a force is a push or a pull on an object. a force results
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• A FORCE is a push or a pull on an object.
• All forces have strength and direction.
• The strength of a force is measured in Newtons (N).
• A force results when two or more objects INTERACT with
each other. When the objects stop interacting, there is no
more force.
• Some forces result when two objects physically touch each
other. These forces are called CONTACT FORCES.
• Some forces result even when two objects do not physically
touch each other. These forces are called NON-CONTACT
FORCES.
© Stephanie Elkowitz 1 Forces & Motion
•Applied
•Friction
•Gravity
•Magnetic
•Electric
© Stephanie Elkowitz 2 Forces & Motion
CONTACT
FORCES
NON-CONTACT
FORCES
• An applied force is a force applied by a person or object
onto another object.
• An applied force can change the motion of an object. It can
cause an object to move in the same direction as the force.
It can also slow or stop a moving object.
© Stephanie Elkowitz 3 Forces & Motion
• Friction is a force that opposes motion. It works in the
opposite direction of a moving object.
• Friction is a force you must overcome to move a stationary
object.
• Friction is a force that causes moving objects to slow down.
© Stephanie Elkowitz 4 Forces & Motion
• The force of friction exerted by a surface depends on the
smoothness of the surface.
• A smooth surface exerts less friction than a rough surface.
• The surface of objects can be coated with liquid to reduce
friction. Liquid makes the surface smoother. This is why oil
is important to a car engine. The oil decreases friction
between the rubbing parts in the engine.
© Stephanie Elkowitz 5 Forces & Motion
• Only solids exert friction.
• Gases and liquids resist
motion. This resistance is
called drag.
• Air resistance is a type of
drag. It is like “air friction.”
• The direction of air
resistance opposes the
direction of motion.
• Air resistance slows falling
objects. It also slows an
object moving through air,
like planes and cars.
© Stephanie Elkowitz 6 Forces & Motion
• Air resistance does NOT depend on the mass of an object.
• Air resistance depends on the speed, shape and orientation
of an object moving through air.
• A fast moving object experiences more air resistance than a
slow moving object.
• An object shaped and orientated so it has more surface
area in contact with air experiences more air resistance.
• Planes are streamlined to reduce air resistance. This allows
them to fly faster through the air.
• Parachutes are large so to “capture” more air resistance.
Parachutes slow the downward movement of an object
through air.
© Stephanie Elkowitz 7 Forces & Motion
• Gravity, or gravitational
force, is a force of
attraction between two
objects.
• All objects with mass exert
a gravitational force.
• Larger objects exert a
greater gravitational force.
• We only notice the
gravitational force of very
large objects, such as
stars and planets.
© Stephanie Elkowitz 8 Forces & Motion
• Gravity is the force that attracts objects to Earth. It pulls
objects towards the center of Earth.
• Earth’s gravity also keeps the moon in orbit around Earth.
© Stephanie Elkowitz 9 Forces & Motion
• The force of gravity decreases when the distance between
objects increases. This explains why the moon does not fall to
Earth’s surface, but objects in Earth’s lower atmosphere do. The
moon is 238,900 miles away from Earth! At this distance, the
strength of Earth’s gravity is strong enough to keep the moon in
orbit but not so strong that the moon crashes to Earth’s surface.
© Stephanie Elkowitz 10 Forces & Motion
Why doesn’t
gravity cause
the moon to
crash into
Earth?
• The sun’s gravity keeps Earth and other planets orbiting
around sun.
• The Earth is at the perfect distance from the sun (based on
its mass and other factors). The sun’s gravity keeps Earth in
orbit but does not pull Earth so much that the planet
crashes into the sun.
© Stephanie Elkowitz 11 Forces & Motion
• Weight is the result of gravity pulling on an object.
• Weight is NOT the same as mass.
• Mass is the amount of matter in an object. Mass is
measured in kilograms (kg).
• Weight is a measure of gravity’s effect on mass. Weight is
measured in Newtons (N).
© Stephanie Elkowitz 12 Forces & Motion
• The force of gravity is much less on the moon because the
moon is much smaller than Earth. Because gravity is less, a
person’s weight is less. A person can jump higher using the
same amount of force as he would use on Earth since he
has to overcome a lesser force of gravity (weight).
