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Forces and Motion
Book Chapter 12
12.1 - Forces
12.2 - Newton’s 1st and 2nd Laws
12.3 - Newton’s 3rd Law & Momentum
12.4 - Universal Forces
Forces
A force is 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.
Measuring Force
Forces are measured in Newtons (named for
Sir Issac Newton).
One Newton is the force that causes a 1
kilogram mass to accelerate at 1 m/s2.
Combining Forces
Forces are vectors, so you can use an arrow to
represent it. The arrow has to be proportional to
the force and in the same direction.
When you combine forces by adding or subtracting,
you are finding the Net Force.
The net force is the overall force acting on an object
after all forces have been combined.
Balanced and Unbalanced Forces
When the forces on an object are balanced,
the net force is zero and there is no change
in the object’s motion.
When an unbalanced force acts on an object,
the object accelerates.
Friction
Friction is a force that affects motion by slowing an object
down.
It is always opposite to the direction of motion and affects
how much an object can accelerate.
Friction is present because of irregularities in the surfaces
of objects that are in contact with one another.
Fluids like water and air also exert a friction force on a
moving object.
Four Types of Friction
1.
2.
3.
4.
Static Friction – friction force that acts on
objects that are not moving
Sliding Friction – Force that opposes the
motion of an object as it
slides over a surface
Rolling Friction-friction that acts on rolling
objects
Fluid Friction-opposes motion as an object
moves through a fluid
Friction
Direction of motion
FRICTION BETWEEN TIRES AND ROAD
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Gravity
Gravity is an attractive force that acts
between any two masses.
Gravity causes objects to accelerate
downward towards the center of the earth.
Falling and Air Resistance
Falling and Air Resistance
As an object falls faster, the force of air
resistance increases until it equals the
weight of the object. At this time, the net
force equals zero and the object stops
accelerating.
This final speed is called terminal velocity.
Falling and Air Resistance
Discovering the Reason for Motion
Aristotle- Greek scientist who thought that
force was necessary to maintain motion
See Aristotle’s ideas on motion.
Discovering the Reason for Motion
Galileo – Italian
scientist who did
experiments that
helped correct
misconceptions about
force and motion
Discovering the Reason for Motion
Newton – scientist who
introduced several
laws describing force
and motion.
Newton’s 1st Law
Law of Inertia
Everything in motion stays in motion, and
everything at rest stays at rest unless an
outside force acts upon it.
Everything in motion and at rest has inertia.
Law of Inertia
Law of Inertia
Law of Inertia
Mass
Mass determines the amount of inertia
possessed by an object.
Mass is the amount of matter in an object.
Mass is NOT Volume
Volume is the amount of space an object
takes up.
EXAMPLE:
A pillow will have a volume larger than a car
battery but less mass.
Mass is NOT weight
Weight is the force of gravity acting on a body
Objects in space will still have mass but will not
have weight.
Metric Units:
mass - kilogram
weight - Newton
On the surface of the earth, mass and weight are
proportional.
1 kg = 9.8 Newtons
Calculating the weight of an object
Since weight depends on the mass of an object AND
the gravitational pull of the earth, we can say that
Weight = mass x gravity
Or
Weight = mg
The weight of an object is measured in Newtons.
Because weight depends on gravity, it is also a
force.
What is the weight of a 500 gram rock?
1. Convert to standard units
500 grams = .5 kg
2. Remember the constant for gravity is 9.8 m/s2.
3. Plug into the equation and solve.
Weight = mg
Weight = (.5 kg)(9.8 m/s2)
Weight = 4.9 N (Newtons)
Questions
1. Your empty hand is not harmed if it bangs against the wall,
but it is harmed if you are carrying a heavy load. Why?
2. Does a person diet to lose mass or to lose weight?
3. Can the force of gravity on a 1 kg mass ever be greater
than on a 2 kg mass? Explain how.
4. A car at a junk yard is compressed until its volume is less
than 1 cubic meter. Has its mass changed? Has its weight
changed? Has its volume changed? Explain.
5. If you jump up in a bus that is moving at a constant
velocity, will you land farther back in the bus? Explain.
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Newtons 2nd Law of Motion
Law of Acceleration
What causes an object to accelerate?
-or change its state of motion?
