Transcript Force

Chapter 12
Forces and Motion
12.1 Forces

What is a force?
Force-a push or pull that acts on an object
 A force can cause a resting object to move,
or it can accelerate a moving object by
changing the object’s speed or direction

12.1 Forces
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Measuring Force
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Tools are used to measure forces
• Ex: a spring scale in a grocery store
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Units of force
• Force is measured in newtons (N)
• One Newton is the force that causes a 1.0 kg mass to
accelerate at a rate of 1.0 m/s2
• 1.0 N = 1 kgm/s2
• The newton is named after Sir Isaac Newton (16421727), who explained how force, mass, and
acceleration are related
12.1 Forces
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Representing Force
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Force is a vector
• You can use an arrow to represent a force
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Combining Forces

Forces acting in the same direction add
together and forces acting in opposite
directions subtract from one another
• Net Force- the overall force acting on an object
after all the forces are combined
12.1 Forces
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Combining Forces (cont.)
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Balanced Forces- are forces that combine to
produce a net force of zero
• When the forces on an object are balanced, the
net force is zero and there is no change in the
object’s motion

Unbalanced Forces- forces that result when
the net force acting on an object is not equal
to zero
• When an unbalanced force acts on an object, the
object accelerates
12.1 Forces

Friction- a force that opposes the motion
of objects that touch as they move past
each other

There are four main types of friction:
•
•
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Static friction
Sliding friction
Rolling friction
Fluid friction
12.1 Forces

Static Friction- the
friction force that acts
on objects that are not
moving

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Always acts in the
direction opposite to that
of the applied force
Once an object is
moving, static friction no
longer acts on the object

Sliding Friction- a force
that opposes the
direction of motion of
an object as it slides
over a surface

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Sliding friction is less
than static friction, so
less force is needed to
overcome it
Less force is needed to
keep an object moving
than to start it moving
12.1 Forces

Rolling Friction- the friction force that acts
on rolling objects

For a given set of materials, the force of rolling
friction is 100-1000 times less than the force of
static or sliding friction
• Ball bearings are often used in machines to
reduce the amount of friction acting on the parts
• Rolling friction replaces sliding friction
12.1 Forces

Fluid Friction- the force that opposes
motion of an object through a fluid

Fluid friction increases as the speed of the
object moving through the fluid increases
• The faster the object the greater the force of the
fluid friction

Air resistance- fluid friction acting on an
object moving through the air
12.1 Forces
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Gravity- a force that acts between any two
masses
Gravity is an attractive force, that is it pulls
objects together
 Earth’s gravity acts downward toward the
center of the earth
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12.1 Forces

Gravity
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Falling Objects
• Gravity causes objects to accelerate downward (at 9.8
m/s2), whereas air resistance acts in the direction
opposite to the motion and reduces acceleration
• The greater the surface area of an object the more
influence air resistance will have on the object as it
falls
• Terminal velocity- the constant velocity of a falling
object when the force of air resistance equals the force
of gravity
12.1 Forces

Projectile Motion- the motion of a falling
object (projectile) after it is given an initial
forward velocity
Air resistance and gravity are the only forces
acting on a projectile
 The combination of an initial velocity and the
downward vertical force of gravity causes the
ball to follow a curved path

12.2 Newton’s First and Second
Laws of Motion
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Scientists that helped us understand the
relationships between force and motion

Aristotle
• Ancient Greek philosopher (384 BC-322 BC)
• Many discoveries through careful observation and
logical reasoning
• Not always correct
12.2 Newton’s First and Second
Laws of Motion
• Galileo
• Italian scientist (1564-1642)
• Experimented to answer questions
• Newton
• English scientist (1643-1727)
• Built on the work of other scientists, like Galileo
• Lots of work with force and motion
12.2 Newton’s First and Second
Laws of Motion
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Newton’s First Law of Motion
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States that the state of motion of an object
will remain the same until an outside,
unbalanced force acts on it
• An object at rest will stay at rest
• An object in motion will stay in motion
• Inertia- the tendency of an object to resist a
change in its motion
12.2 Newton’s First and Second
Laws of Motion
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Newton’s Second Law of Motion
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States that the acceleration of an object is
equal to the net force acting on the object
divided by the mass of the object
• Acceleration = Net force / mass
• a = F/m
12.2 Newton’s First and Second
Laws of Motion
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Examples using Newton’s second Law
formula
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An automobile with a mass of 2000 kg
accelerates at a traffic light when it turns
green. If the net force acting on the car is
4000 N, what is the car’s acceleration?
• a = F/m
• a=
• F = 4000 N
• m = 2000 kg
12.2 Newton’s First and Second
Laws of Motion
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Weight and Mass
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Weight and mass are NOT the same thing!
Weight- the measure of the force of gravity acting on
an object
Mass is a measure of the inertia of an object, or you
can say that mass is the amount of matter in an
object
Weight is the product of an object’s mass and the
acceleration of the object due to the force of gravity
acting on it
12.2 Newton’s First and Second
Laws of Motion