© Stephanie Elkowitz 13 Forces & Motion
Why can an
astronaut
jump so
high on the
moon?
THINK ABOUT IT...
The force of gravity is greater on Jupiter because Jupiter is
larger than Earth.
Would a person’s weight on Jupiter be greater, less than or the
same as his weight on Earth?
Would a person’s mass on Jupiter be greater, less than or the
same as his mass on Earth?
© Stephanie Elkowitz 14 Forces & Motion
THINK ABOUT IT...
The force of gravity is greater on Jupiter because Jupiter is
larger than Earth.
Would a person’s weight on Jupiter be greater, less than or the
same as his weight on Earth?
IT WOULD BE GREATER.
Would a person’s mass on Jupiter be greater, less than or the
same as his mass on Earth?
IT WOULD BE THE SAME.
© Stephanie Elkowitz 15 Forces & Motion
You can calculate weight using the equation:
Weight (Fg) = Mass (m) × Gravity (g)
Gravity on Earth is 9.8 m/s2
Example:
Mass = 10 kg Weight = Mass × Gravity
Gravity = 9.8 m/s2 Weight = 10kg × 9.8 m/s2
Weight = 98 N
© Stephanie Elkowitz 16 Forces & Motion
TRY IT:
What is the weight of a 50 kg person on Earth? On the moon?
© Stephanie Elkowitz 17 Forces & Motion
Formula: Fg = m × g
Gravity on Earth = 9.8 m/s2
Gravity on Moon = 1.6 m/s2
TRY IT:
What is the weight of a 50 kg person on Earth? On the moon?
Earth: Moon:
Fg = m × g Fg = m × g
Fg = 50 kg × 9.8 m/s2 Fg = 50 kg × 1.6 m/s2
Fg = 490 N Fg = 80 N
© Stephanie Elkowitz 18 Forces & Motion
Formula: Fg = m × g
Gravity on Earth = 9.8 m/s2
Gravity on Moon = 1.6 m/s2
• A magnetic force is an invisible force created by magnets. It
is a force that pushes or pulls magnetic objects towards or
away from the magnet.
• The area of a magnetic force around a magnet is called the
magnetic field.
© Stephanie Elkowitz 19 Forces & Motion
• An electric force is an invisible force created by electrically
charged objects. Objects with an electric charge are
attracted to or repelled by other objects with an electric
charge. This attraction and repulsion is called electric force.
• The area of electric force around an electrically charged
object is called an electric field.
© Stephanie Elkowitz 20 Forces & Motion
• Tension is created when two objects pull on a rope, string,
wire or cable in opposite directions.
• Tension is the force created in the wire when objects pull on
the wire. Tension pulls on the objects equally towards the
center of the wire.
• If a person pulls on a wire anchored to a wall, tension in the
wire pulls back on the person towards the wall.
© Stephanie Elkowitz 21 Forces & Motion
• Spring force is a force created by a stretched or compressed
spring.
• When a spring is compressed, it wants to push outwards to
its neutral/resting position.
• When a spring is stretched, it wants to pull inward to its
neutral/resting position.
© Stephanie Elkowitz 22 Forces & Motion
• A fluid pushes upward on an
immersed object. This force is
called buoyancy or buoyant force.
• If buoyancy is equal to or greater
than the weight of an immersed
object, the object will float.
• If buoyancy is less than the weight
of the object, the object will sink.
• Buoyant force is directly related to
the density of the fluid and how
much fluid is displaced, or moved,
by an immersed object.
© Stephanie Elkowitz 23 Forces & Motion
• The normal force is a force exerted by
a surface on an object resting on that
surface.
• On a level surface, the normal force
is equal and opposite to the weight of
the object.
• The normal force explains why a book
resting on a table does not move. The
force of gravity (weight) pulls the
book down to the surface. A force
acts in an equal and opposite
direction so that the book does not
move. This force is the normal force.
© Stephanie Elkowitz 24 Forces & Motion
• When the forces on an object are equal and balanced, the
object’s motion does not change. If the object is at rest, it
will stay at rest. If the object is moving, it will continue
moving in the same direction with the same speed.
© Stephanie Elkowitz 25 Forces & Motion
• When the forces on an object are NOT equal and balanced,
the object’s motion will change. The objects speed, position
and/or direction will change.
