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Forces and Motion
Prentice Hall
Physical Science: Concepts in Action
Chapter 12
May the Force be with you
 Forces exist all around us
 There are 4 types of forces:
gravitational,
electromagnetic and strong
and weak nuclear forces
 Which do you think would be
the strongest?
 Gravity is actually the weakest
but has a far reaching effect
and nuclear forces (attractions)
are the strongest with an
effect only on things nearby
What is Force?
 Force: a push or pull that acts on an
object by changing its speed or direction
 Can cause a resting object to move
 Can accelerate a moving object
How is force measured?
 Spring scale
 Stretch of the spring depends on the mass
of the object acting on it
 Ex: vegetable scale at the grocery store
 Unit of Force
kg  m
 Newton (N)
1
N

 1 kg to accelerate 1 m/s2
s
 (named after Sir Isaac Newton)
2
How is force represented?
 Use arrows
 Direction
 Strength
 Length represents
strength or
magnitude
 The scale with more apples, greater mass, has a longer
arrow. The arrow is pointed downward due to mass is
below the balance pulling downwards.
Combining Forces
 Have you ever tried to push a broken down
car alone?
 Did you ask for help? Why?
 Forces in the same direction are added
together
 Force in the opposite direction are
subtracted
 Net Force the overall force acting on an object
after all the forces are combined
Balanced vs. Unbalanced Forces
 Balanced
 Combine to produce a net force of zero
 No change in the object’s motion
 Ex: tug of war, arm wrestling
 Unbalanced
 Net force equals the size of the larger
force minus the size of the smaller force
 Net force does not equal zero
 Causes an object to accelerate
Representing Forces
 Forces can add together
or subtract from one
another
 A- 2 forces acting in the
same direction add
 B- forces in opposite
directions subtract from
one another
 C – forces that are equal in
size and opposite in
direction result in no net
force
Friction
 All moving objects are subject to friction
 Friction is a force that opposes the motion of
objects that touch as they move past each other
 Acts at the surface where objects are in contact
 4 types of friction:
 Static friction
 Sliding friction
 Rolling friction
 Fluid friction
4 Types of Friction
 Static friction
 Force that acts on objects that are not
moving
 Always acts in the direction opposite
to that of the applied force
 Ex: taking a step
 Sliding friction
 Force that opposes the direction of
motion of an object as it slides over a
surface
 Less force is needed to keep an
object moving than to start it moving
 Ex: pushing a potted plant
4 Types of Friction
 Rolling friction
 The friction force that acts on rolling
objects
 Causes change in shape at the point of
rolling contact
 Ex: furniture
 Fluid friction
 The force that opposes the motion of an
object through fluid
 Increases the speed of the object
moving through the fluid
 Ex: cake batter
 Fluids (gas and liquids)
 Fluid friction acting on an object moving
through the air is known as air resistance
Gravity
 Gravity is a force that acts
between two masses
 Attractive force that pulls objects
together
 Earth’s gravity exerts a force of
attraction on every other object
that is near Earth
 Acts downwards towards the
center of the earth
Because the boulder is at rest, what do you
Know about the net force acting on it?
Gravity and Falling Objects
 Gravity causes objects to
accelerate downward
 Air resistance (fluid friction) acts
in the direction opposite to the
motion and reduces acceleration
 The flying squirrel to the right
takes advantage of air resistance
to slow its fall by increasing it’s
surface area.
 Terminal velocity is the constant
velocity of a falling object when
the force of air resistance equals
the force of gravity. (in other
words, top speed)
Projectile Motion
 Projectile Motion: motion of a falling object after given an
initial forward velocity
 Air resistance and gravity are the only forces
 The combination of an initial velocity and the downward
vertical force of gravity causes the ball to follow a curved path
The 2 balls fall
with the same
acceleration and
hit the ground at
the same time
12:2 Newton’s
st
1
Law of Motion
 Sir Isaac Newton studied motion and gave us the basic laws
of motion
 The 1st Law of Motion states that the motion of an object
does not change as long as the net force acting on the
object is zero.
 Sometimes also called the Law of Inertia
 Inertia Tendency of an object to resist change in its motion
 In other words… An object at rest stays at rest, an object in
motion stays in motion at the same direction and speed (until
something acts on it)
Newton’s
st
1
Law of Motion
Crash Dummy: Example of inertia
Newton’s
nd
2
Law of Motion
 How do unbalanced forces affect the motion of an object?
 It causes the velocity to change (accelerate)
 Ex: you apply a net force to a ball when you throw it
 The harder you throw, the more the ball accelerates.
 The acceleration is directly proportional to the net force acing on
it.
 Newton’s Second Law of Motion The acceleration of an object
is equal to the net force acting on it divided by the objects
mass
Newton’s
nd
2
Law of Motion
 Mass is a measure of inertia of an object and depends on
the amount of matter the object contains
 If you double the mass of an object, it cuts the acceleration in
half
 The acceleration of an object is always in the same direction as
the net force
 Newton’s 2nd Law applies when a net force acts in the opposite
direction of object’s motion
 Force produces deceleration and reduces speed
 Ex. Seat belts
 Force decelerates the passenger in order to prevent injury
 Units for Acceleration are equivalent (mean the same thing)
 N/kg=m/s2
Newton’s
nd
2
Law of Motion
Weight and Mass
 Are weight and mass the same?
 Weight & Mass are Different
 Weight The force of gravity acting on an
object
 Product of the mass and acceleration due
to gravity
 Unit is Newtons (N)
Weight and Mass
Newton’s
nd
2
Law of Motion
•How would the acceleration of a chain of two carts compare
with the acceleration of a single cart if the same force acted
on both?
