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Chapter 4 The Laws of Motion

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Chapter 4 The Laws of Motion - PowerPoint PPT Presentation


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Chapter 4 The Laws of Motion. Sir Isaac Newton (1643 – 1727). Newtonian mechanics Describes motion and interaction of objects Applicable for speeds much slower than the speed of light Applicable on scales much greater than the atomic scale

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Presentation Transcript
slide1

Chapter 4

The Laws of Motion

slide2

Sir Isaac Newton

(1643 – 1727)

  • Newtonian mechanics
  • Describes motion and interaction of objects
  • Applicable for speeds much slower than the speed of light
  • Applicable on scales much greater than the atomic scale
  • Applicable for inertial reference frames – frames that don’t accelerate themselves
slide3

Force

  • What is a force?
  • Colloquial understanding of a force – a push or a pull
  • Forces can have different nature
  • Forces are vectors
  • Several forces can act on a single object at a time – they will add as vectors
slide4

Force superposition

  • Forces applied to the same object are adding as vectors – superposition
  • The net force – a vector sum of all the forces applied to the same object
slide5

Newton’s First Law

  • If the net force on the body is zero, the body’s acceleration is zero
slide6

Newton’s Second Law

  • If the net force on the body is not zero, the body’s acceleration is not zero
  • Acceleration of the body is directly proportional to the net force on the body
  • The coefficient of proportionality is equal to the mass (the amount of substance) of the object
slide7

Newton’s Second Law

  • SI unit of force kg*m/s2 = N (Newton)
  • Newton’s Second Law can be applied to all the components separately
  • To solve problems with Newton’s Second Law we need to consider a free-body diagram
  • If the system consists of more than one body, only external forces acting on the system have to be considered
  • Forces acting between the bodies of the system are internal and are not considered
slide8

Chapter 4

Problem 12

Two forces are applied to a car in an effort to move it. (a) What is the resultant of these two forces? (b) If the car has a mass of 3 000 kg, what acceleration does it have? Ignore friction.

slide9

Newton’s Third Law

  • When two bodies interact with each other, they exert forces on each other
  • The forces that interacting bodies exert on each other, are equal in magnitude and opposite in direction
slide10

Forces of different origins

  • Gravitational force
  • Normal force
  • Tension force
  • Frictional force (friction)
  • Drag force
  • Spring force
slide11

Gravity force (a bit of Ch. 7)

  • Any two (or more) massive bodies attract each other
  • Gravitational force (Newton\'s law of gravitation)
  • Gravitational constant G = 6.67*10 –11 N*m2/kg2 = 6.67*10 –11 m3/(kg*s2) – universal constant
slide13

Gravity force at the surface of the Earth

  • The apple is attracted by the Earth
  • According to the Newton’s Third Law, the Earth should be attracted by the apple with the force of the same magnitude
slide14

Weight

  • Weight (W) of a body is a force that the body exerts on a support as a result of gravity pull from the Earth
  • Weight at the surface of the Earth: W = mg
  • While the mass of a body is a constant, the weight may change under different circumstances
slide15

Tension force

  • A weightless cord (string, rope, etc.) attached to the object can pull the object
  • The force of the pull is tension ( T )
  • The tension is pointing away from the body
slide17

Normal force

  • When the body presses against the surface (support), the surface deforms and pushes on the body with a normal force (n) that is perpendicular to the surface
  • The nature of the normal force – reaction of the molecules and atoms to the deformation of material
slide18

Normal force

  • The normal force is not always equal to the gravitational force of the object
slide21

Chapter 4

Problem 30

An object with mass m1 = 5.00 kg rests on a frictionless horizontal table and is connected to a cable that passes over a pulley and is then fastened to a hanging object with mass m2 = 10.0 kg, as shown in the Figure. Find the acceleration of each object and the tension in the cable.

slide22

Frictional force

  • Friction ( f) - resistance to the sliding attempt
  • Direction of friction – opposite to the direction of attempted sliding (along the surface)
  • The origin of friction – bonding between the sliding surfaces (microscopic cold-welding)
slide23

Static friction and kinetic friction

  • Moving an object: static friction vs. kinetic
slide24

Friction coefficient

  • Experiments show that friction is related to the magnitude of the normal force
  • Coefficient of static frictionμs
  • Coefficient of kinetic frictionμk
  • Values of the friction coefficients depend on the combination of surfaces in contact and their conditions (experimentally determined)
slide27

Chapter 4

Problem 49

Find the acceleration reached by each of the two objects shown in the figure if the coefficient of kinetic friction between the 7.00-kg object and the plane is 0.250.

slide29

Answers to the even-numbered problems

Chapter 4

Problem 6

7.4 min

slide30

Answers to the even-numbered problems

Chapter 4

Problem 26

4.43 m/s2 up the incline, 53.7 N

slide31

Answers to the even-numbered problems

  • Chapter 4
  • Problem 40
  • 55.2°;
  • (b) 167 N
slide32

Answers to the even-numbered problems

  • Chapter 4
  • Problem 50
  • 18.5 N;
  • (b) 25.8 N
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