1. The Laws of Motion
Physics 2053
Lecture Notes
The Laws of Motion

2. The Laws of Motion Topics 4-01 Force 4-02 Newton’s First Law
4-03 Newton’s Second Law
4-04 Newton’s Third Law
4-05 Applications of Newton’s Laws
4-06 Forces of Friction
The Laws of Motion

3. Gravitational Unlimited
Force
Types Range
Size
100
106
1020
1035
Gravitational Unlimited
Electromagnetic Unlimited
Weak Nuclear  m
Strong Nuclear  m
The Laws of Motion

4. The magnitude of a force can be measured using a spring scale.
Newton’s First Law
A force is a push or pull.
An object at rest needs a force to get it moving; a moving object needs a force to change its velocity.
The magnitude of a force can be measured using a spring scale.
The Laws of Motion

5. Newton’s first law is often called the law of inertia.
Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.
If no external force acts
The Laws of Motion

6. When you sit on a chair, the resultant force on you is A) zero. B) up.
Newton’s First Law
When you sit on a chair, the resultant force on you is
A) zero.
B) up.
C) down.
D) depending on your weight.
The Laws of Motion

7. Force is a vector, so SF = ma is true along each coordinate axis.
Newton’s Second Law
Newton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass.
Force is a vector, so SF = ma is true along each coordinate axis.
Units of Force
System Mass Acceleration Force
SI kg m/s N = kg m/s2
British slug ft/s lb = slug ft/s2
The Laws of Motion

8. Newton’s Second Law Reading on scale
A man stands on a scale
inside a stationary elevator.
Forces acting on the man N mg
Reading
on scale
The Laws of Motion

9. Newton’s Second Law Reading on scale
When Moving Upward With
Constant Velocity
Forces acting on the man v N mg
Reading
on scale
The Laws of Motion

10. Newton’s Second Law Reading on scale
When Moving Upward With
Constant Acceleration
Forces acting on the man a N mg
Reading
on scale
The Laws of Motion

11. Newton’s Second Law Reading on scale
When Moving Downward With
Constant Acceleration
Forces acting on the man a N mg
Reading
on scale
The Laws of Motion

12. A constant net force acts on an object. Describe the
Newton’s Second Law
A constant net force acts on an object. Describe the
motion of the object.
A) constant acceleration
B) constant speed
C) constant velocity
D) increasing acceleration
The Laws of Motion

13. Newton’s Second Law
A constant force F acts on a block of mass m. which is initially at rest. Find the velocity of the block after time Dt.
vo = 0
Dt = 5 s F
v = ?
F = 20 N m
m = 5 kg
The Laws of Motion

14. Newton’s Second Law (Problem)
What average force is required to stop an 1100 kg car in 8.0 s if the car is travelling at 95 km/h? F
Newton’s 2nd Law
The Laws of Motion

15. A net force F accelerates a mass m with an acceleration a.
Newton’s Second Law
A net force F accelerates a mass m with an acceleration a.
If the same net force is applied to mass 2m, then the
acceleration will be
A) 4a.
B) 2a.
C) a/2
D) a/4
The Laws of Motion

16. Newton’s Second Law (Problem)
The cable supporting a 2,125 kg elevator has a maximum
strength of 21,750 N. What maximum upward
acceleration can it give the elevator without breaking?
Newton’s 2nd Law
Tmax a m mg
The Laws of Motion

17. Newton’s Second Law (Problem)
How much tension must a rope withstand if it is used to
accelerate a 1200 kg car vertically upward at 0.80 m/s2.
Newton’s 2nd Law a
The Laws of Motion

18. Universal Gravitational Constant
Newton’s Second Law
Gravitational Force:
Gravitational Force is the mutual force of attraction between any two objects in the Universe. m F R M
Universal Gravitational Constant
The Laws of Motion

19. The gravitational force between two objects is proportional to
Newton’s Second Law
The gravitational force between two objects is
proportional to
A) the distance between the two objects.
B) the square of the distance between the two objects.
C) the product of each objects mass.
D) the square of the product of each objects mass.
The Laws of Motion

20. Two objects attract each other gravitationally. If the
Newton’s Second Law
Two objects attract each other gravitationally. If the
distance between their centers is cut in half, the
gravitational force
A) is cut to one fourth.
B) is cut in half.
C) doubles.
D) quadruples
The Laws of Motion