• The change in an object’s motion depends on the net force
acting on the object and the mass of the object.
© Stephanie Elkowitz 26 Forces & Motion
• The combined result of all forces acting on an object is
called the net force.
• If two forces are acting in opposite directions, the net force
is the difference between the two forces.
© Stephanie Elkowitz 27 Forces & Motion
Net Force = 10 N – 5 N
Net Force = 5 N to the RIGHT
• The strength of force is directly related to an object’s mass
and acceleration.
• You can calculate the force of an object using the equation:
Force (F) = Mass (m) × Acceleration (a)
• This equation tells us the force of an object depends on
how massive the object is and how much the object is
accelerating.
• Objects with a greater mass have greater force.
• Objects with a greater acceleration have greater force.
© Stephanie Elkowitz 28 Forces & Motion
A truck has a mass of 2,000 kg. It is accelerating 20 m/s2.
What is the truck’s force?
A car has a mass of 1,500 kg. It is accelerating at 20 m/s2.
What is the car’s force?
© Stephanie Elkowitz 29 Forces & Motion
A truck has a mass of 2,000 kg. It is accelerating 20 m/s2.
What is the truck’s force?
F = m × a F = 2,000 kg × 20 m/s2
F = 40,000 N
A car has a mass of 1,500 kg. It is accelerating at 20 m/s2.
What is the car’s force?
F = m × a F = 1,500 kg × 20 m/s2
F = 30,000 N
© Stephanie Elkowitz 30 Forces & Motion
• Motion is the movement of an object.
• An object moves when unbalanced forces act on the object.
• Pushing or pulling an object will change an object’s position
and/or direction.
© Stephanie Elkowitz 31 Forces & Motion
• The motion of an object is described with respect to some
other object or position.
• The motion of an object is described by its position,
direction of motion and speed.
– Position: on top of, next to, over, under
– Direction: up/down, left/right, north/south
– Speed: miles per hour (mph), meters per second (m/s)
© Stephanie Elkowitz 32 Forces & Motion
• Velocity describes the speed and direction of an object’s
motion
• Speed is the distance traveled in a certain amount of time
• Direction is the way or path an object moves
• You can calculate velocity using the equation:
velocity (v) = distance (d) ÷ time (t)
• Velocity is measured in meters/second (m/s)
EXAMPLE:
A car travels east 100 meters in 2 seconds.
Velocity = distance ÷ time
Velocity = 100 meters ÷ 2 seconds
Velocity = 50 m/s east
© Stephanie Elkowitz 33 Forces & Motion
• Acceleration describes the change in an object’s velocity
• Objects that speed up have a positive acceleration
• Objects that slow down have a negative acceleration
(this is also called deceleration)
• You can calculate acceleration using the equation:
acceleration (a) = change in velocity (v) ÷ time (t)
• Acceleration is measured in meters per second2 (m/s2)
EXAMPLE:
A plane’s velocity changes from 0 to 100 m/s in 5 seconds.
Acceleration = change in velocity ÷ time
Acceleration = (100 m/s – 0 m/s) ÷ 5 seconds
Acceleration = 20 m/s2
© Stephanie Elkowitz 34 Forces & Motion
• Graphs can be used to describe the motion of an object
• A distance vs. time graph shows velocity
• A velocity vs. time graph shows acceleration
• What do the following graphs show?
© Stephanie Elkowitz 35 Forces & Motion
© Stephanie Elkowitz 36 Forces & Motion
Zero velocity
because there’s no
change in distance
over time
Constant velocity
because distance
directly increases
over time
Increasing velocity or
acceleration because
distance exponentially
increases over time
Zero acceleration
because velocity
does not change
over time
Positive acceleration
because velocity
increases over time
Negative acceleration
(deceleration)
because velocity
decreases over time
• Some objects move in an
expected or cyclical way
• Patterns of motion can
help you predict the
position, speed and
direction of an object
• Examples
– Sliding/Linear Motion
– Spinning/Rotation
– Circular/Revolution
– Rolling
– Periodic (swinging,
rocking, vibrating)
© Stephanie Elkowitz 37 Forces & Motion
Periodic Motion
• Sliding/Linear Motion
– Motion along a straight line or path
• Spinning/Rotation
– Movement of an object around a fixed point
• Circular/Revolution
– Movement of one object in a circular path around a second object
• Rolling
– Combination of linear motion and rotation; an object spins as it moves along a straight path
© Stephanie Elkowitz 38 Forces & Motion
© Stephanie Elkowitz 39 Forces & Motion
• Periodic Motion
– A group of motion patterns
– A motion that recurs over and over in a regular and repeating way
– Examples: rocking, bouncing, vibrating, swinging
• Isaac Newton was a
scientist and
mathematician who lived
1643 – 1727.