Formula to calculate weight
• Weight = mass (m) x acceleration due to
gravity (g)
• W = mg
• The unit for weight must be in newtons
• g = 9.8 m/s2
• Mass must be in kg
12.3 Newton’s Third Law of
Motion and Momentum
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Newton’s Third Law
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States that whenever one object exerts a
force on a second object, the second object
exerts an equal and opposite force on the
first object
• These forces are called action and reaction forces
12.3 Newton’s Third Law of
Motion and Momentum
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Action and Reaction Forces
• Examples
• Walking/running
• When you push off the floor with your foot your foot
applies an action force on the floor, the floor pushes
back on your foot with the reaction force
• Swimming
• Your arms push against the water--> action force
The water pushes the swimmer forward--> reaction
force
• These forces are equal in value and opposite in
direction
12.3 Newton’s Third Law of
Motion and Momentum
• Action-Reaction forces do not always cancel
each other out
• Action-reaction forces are equal and opposite-so why does the swimmer move? Why don’t the
forces result in a net force of zero?
-The action and reaction forces act on different
objects
-the action force acts on the water and the
-reaction force acts on the swimmer
12.3 Newton’s Third Law of
Motion and Momentum
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Momentum
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Momentum- the product of an object’s mass
and its velocity
• An object with a large momentum is hard to stop
An object has a large momentum if the
product of its mass and velocity is large
 Formula to calculate momentum
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• Momentum = mass x velocity
• Mass must be in kg
• Velocity must be in m/s
12.3 Newton’s Third Law of
Motion and Momentum
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Example problem
• Which has a larger momentum, a wagon full of
logs (mass of 62 kg, speed of 12 m/s) or a 5th
grader on a sled (mass of 47 kg, speed of 27
m/s)?
• Momentum = mass x velocity
• Momentum1 =
Momentum = (62)(12)
• Mass1 = 62 kg
= 744 kgm/s
• Velocity1 = 12 m/s
Momentum = (47)(27)
• Momentum2 =
= 1269 kgm/s
• Mass2 = 47 kg
The kid on the sled has
• Velocity2 = 27 m/s
the greater momentum!
12.3 Newton’s Third Law of
Motion and Momentum
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Conservation of Momentum
Means overall momentum doesn’t
increase or decrease
 A closed system must be in place for
momentum to be conserved
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• Closed system means that other objects
and/or forces can not enter or leave a
system
• Objects within the system can exert forces on
each other
12.3 Newton’s Third Law of
Motion and Momentum
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Law of Conservation of Momentum
• States that if no net force acts on a system, then
the total momentum of the system does not
change
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In a closed system, the loss of momentum of
one object equals the gain in momentum of
another object--momentum is conserved
12.4 Universal Forces
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There is evidence that there are four forces that
exist throughout the universe
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Electromagnetic Forces
Nuclear Forces
• Strong Force
• Weak Force
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Gravitational Force
All these forces act over a distance between
particles (means particles don’t have to be
touching for these forces to work)
All these forces are affected by the distance
between the particles
12.4 Universal Forces
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Electromagnetic Forces
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Electric and magnetic force are two aspects
of the electromagnetic force
• Electromagnetic force- is associated with charged
particles
• Electric force and magnetic force are the only
forces that can both attract AND repel
12.4 Universal Forces
• Electric Force
• Act between charged objects
• Opposite/unlike charges attract, like charges repel
• Magnetic Force
• Act on certain metals, on the poles of magnets, and on
moving charges
• Opposite/unlike poles attract, like poles repel
• Magnets have two poles--north and south
12.4 Universal Forces
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Nuclear Forces
Act on the nucleons (particles in the nucleus
of an atom) in an atom
 Because of the fact that like charges repel
(electric force-protons are positively charged)
and the protons are close together, you may
think that the nucleus couldn’t exist

• Because the of the nuclear forces they can
12.4 Universal Forces
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Two forces, the strong nuclear force and the
weak nuclear force, act within the nucleus to
hold it together
• Strong nuclear force overcomes the electric
repulsion between the protons
• Weak nuclear force is involved in radioactive
decay
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The nuclear forces are attractive forces
12.4 Universal Forces
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Strong Nuclear Force
• …is a powerful force of attraction that acts only on
neutrons and protons in the nucleus, holding them
together
• The range over which this force acts is 10-15 m
(about the diameter of a proton)
• Acts over very short distances, but is 100 times stronger
than the electric force of repulsion at that distance
12.4 Universal Forces
• Weak Nuclear Force
• …an attractive force that acts only over a short range
(10-18 m)
• Is involved in radioactive processes
12.4 Universal Forces
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Gravitational Force
Weakest universal force at close distances,
but most effective over great distances
 …an attractive force that act between any
two masses
 Newton’s Law of Universal Gravitation states
that every object in the universe attracts
every other object
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12.4 Universal Forces
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Gravity acts over large distances
• The gravitational force between two objects is
proportional to their masses and decreases
rapidly as the distance between the masses
increases
• The greater the mass of the objects, the greater their
gravitational forces
12.4 Universal Forces
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Centripetal force- a center-directed force that
continuously changes the direction of an object to
make it move in a circle
• Results in a circular path
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The moon’s gravitational pull on the Earth results in
two bulges in the earth’s oceans
• One is on the side of the Earth closest to the moon and the
second is on the side of the Earth farthest from the moon
• Because the Earth rotates once in a day this results in two
high and two low tides per day on Earth