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Newton's Laws of Motion Dr. Robert MacKay Clark College, Physics.

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1 Newton's Laws of Motion Dr. Robert MacKay Clark College, Physics

2 Introduction Newtons 3 laws of motion 1. Law of inertia 2. Net Force = mass x acceleration ( F = M A ) 3. Action Reaction Newtons Universal Law of Gravity

3 Isaac Newton 1689 Isaac Newton 1702 Isaac Newton 1726 Isaac Newton Knighted by Queen Anne 1705

4 Other topics Why do objects accelerate? Why do objects not accelerate? Forces in Balance (Equilibrium) Forces out of Balance Friction Air resistance Terminal Velocity

5 Law of inertia (1st Law) Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. acceleration = 0.0 unless the objected is acted on by an unbalanced force

6 Law of inertia (1st Law) Inertia (The intrinsic tendency of an object to resist changes in motion) Mass is a measure of an objects inertia Mass is also a measure of the amount of an objects matter content. (i.e. protons, neutrons, and electrons) Weight is the force upon an object due to gravity

7 Newtons 2nd Law Net Force = Mass x Acceleration F = M A

8 Newtons Law of Action Reaction (3rd Law) You can not touch without being touched For every action force there is and equal and oppositely directed reaction force

9 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M=2.0 kgF=? A= 6.0 m/s 2 F=M A = 2.0 kg x 6.0 m/s 2 =12.0 Newtons = 12.0 N

10 An object experiences a net force and exhibits an acceleration in response. Which of the following statements is always true? (a) The object moves in the direction of the force. (b) The acceleration is in the same direction as the velocity. (c) The acceleration is in the same direction as the force. (d) The velocity of the object increases.

11 An object experiences a net force and exhibits an acceleration in response. Which of the following statements is always true? (a) The object moves in the direction of the force. (b) The acceleration is in the same direction as the velocity. X(c) The acceleration is in the same direction as the force. (d) The velocity of the object increases.

12 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M=2.0 kgF=6.0 N A= ? A=F / M = ? m/s 2

13 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M=2.0 kgF=6.0 N A= 3.0 m/s 2 A=F / M = 3.0 m/s 2

14 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M=?F= 10.0 N A= 20.0 m/s 2 M= F/A = ? kg

15 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M=?F= 10.0 N A= 20.0 m/s 2 M= F/A = 0.50 kg

16 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 8.0 kgF= ? N A= 10.0 m/s 2 F=M A = 80.0 N

17 Newtons 2nd Law Net Force = Mass x Acceleration F = M A F= ? N F= weight A= 10.0 m/s 2 F=M A = ? N M= 8.0 kg

18 Newtons 2nd Law Net Force = Mass x Acceleration F = M A F= 80 N F= weight A= 10.0 m/s 2 F=M A = 80 N M= 8.0 kg

19 Weight W = m g g = 10 m/s 2 weight is the force due to the gravitational attraction between a body and its planet W= ? m= 6.0 kg

20 Weight W = m g g = 10 m/s 2 weight is the force due to the gravitational attraction between a body and its planet W= 60N m= 6.0 kg

21 Question 1: A force of 45 N pushes horizontally on a 15 kg crate resting on a level frictionless surface. (Actually the crate has real good tiny wheels) What is the acceleration of the crate? m/s/s m/s/s m/s/s m/s/s

22 Question 1: A force of 45 N pushes horizontally on a 15 kg crate resting on a level frictionless surface. (Actually the crate has real good tiny wheels) What is the acceleration of the crate? 1.X 3.0 m/s/s m/s/s m/s/s m/s/s

23 Question 2: A 15.0 kg crate is in contact with a 30.0 kg crate on a level frictionless surface as shown. If the 15.0 kg mass is pushed with a force of 45.0 N what is the acceleration of the two masses? A. 1.0 m/s/s B. 1.5 m/s/s C. 2.0 m/s/s D. 3.0 m/s/s

24 Question 2: A 15.0 kg crate is in contact with a 30.0 kg crate on a level frictionless surface as shown. If the 15.0 kg mass is pushed with a force of 45.0 N what is the acceleration of the two masses? A. 1.0 m/s/s B. 1.5 m/s/s C. 2.0 m/s/s D. 3.0 m/s/s

25 Question 3: A 15.0 kg crate is in contact with a 30.0 kg crate on a level frictionless surface as shown. If the 15.0 kg mass is pushed with a force of 45.0 N what is the force that the 15.0 kg mass exerts on the 30.0 kg mass? N N N N

26 Question 3: A 15.0 kg crate is in contact with a 30.0 kg crate on a level frictionless surface as shown. If the 15.0 kg mass is pushed with a force of 45.0 N what is the force that the 15.0 kg mass exerts on the 30.0 kg mass? N N N N The contact force is what gives the 30 kg crate its acceleration of 1 m/s/s. Thus this contact force must be equal to 30kg(1 m/s/s)=30 N

27 Weight W = m g W= ? 8.0 kg

28 Weight W = m g W= 80 N 8.0 kg

29 A baseball of mass m is thrown upward with some initial speed. A gravitational force is exerted on the ball (a) at all points in its motion (b) at all points in its motion except at the highest point (c) at no points in its motion

30 A baseball of mass m is thrown upward with some initial speed. A gravitational force is exerted on the ball (a) at all points in its motion (b) at all points in its motion except at the highest point (c) at no points in its motion

31 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 5.0 kg F= N A= ? m/s 2 Net Force = ? D =120.0 N

32 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 5.0 kg F= N 2. A= 6.0 m/s 2 1. Net Force = 30.0 N D =120.0 N

33 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 5.0 kg F= N A= 20.0 m/s 2 1.Net Force =?? 2. D=??