21. Two objects, with masses m1 and m2, are originally a
Newton’s Second Law
Two objects, with masses m1 and m2, are originally a
distance r apart. The magnitude of the gravitational force
between them is F. The masses are changed to 2m1 and
2m2, and the distance is changed to 4r. What is the
magnitude of the new gravitational force?
A) F/16
B) F/4
C) 16F
D) 4F
The Laws of Motion

22. Newton’s Second Law
“g” in terms of G g m F R M
The Laws of Motion

23. Newton’s Second Law
Mass is the measure of inertia of an object. In the SI system, mass is measured in kilograms.
Mass is not weight:
Mass is a property of an object. Weight is the force exerted on that object by gravity.
If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.
Gravitational mass mg
mg = mi
Inertial mass mi
The Laws of Motion

24. Newton’s Second Law
Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight is: m
Weight = mg
The Laws of Motion

25. A) both measure the same thing. B) are exactly equal.
Newton’s Second Law
Mass and weight
A) both measure the same thing.
B) are exactly equal.
C) are two different quantities.
D) are both measured in kilograms.
The Laws of Motion

26. A stone is thrown straight up. At the top of its path, the
Newton’s Second Law
A stone is thrown straight up. At the top of its path, the
net force acting on it is
A) equal to its weight.
B) greater than its weight.
C) greater than zero, but less than its weight.
D) instantaneously equal to zero.
The Laws of Motion

27. Action/Reaction Forces
Newton’s Third Law
Force block exerts
downward on table top
Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. F1
Force table exerts
upward on block F2
Action/Reaction Forces
The Laws of Motion

28. Newton’s Third Law
Force block exerts
downward on table top N mg
An object at rest must have no net force on it. If it is sitting on a table, the object exerts a downward force mg on the surface of the table. m
The surface of the table exerts an upward force on the block, called the normal force. It is exactly as large as needed to balance the force from the object.
Force table exerts
upward on block
The Laws of Motion

29. If an upward force F is applied to the block, the magnitude of
Newton’s Third Law
Force block exerts
downward on table top F
If an upward force F is applied
to the block, the magnitude of
the normal force is N mg
Force table exerts
upward on block
The Laws of Motion

30. If a downward force F is applied to the block, the magnitude of
Newton’s Third Law
Force block exerts
downward on table top F
If a downward force F is applied
to the block, the magnitude of
the normal force is N m mg
Force table exerts
upward on block
The Laws of Motion

31. Action-reaction forces A) sometimes act on the same object.
Newton’s Third Law
Action-reaction forces
A) sometimes act on the same object.
B) always act on the same object.
C) may be at right angles.
D) always act on different objects.
The Laws of Motion

32. A 40,000 kg truck collides with a 1500 lb car and causes
Newton’s Third Law
A 40,000 kg truck collides with a 1500 lb car and causes
a lot of damage to the car.
A) the force on the truck is greater then the force on the car.
B) the force on the truck is equal to the force on the car.
C) the force on the truck is smaller than the force on the car.
D) the truck did not slow down during the collision.
The Laws of Motion

33. A golf club hits a golf ball with a force of 2,400 N.
Newton’s Third Law
A golf club hits a golf ball with a force of 2,400 N.
The force the golf ball exerts on the club is
A) slightly less than 2400 N.
B) exactly 2400 N.
C) slightly more than 2400 N.
D) close to 0 N.
The Laws of Motion

34. Applications of Newton’s Laws
A block of mass m moving with a speed vo is brought to rest
by a constant force F. Find the distance the block moves.
vo = 20 m/s
F = -10 N
v = 0 F
m = 5 kg m Dx
Newton’s 2nd Law
The Laws of Motion

35. Solving Problems with Newton’s Laws
Find the acceleration of
the two block system
Forces on m1 N T m1
m1g T m2 a
m2g
Forces on m2
The Laws of Motion

36. Solving Problems with Newton’s Laws
Find the acceleration of
the two block system
Mass 1
Mass 2 T T m2 m1
m2g
m1g
The Laws of Motion

37. Solving Problems with Newton’s Lawsy x N
mg sin(q)
mg cos(q) mg q q
The Laws of Motion

38. A pair of fuzzy dice is hanging by a string from your
Problem
A pair of fuzzy dice is hanging by a string from your
rear-view mirror. While you are accelerating from a
stoplight to 24 m/s in 6.0 s, what angle does the string
make with the vertical?
The acceleration of the dice a
Newton’s 2nd Law
The Laws of Motion

39. Solving Problems with Newton’s Laws
Three mass system - find acceleration T1 T2 F m 2m 3m
Find the tensions in the string.
First find the acceleration of the system.
The Laws of Motion

