Forces and changes in motion

1
Forces and changes in motion

• Motion and Forces

2
MOTION
Definition ?
It is a change of position / location in
relation with stationary or moving objects or
reference point
3
Laws of Motion

• They describe the motion of objects in terms of
• their MASS and
• the FORCES acting on them

• Mass the amount of matter that an object
  contains is a measure of the INERTIA of an
  object or its resistance to a change in
  motion.metric unit of mass is gram, kilogram,
  centigram...

4
(No Transcript)
5
Laws of Motion

• Force a PUSH or a PULL is always acting on an
  object even if the object is either at rest or
  moving. metric unit of force is Newton

6
The forces are BALANCED
7

• To move an object at rest or change the motion of
  a moving object i.e. accelerate, the forces
  acting on it must be ?

UNBALANCED

• The object will then accelerate in the direction
  of the greater force

Net force is 0, so no motion
8
Fg and the push from the table balance each
other. However, there is no force present to
balance the force of friction. As the book
moves to the right, friction acts to the left to
slow the book down. This is an unbalanced force.
9
1st Law of Motion or Law of Inertia

• An object at REST, will remain at rest AND an
  object in motion will stay in motion unless acted
  upon by an unbalanced force? in other words,
  objects keep on doing what they are doing.

10
Newton's Second Law
To accelerate an object, a ______ must cause its
__________ to change.
FORCE
VELOCITY
The 2nd law of Motion, describe the relationship
between Mass, Force and Acceleration
11
Net Force Mass
or F mA
Acceleration
The metric unit for Force is kg x m/sec2 or
NEWTON
12
ACCELERATION

• VELOCITY is speed in a given direction
• Acceleration is the rate of change of
  VELOCITYhow much velocity changes over time
• You can accelerate 3 different waysall
  requiring a F
• Increasing your speed speeding up accelerating
• Decreasing your speed slowing down
  decelerating
• Changing direction

13
Newton's Third Law
This law is NOT in terms of _______ and
__________ it is acting on 2 different __________
CAUSE
EFFECT
OBJECTS
According to Newton, whenever objects A and B
interact with each other, they exert forces
upon each other.
14
REACTION
ACTION
15
3rd Law in NATURE
Flying gracefully through the air, birds depend
on Newtons third law of motion,
in other words it depends on the action-reaction
force PAIR
16
A person is walking in the aisle of a moving
train. When compared to which reference point is
the person NOT moving?

• A passenger in a seat on the train
• A passenger walking in the opposite direction
• A cell phone clipped to the persons belt
• The train station where the person boarded the
  train

17
The seat belt in a car worksin opposition to

• Inertia
• Acceleration
• Velocity
• Mass

18
The relationship between mass, acceleration, and
force is described by

• Newtons First law of motion
• Newtons Second law of motion
• Newtons Third law of motion
• The law of inertia

19
Sam pushes a wheelbarrow that has a mass of 5kg
with an acceleration of 2m/s2. Sandra pushes a
wheelbarrow with a mass of 9kg with an
acceleration of 1m/s2. What are the respective
forces applied by Sam and Sandra?

• 2.5N, 9N
• 10N, 9N
• 0.4N, 0.1N
• None of the above

20
Calculating and graphing motion

• Motion and Forces

21
Speed

• Speed is the DISTANCE covered by an object over a
  certain amount of TIME
• S D / T
• Units could be

D
T
S
meters/second meters per second or m/s
kilometers/hour centimeter/year
22
Constant / Average SPEED

• Constant ? equal distances covered in the same
  amount of time

23
Instantaneous SPEED

• Instantaneous ? speed at one moment, at one point
  in time

increase
decrease
increase
24
Graphing SPEED

• A graph with time on the x axis (horizontal), and
  distance on the y axis (vertical) is called a
  DISTANCE-TIME GRAPH.

y
DISTANCE
x
TIME
25

• Why is plotting distance over time a
  representation of SPEED??
• Remember the equation S D / T
• Remember the equation for slope Rise / Run
• So Rise / Run is D / T or SPEED

26
STRAIGHT LINE ??

• Any straight line implies that the object covers
  the same distance in the same amount of time
  therefore it represents CONSTANT SPEED

y
DISTANCE
x
TIME
27

• The STEEPNESS of the slope will give us valuable
  information regarding the motion of the object
  being studied

Faster or Slower??
y
DISTANCE
Slower or Faster??
x
TIME
28

• WHAT ABOUT A HORIZONTAL LINE??