34 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 5.0 kg F= N A= 20.0 m/s 2 1.Net Force = 100N 2.Using MA 2. D=50 N

35 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 5.0 kg F= N A= 0.0 m/s 2 Net Force = ? D=?

36 Newtons 2nd Law Net Force = Mass x Acceleration F = M A M= 5.0 kg F= N A= 0.0 m/s 2 Net Force = 0.0 D=150.0 N

37 Which of the following statements is most correct? (a) It is possible for an object to have motion in the absence of forces on the object. (b) It is possible to have forces on an object in the absence of motion of the object. (c) Neither (a) nor (b) is correct. (d) Both (a) and (b) are correct.

38 Which of the following statements is most correct? (a) It is possible for an object to have motion in the absence of forces on the object. (b) It is possible to have forces on an object in the absence of motion of the object. (c) Neither (a) nor (b) is correct. (d) Both (a) and (b) are correct.

39 An object experiences no acceleration. Which of the following cannot be true for the object? (a) A single force acts on the object. (b) No forces act on the object. (c) Forces act on the object, but the forces cancel.

40 An object experiences no acceleration. Which of the following cannot be true for the object? (a) A single force acts on the object. (b) No forces act on the object. (c) Forces act on the object, but the forces cancel.

41 On Earth, where gravity is present, an experiment is performed on a puck on an air hockey table, with negligible friction. A constant horizontal force is applied to the puck and its acceleration is measured. The experiment is performed on the same puck in the far reaches of outer space where both friction and gravity are negligible. The same constant force is applied to the puck and its acceleration is measured. The pucks acceleration in outer space will be a) greater than its acceleration on Earth, b) less than its acceleration on Earth, c) exactly the same as its acceleration on Earth, d) infinite since neither friction nor gravity are holding it back? (end of section 5.5) QUICK QUIZ 5.3

42 On Earth, where gravity is present, an experiment is performed on a puck on an air hockey table, with negligible friction. A constant horizontal force is applied to the puck and its acceleration is measured. The experiment is performed on the same puck in the far reaches of outer space where both friction and gravity are negligible. The same constant force is applied to the puck and its acceleration is measured. The pucks acceleration in outer space will be a) greater than its acceleration on Earth, b) less than its acceleration on Earth, c) exactly the same as its acceleration on Earth, d) infinite since neither friction nor gravity are holding it back? (end of section 5.5) QUICK QUIZ 5.3

43 Newtons 2nd Law Friction depends on surfaces in contact (roughness) contact force pushing surfaces together M= 5.0 F= N A= 0.0 m/s 2 Net Force = ? Friction ?

44 f s,max Applied Force=Static frictional force F=fs Static friction kinetic friction F f s,max = s N f K = K N (sliding friction) f K = K N F net =P-f K P f

45

46 Air ResistanceForce Depends on: velocity Air density Shape and aerodynamics of object

47 Terminal Velocity When air resistance force balances an objects weight w Air Drag Acceleration= 0.0 ===> Terminal velocity

48 Terminal Velocity w Air Drag Acceleration = 0.0 ===>Terminal velocity w Air Drag 80 kg10 kg which has the greatest force of air resistance?

49 Not Terminal Velocity Acceleration = ? W = ? Air Drag = 240N 80 kg which has the greatest force of air resistance?

50 Not Terminal Velocity Acceleration = ? W = 800 N Air Drag = 240N 80 kg +x

51 Not Terminal Velocity Acceleration = ? W = 800 N Air Drag = 240N 80 kg +x F = m a +800N - 240N= 80kg a +560N=80kg a a=7.0 m/s2 Down

52 1.A falling elephant, or 2.A falling feather? Which encounters the greater force of air resistance

53 1. A falling elephant, or 2.A falling feather? Which encounters the greater force of air resistance

54 Two smooth balls of exactly the same size, one made of wood and the other of iron, are dropped from a high building to the ground below. The ball to encounter the greater force of air resistance on the way down is 1. the wooden ball. 2. the iron ball. 3. Neither. The force is the same.

55 Two smooth balls of exactly the same size, one made of wood and the other of iron, are dropped from a high building to the ground below. The ball to encounter the greater force of air resistance on the way down is 1. the wooden ball. 2. the iron ball. 3. Neither. The force is the same.

56 1. 10 m/s/s down. 2. zero 3. increasing Just after jumping off a bridge, her acceleration is

57 1. 10 m/s/s down. 2. zero 3. increasing Just after jumping off a bridge, her acceleration is

58 1. increases. 2. decreases. 3. remains the same. As she falls faster and faster through the air, her acceleration

59 1. increases. 2. decreases. 3. remains the same. As she falls faster and faster through the air, her acceleration

60 1. zero. 2. decreasing 3. Equal to gravity. When she reaches terminal velocity her acceleration is

61 1. zero. 2. decreasing 3. Equal to gravity. When she reaches terminal velocity her acceleration is

62 Whos falling faster? 1.Heavy 2.Light 3.Both falling at same speed

63 Whos falling faster? 1.Heavy 2.Light 3.Both falling at same speed


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