40. Solving Problems with Newton’s Laws
Three mass system - find T2 T1 T2 F m 2m 3m
The Laws of Motion

41. Solving Problems with Newton’s Laws
Three mass system - find T1 T1 T2 F m 2m 3m
The Laws of Motion

42. Solving Problems with Newton’s Laws
Three mass system - find T1 Or T2 T1 F m 2m 3m
The Laws of Motion

43. You are standing in a moving bus, facing forward, and
Forces of Friction
You are standing in a moving bus, facing forward, and
you suddenly fall forward. You can imply from this that
the bus's
A) velocity decreased.
B) velocity increased.
C) speed remained the same, but it's turning to the right.
D) speed remained the same, but it's turning to the left.
The Laws of Motion

44. Force of Static Friction
Forces of Friction
Friction:
Force of Static Friction N F fs m mg
The Laws of Motion

45. Force of Kinetic Friction v
Forces of Friction
Friction:
Force of Kinetic Friction v N F fk m mg
The Laws of Motion

46. Coefficients of Friction
Forces of Friction
Coefficients of Friction
Steel on steel
Aluminum on steel
Copper on steel
Rubber on concrete
Wood on wood
Glass on glass
Waxed wood on wet snow
Waxed wood on dry snow
Metal on metal (lubricated)
Ice on ice
Teflon on Teflon
The Laws of Motion

47. Forces of Friction (Problem)
Suppose that you are standing on a train accelerating
at 2.0 m/s2. What minimum coefficient of static friction
must exist between your feet and the floor if you are not
to slide?
Frictional force
Newton’s 2nd Law a N mg
The Laws of Motion

48. The force that keeps you from sliding on an icy sidewalk is A) weight.
Forces of Friction
The force that keeps you from sliding on an icy sidewalk is
A) weight.
B) kinetic friction.
C) static friction.
D) normal force.
The Laws of Motion

49. Pulling a block with constant speed
Forces of Friction
Pulling a block with constant speed
Pulling a block x y N q F N f mk F q f m mg mg
The normal force
A man drags a 20 kg crate at constant speed across a floor by pulling on a rope inclined at 35 degrees above the horizontal. The tension in the rope is 40 N.
Find the magnitude of the
a) normal force exerted on the crate by the floor.
b) frictional force acting on the crate.
c) coefficient of kinetic friction between the crate and floor.
The Laws of Motion

50. Pulling a block with constant speed
Forces of Friction
Pulling a block with constant speed y N F N mk F q f q m f x mg mg
The frictional force
A man drags a 20 kg crate at constant speed across a floor by pulling on a rope inclined at 35 degrees above the horizontal. The tension in the rope is 40 N.
Find the magnitude of the
a) normal force exerted on the crate by the floor.
b) frictional force acting on the crate.
c) coefficient of kinetic friction between the crate and floor.
The Laws of Motion

51. Pulling a block with constant speed
Forces of Friction
Pulling a block with constant speed N y F N mk F q f q m f x mg mg
The coefficient of friction
A man drags a 20 kg crate at constant speed across a floor by pulling on a rope inclined at 35 degrees above the horizontal. The tension in the rope is 40 N.
Find the magnitude of the
a) normal force exerted on the crate by the floor.
b) frictional force acting on the crate.
c) coefficient of kinetic friction between the crate and floor.
The Laws of Motion

52. Forces of Friction (Problem)
The coefficient of static friction between hard rubber and
normal street pavement is about On how steep a hill
(maximum angle) can you leave a car parked? q q
The Laws of Motion

53. Forces of Friction (Problem)
Drag-race tires in contact with an asphalt surface have a
very high coefficient of static friction. Assuming a constant
acceleration and no slipping of tires, estimate the coefficient
of static friction needed for a drag racer to cover 1.0 km
in 12 s, starting from rest.
Newton’s 2nd Law
The Laws of Motion

54. A brick and a feather fall to the earth at their respective
Forces of Friction
A brick and a feather fall to the earth at their respective
terminal velocities. Which object experiences the greater
force of air friction?
A) the feather
B) the brick
C) Neither, both experience the same amount of air friction.
D) It cannot be determined because there
is not enough information given.
The Laws of Motion

55. Which of Newton's laws best explains why motorists should buckle-up?
Newton’s First Law
Which of Newton's laws best explains why motorists
should buckle-up?
A) the first law
B) the second law
C) the third law
D) the law of gravitation
The Laws of Motion

56. END