Time increases but distance remains the same
object is
NOT MOVING stopped
y
DISTANCE
x
TIME
29
Calculating Acceleration and Slope

• If you are speeding up, your velocity increases
  with time your Vf is greater than your
  ViPositive acceleration Positive Slope
• If you are slowing down, your velocity decreases
  with time your Vf is less than your ViNegative
  acceleration Negative Slope deceleration

30
Calculating Acceleration and Slope

• The equation for Acceleration is A (final
  Velocity initial Velocity) / Time (Vf
  Vi) / T
• Units are meter per second squared
  m/sec2m/s/s ? m/s2

31
Representing AccelerationDistance-Time graph
SLOWING DOWN
NO ACCELERATION
DISTANCE
(m)
SPEEDING UP
(s)
TIME
32
Representing AccelerationVelocity-Time graph or
simply Speed vs. Time
VELOCITY
TIME
33
Based on the table of data, how would you
describe the balls motion?

• Constant velocity
• Negative acceleration
• Positive acceleration
• Negative velocity

34
Peter runs in one direction at 1.5m/s. He then
turns around and runs in the opposite direction
at 2m/s. The entire trip takes 5s. What is
Peters average acceleration?

• 0.1m/s2
• - 0.1m/s2
• 0.7m/s2
• - 0.7m/s2

1.5m/s -2m/s -0.5m/s for overall
velocity -0.5m/s / 5s -0.1m/s2
35
John is driving his car along a straight road. He
then slows down so that he has a negative
acceleration. Which graph best describes his
motion?

• A curve up on a distance-time graph
• A curve down on a distance-time graph
• An horizontal line on a velocity-time graph
• A straight, oblique line on a distance-time graph

36
Electric forces

• Motion and Forces

37
Electric Forces

• The sub-atomic particles are charged and interact
  with each other to produce an electrical force
• like charges produce repulsive forces force of
  repulsion
• opposite charges produce attractive forces
  force of attraction

The condition of space around an object is
changed by the presence of an electrical charge.
The electrical charge produces a force field
called an ELECTRIC FIELD
38
The direction of the arrows represent the effect
of the field on a POSITIVE CHARGE
39
As the distance increases between charges, the
intensity of the field will
decrease
As the number of charges increase, the intensity
of the field will
increase.
40

• Coulombs law
• 1785

where, F is the force, k is a constant and has
the value of 9.00 x 109 Newton?meters2/coulomb2
(9.00 x 10 9 N?m2/C2), q1 represents the
electrical charge of object 1 and q2 represents
the electrical charge of object 2, and d is the
distance between the two objects.
Which factor has the greatest impact on the
magnitude of the force? Charges or distance
41
Static Electricity

• When electric charges build up on a surface,
  due to friction for example,
• the charges are at rest STATIC ELECTRICITY.
• When static charges move to another, the
  phenomenon is calledSTATIC DISCHARGE

42
LIGHTNING
43
Detection of electrostatic charges
An electroscope is a device that detects static
electricity. Separating thin metal leaves Au,
Al indicate the presence of charges, but does
not differentiate if the object under
investigation is positively or negatively
charged. In both cases, the leaves will REPEL
44
(No Transcript)
45
A student touches a glass rod to the metal knob
at the top of an electroscope. The leaves do not
spread apart. What conclusion can be drawn?

• The glass rod has a positive charge
• The glass rod has a negative charge
• The glass rod has no charge and is neutral
• The glass rod is positively charged and the
  leaves are negatively charged.

46
What is Coulombs law used to find?

• The magnitude of the electrical force
• The distance an electric force is in effect
• The charge of an electric force
• The strength of an ionic bond

47
When taking laundry out of a dryer, Sarah sees
tiny sparks when separating some of the clothes.
What causes those sparks?