NET FORCE
What is a Net Force?
A net force is the combination of all forces acting on
an object
Applied Forces
Net Force
15 N
5N
10 N
5N
5N
10 N
Equilibrium
Applied Forces
Net Force
0N
5N
5N
If the net force is zero, the motion of the object will not
change.
This is a condition called EQUILIBRIUM.
If an object is in equilibrium, we have to go back to the
Law of Inertia.
Acceleration
Acceleration is directly related to the net
force.
Acceleration ~ Net Force
The larger the net force is acting on an object, the
greater the acceleration of the object.
Acceleration
Acceleration is inversely proportional to the
mass of an object
Acceleration ~
1
mass
Try to push a 250 pound crate. Now try to push a 25
pound crate. Which could you move faster?
Newton’s 2nd Law of Motion
The acceleration of an object is directly
proportional to the net force acting on it, is
in the same direction as the net force, and
inversely proportional to the mass of the
object.
a=
F
m
OR
F = ma
F
m
a
Newton’s 3rd Law of Motion
Action / Reaction
Law of Interaction
Newton realized that force is not a thing in
itself, but part of an interaction between
one thing and another.
Forces always occur in pairs
One force is called the action force and the
other the reaction force.
Action: Object A exerts a force on Object B.
Reaction: Object B exerts an equal and
opposite force on Object A.
Newton’s 3rd Law
For each action force, there is an equal and
opposite reaction force.
Whenever one object exerts a force on a
second object, the second object exerts an
equal and opposite force on the first object.
Action / Reaction Forces
Action: Tire pushes road
Reaction: Road pushes tire
Action: Rocket pushes gas
Reaction: Gas pushes rocket
Action: Earth pulls ball
Reaction: Ball pulls Earth
Action / Reaction Forces
Consider the apple at
rest on the table. If
we call the
gravitational force
exerted on the apple
action, what is the
reaction force? Are
there any other
action/reaction forces
present?
Action / Reaction Forces
If a cement truck and a car have a head-on collision, which
vehicle will experience the greater impact force?
A. The cement truck
B. The car
C. Both the same
D. ….it depends on other factors
Momentum
Momentum is inertia in motion.
Momentum = (mass)(velocity)
It is harder to stop a large truck than a small
car when both are moving at the same
speed because the truck has a larger mass
and larger momentum
Calculating Momentum
Example
Which would be harder to stop….a .25 kg ball
thrown at you at 40 m/s or a 7 kg bowling ball
rolled at 1 m/s?
kgm
p  mv  (.25kg)( 40m / s )  10
s
kgm
p  mv  (7 kg)(1m / s )  7
s
It would be harder to stop the baseball; it has more momentum.
Law of Conservation of Momentum
In the absence of an external force, the total
momentum of a system remains unchanged.
This is an extension of Newton’s 3rd Law
(Mass)(Velocity before) = (Mass)(Velocity after)
See an example of the Law of Conservation of Momentum (and Energy)
Conservation of Momentum
Conservation of Momentum
Universal Forces
Electromagnetic forces, Nuclear forces, and
Gravitational forces are all universal forces.
Electromagnetic force is associated with charged
particles.
Electric force and magnetic force are the only
forces that can both attract and repel.
Nuclear Forces
Two forces, the strong nuclear force and the weak
nuclear force, act within the nucleus to hold it
together.
The strong nuclear force acts only on the neutrons
and protons in the nucleus.
The weak nuclear force acts over a short range and
affects all particles, not just protons and
neutrons.
Gravitational Force
Gravitational force is an attractive force that acts
between two masses.
The bigger the masses and the closer they are, the
more the attraction force.
Newton’s Law of Universal Gravitation states that
every object in the universe attracts every other
object.
Gravitational Force
Gravity is the weakest universal force, but it
is the most effective over long distances.
Earth’s gravitational force keeps the moon in
a nearly circular orbit.
The gravitational pull of the moon on the
Earth causes ocean tides.
Satellites
When an artificial
satellite is put into
orbit, its inertia and
the pull of the Earth
keep it in orbit. If the
satellite is too close
to the atmosphere,
friction slows it down
and it crashes into
the Earth.
Satellite without Gravity
Launch speed less
than 8000 m/s
Launch speed equals
8000 m/s