•When the same force acts, the single cart accelerates 8x
faster than the chain of eight carts
•As mass increases, acceleration has to decrease in
proportion to the increase in mass
Newton’s
nd
2
Law of Motion
•How would the force have to change in order to have the
same acceleration for the eight carts as for one cart?
•The force would have to be 8x greater
•How would another force directed to the left on the cart
affect the cart’s acceleration?
•The acceleration would depend on the net force.
•The net force would be the force acting to the right minus
the force acting to the left.
Newton’s
nd
2
Law of Motion
1.A boy pushes forward a cart of groceries with a
total mass of 40.0 kg. What is the acceleration of
the cart if the net force on the cart is 60.0 N?
a=F/m
= 60.0 N/40.0 kg
= 1.50 m/s2
• 2.What is the upward acceleration of a helicopter
with a mass of 5000 kg if a force of 10,000 N acts
on it in an upward direction?
a=F/m
= 10000 N/5000 Kkg
= 2 m/s2
Newton’s
nd
2
Law of Motion
3.An automobile with a mass of 1200 kg accelerates at a rate
of 3.0 m/s2 in the forward direction. What is the net force
acting on the automobile? (Hint: Solve the acceleration
formula for force.)



a=F/m F=ma
= 1200 kg(3.0 m/s2)
= 3600 N
• 4.A 25-N force accelerates a boy in a wheelchair at
0.5 m/s2 What is the mass of the boy and the wheelchair?
(Hint: Solve Newton's second law for mass.)
 a=F/m m=F/a
 = 25 N/0.50 m/s2
 = 50 k/=g
Newton’s
rd
3
Law of Motion &
Momentum
 Newton’s 3rd Law – when an object exerts a force
on a second object, that object exerts an equal and
opposite force on the first object
 A force cannot exist alone. Forces always exist in pairs.
 These are called action and reaction pairs (forces)
Action and Reaction Forces
 Pressing your hand against a wall produces a pair of
forces
 Your hand exerts a force on the wall – the action force
 The wall exerts an equal and opposite force against your
hand – the reaction force
 Using a hammer to drive a nail into a piece of wood
produces force pairs
 Hammer applies force to the nail
 Action forces drives nail in wall
 Nail exerts equal and opposite force
 This force brings the motion of the
hammer to a stop
Action and Reaction Forces
Do Not cancel
 These forces that are produced by the two objects are
action and reaction forces.
 Sometimes action and reaction forces result in motion –
sometimes they don’t.
 Action and reaction forces do NOT cancel themselves
out.
 Only when equal and opposite forces act on the same
object do they result in a net force of zero
Momentum
 Momentum is the product of an object’s mass and its
velocity
 An object at rest has a momentum of zero.
 Momentum = Mass x Velocity
 Measured in kilogram-meters/second (kg m/s)
 p = mv (p is momentum)
 An object has a large momentum if the product of it’s
mass and velocity is large
Law of Conservation of
Momentum
 What happens to momentum when objects collide?
 Momentum is conserved – the total momentum does not
increase or decrease
 If no net force acts on a system, then the total momentum
of the system does not change
 A ‘system’ is part of the world under investigation (ie two cars
colliding)
 The rest of the world around the system is called the
‘surroundings’
 A closed system of objects means that there are no outside
forces or objects acting on the objects in the system.
 In a closed system, the loss of momentum by one object
equals the gain of momentum by the second object.
Momentum
Universal ForcesElectromagnetic
 Observations of planets, stars or other galaxies
suggest that the four forces exist throughout the
universe
 Electromagnetic force is associated with charged
particles
 Electric and magnetic force are 2 different parts of the
electromagnetic force
 Electric and magnetic are the only forces that both
attract and repel
Electric and Magnetic Forces
 Electric forces act between charged objects such as
electrons or protons
 Objects with opposite charges attract
 Objects with like charges repel
 Clothes often acquire electric charges in the dryer
 Clothes with opposite charges tend to cling together
 Magnetic forces act on some metals, poles of magnets
and moving charges
Nuclear Forces
 If the nucleus of an atom has protons (+)
packed into a small space, why don’t they
repel each other and break apart the
nucleus?
 Two forces, the strong-nuclear force and
the weak-nuclear force act within the
nucleus to hold it together.
 The strong NF overcomes the electric
force of repulsion between the protons in
the nucleus
 Acts only on neutrons and protons. 100x
strong than repulsive forces
 The weak nuclear force is involved in
radioactive processes and is weaker than
the strong NF
Gravitational Force
 Gravitational force an attractive force that acts between
any two masses
 Weakest universal force
 Most effective over large distances
 Newton’s law of universal gravitation
 Says that every object in the universe attracts every other
object
 The greater the mass of an object, the greater the
gravitational force.
 The more distance between two objects, the less the
gravitational force
 Gravitational forces quicklab
Centripetal Force
 How is the moon kept in orbit around earth? It has inertia
and should move in a straight path until acted on by a force
 Earth’s gravitational force continuously pulls the object toward
it
 Centripetal force: center-directed force that constantly
changes the direction of an object and causes it to move in
a circle
Centripetal Force
 If the Earth causes the moon to stay in a circular orbit,
what force does the moon apply on the earth?
 Gravitational pull from moon causes two bulges in earth’s
oceans
 Earth rotates 2x a day below these bulges resulting in two
high and two low tides per day on Earth
 Examples affected by centripetal force:
 Satellites
 Examples (there are too many to list) of satellite uses
include:
 communication
 tracking Earth’s weather and climate