• Static discharge
• Magnetic attraction
• An electroscope
• Surface tension

48
How would you represent the electrical field
surrounding a positive charge?

• Inward
• Concentric circles
• Outward
• Alternative arrows inward and outward

49
Electricity and magnetism

• Motion and Forces

50
Electromagnetic Force

• Electricity and magnetism are 2 aspects of a
  single force i.e. EM force
• Moving electric charges produce magnetic
  fields/forces
• Moving magnets produce electric fields/forces

Alike repel, opposite attract
51
4 UNIVERSAL FORCES

• EM force holds the atoms and molecules together,
  involved in CHEMICAL REACTIONS. strong and
  exerted over long distance
• Gravitational weak but exerted over long
  distance responsible for the ATTRACTION b/t 2
  masses
• Strong nuclear exerted over short distance
  responsible for holding the nucleus subatomic
  particles together and involved in NUCLEAR
  FISSION.
• Weak nuclear exerted over short distance
  involved in RADIOACTIVE DECAY.

52
Relative Strength of Universal Forces

• 1
• 1/137
• 10-6
• 6 X 10-39

• STRONG
• ELECTROMAGNETIC
• WEAK
• GRAVITY

53
What are Magnetic Field Lines?

• Magnetic Field Lines extends outward from the
  North pole, and loops around the magnet to the
  South pole/end.
• Magnetic force is stronger at the poles

Notice the convention arrows leave N, pointing
to S
54
Current generating a magnetic field

• The combined spinning and orbiting motions of
  the e- create tiny magnetic fields.
• An electric current flowing through a wire is
  surrounded by a magnetic field directed around
  the wire.

55
Current generating a magnetic field

While the e- flow from the (-)ve pole to the
()ve pole of a battery, the conventional
direction of current is in the opposite direction.
-
56
Right-hand rule
57

• Electromagnetic Induction
• The relative motion of a coil of wire and a
  magnetic field generates an electric current
  flow of e-.

The direction of the induced current in the coil
of wire depends on the direction of motion of the
magnet
58
APPLICATIONS

• Electromagnet
• temporary, strong magnet that can be turned
  on/off
• a current-carrying wire wrapped around an iron
  core.

Its strength increases with 1. more coils, 2.
bigger iron core
59
APPLICATIONS

• MOTOR
• converts electrical energy into mechanical
  energy
• electromagnet placed b/t the poles of a
  permanent magnet
• by applying a current to the electromagnet, the
  temporary magnetic field constantly changes
  direction

60
(No Transcript)
61
Motor
62
APPLICATIONS

• GENERATOR
• converts mechanical energy into electrical
  energy
• mechanical energy from wind or flowing water
  spin the permanent magnet causing an electrical
  current to flow in the wire coil.

63
(No Transcript)
64
APPLICATIONS

• TRANSFORMER
• allows to either reduce step-down or increase
  step-up the voltage electrical energy
  available for moving a charge
• consists of an iron core with a primary and
  secondary coils.
• a current flowing in the primary coil magnetizes
  the iron core which in turn produces a current in
  the secondary coil.

65
(No Transcript)
66
(No Transcript)
67
A grocery store checks the details of a credit
card by having customers move a magnetic strip on
the card past a wire coil. Which principle
explains this process?

• An electrical current produces mechanical energy
• A permanent magnet produces a changing magnetic
  field
• A changing magnetic field produces an electrical
  current
• Mechanical energy produces a changing
  gravitational field.

68
A loudspeaker contains a permanent magnet and an
electromagnet. The electromagnet changes
electrical energy into mechanical energy to
vibrate a speaker cone. Which of the following
BEST describes what happens in the loudspeaker?

• Moving electric charges in the electromagnet
  create a magnetic field
• The permanent magnet causes the electric current
  to alternate direction
• The field of the permanent increases voltage in
  the electromagnet
• The electromagnet generates electricity by
  pushing against the permanent magnet.

69
SPEAKERCONE
70
Lisa has a copper wire coil, a permanent magnet,
and an iron rod. What can she do with these items
to produce an electrical current?

• Place the wire near the north pole of the
  permanent magnet
• Move the permanent magnet rapidly through the
  coil.
• Touch the ends of the wire to the poles of the
  magnet
• Bring the permanent magnet near the end of the
  iron rod.

71
Conserving matter and energy

• Energy Transformations

72
(No Transcript)