

14. Newton’s First Law • Newton’s First Law of Motion • “Law of Inertia” • Inertia • tendency of an object to resist any change in its motion • increases as mass increases

15. Some Examples from Real Life A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion. Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

16. More Examples from Real Life A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them. On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

17. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

18. Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction.

19. There are four main types of friction: Sliding friction: ice skating Rolling friction: bowling Fluid friction (air or liquid): air or water resistance Static friction: initial friction when moving an object What is this unbalanced force that acts on an object in motion? Friction!

20. Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

21. What does this mean? Review… Basically, an object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

22. Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

23. 2nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.

24. Newton's Laws of Motion Second law: The greater the force applied to an object, the more the object will accelerate. It takes more force to accelerate an object with a lot of mass than to accelerate something with very little mass. The player in black had more acceleration thus he hit with a greater amount of force

25. 2nd Law F = m x a What is Force? It is any push or pull on an object!!

27. Newton's Laws of Motion Second law: • The greater the force, the greater the acceleration • The greater the mass, the greater the force needed for the same acceleration • Calculated by: F = ma • (F = force, m = mass, a = acceleration)

28. Examples of Newton’s 2nd Law a) hitting a baseball, the harder the hit, the faster the ball goes b) accelerating or decelerating a car c) The positioning of football players - massive players on the line with lighter (faster to accelerate) players in the backfield d) a loaded versus an unloaded truck

29. Examples of Newton’s 2nd Law

30. Newton’s Second Law • Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma

31. Newton’s Second Law Force equals mass times acceleration. F = ma Acceleration: a measurement of how quickly an object is changing speed.

32. 2nd Law When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N). One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

33. 2nd Law (F = m x a) How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? Write the formula F = m x a Fill in given numbers and units F = 1400 kg x 2 meters per second/second Solve for the unknown 2800 kg-meters/second/second or2800 N

34. Newton’s 2nd Lawproves that different masses accelerate to the earth at the same rate, but with different forces. We know that objects with different masses accelerate to the ground at the same rate. However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

35. ConceptTest • Is the following statement true or false? • An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. • False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.

36. Check Your Understanding 1. What acceleration will result when a 12 N net force applied to a 3 kg object? A 6 kg object? 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass. 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

37. Newton's Laws of Motion third law: For every action force, there is an equal and oppositereaction force. (Forces are always paired)

38. 3rd Law According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

40. Newton’s 3rd Law in Nature Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

41. 3rd Law Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

42. Examples of Newton’s 3rd Law • rockets leaving earth • guns being fired • c) two cars hit head on • d) astronauts in space • e) pool or billiards • f) jumping out of a boat onto • the dock • g) sprinklers rotating

43. Examples of Newton’s 3rd Law Newton’s third law: "For every action, there is an equal and opposite reaction." When you fire a gun you feel the recoil. Some of the funniest things in cartoons follow physics that have been exaggerated or just plain ignored. Wyle Coyote hangs suspended in space over that canyon for a lot longer than an object would in reality, but it is the anticipation of the drop and Wyle's facial recognition of the upcoming pain that is so classically cartooney. So some laws are stretched for comical effect.

44. Other examples of Newton’s Third Law The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

45. 3rd Law Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

46. Examples of Newton’s 3rd Law

47. Newton’s Third Law • Newton’s Third Law of Motion • When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

48. NO!!! Newton’s Third Law • Problem: • How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force? • Aren’t these “balanced forces” resulting in no acceleration?

49. Newton’s Third Law • Explanation: • forces are equal and opposite but act on different objects • they are not “balanced forces” • the movement of the horse depends on the forces acting on the horse

50. Newton’s Third Law • Action-Reaction Pairs • The hammer exerts a force on the nail to the right. • The nail exerts an equal but opposite force on the hammer to the left.