• He developed three laws of
motion to describe how
forces interact with objects
and cause motion.
• Newton also made
important findings about
gravity and how to calculate
the gravitational force
between two objects. © Stephanie Elkowitz 40 Forces & Motion
What does this mean?
This means that objects
want to keep on doing
what they are doing.
Objects resist changes to
their state of motion. If
there are no unbalanced
forces, an object will
maintain its state of
motion.
© Stephanie Elkowitz 41 Forces & Motion
An object at rest will
remain at rest unless
acted on by unbalanced
forces. An object in
motion continues in
motion with the same
speed and direction
unless acted on by
unbalanced forces.
• Newton’s 1st law is also called the “Law of Inertia.”
• Inertia is the tendency to resist change in motion.
• Inertia explains why it takes time for a car to come to a
stop. A car moving forward wants to continue its motion.
When the driver pushes on the breaks, the car and the
passengers inside the car want to continue moving forward.
© Stephanie Elkowitz 42 Forces & Motion
• Momentum is the quantity of motion an object has.
• The momentum of an object depends on the object’s mass
and velocity.
• A heavy and fast moving object has more momentum than a
lightweight and slow moving object.
• More force is needed to change the motion of a heavy and
fast moving object than a lightweight and slow moving object
because it has more momentum.
© Stephanie Elkowitz 43 Forces & Motion
© Stephanie Elkowitz 44 Forces & Motion
Two football players are running
down the field with the same
speed. One player has a mass of 70
kg. The other has a mass of 100 kg.
Which player has more
momentum?
Which player is harder to tackle?
Two football players are running
down the field with the same
speed. One player has a mass of 70
kg. The other has a mass of 100 kg.
Which player has more
momentum?
The 100 kg player.
Which player is harder to tackle?
The 100 kg player because he has
more momentum.
© Stephanie Elkowitz 45 Forces & Motion
What does this mean?
This means that more
force is needed to move
heavier objects. This law
also explains what
happens when you apply
an equal force to a heavy
and a lightweight object –
the lightweight object
moves (accelerates) more.
This law establishes the
equation F = ma. © Stephanie Elkowitz 46 Forces & Motion
An object accelerates
when a force acts on an
object with mass. The
greater the mass of the
object being
accelerated, the more
force needed to
accelerate the object.
• Example: If a boy applies the same force to each wagon, the
wagon that has twice as much mass will accelerate half as
fast. It would take twice the amount of force to accelerate
the wagon with 20 kg the same as the wagon with 10 kg.
© Stephanie Elkowitz 47 Forces & Motion
What does this mean?
This means there is a
force equal in size but
opposite in direction for
every force. In other
words, when one object
pushes on a second
object, the second object
pushes back on the first
object in the opposite
direction equally hard.
© Stephanie Elkowitz 48 Forces & Motion
For every action
there is an equal
and opposite
reaction.
• Example: When a rocket
blasts off, the force of its
powerful engines pushes
down on Earth’s surface.
This is the action. The
reaction is that Earth’s
surface pushes the rocket
upward with an equally
strong force. This causes
the rocket to move upward
into space.
© Stephanie Elkowitz 49 Forces & Motion
• A collision is an interaction between two objects that
physically come into contact with each other.
• A collision does not necessarily involve an accident – it is
any event where two objects bump into each other.
• Newton’s 3rd Law describes what happens during a collision.
The force exerted by one object is equal and opposite to the
force exerted by the second object.
© Stephanie Elkowitz 50 Forces & Motion
• Example: If a pool stick collides
with a pool ball, the force
exerted by the stick onto the
ball is equal and opposite to
the force exerted by the ball
onto the stick.
• Momentum is conserved during
a collision. This means the total
momentum of the two objects
before the collision equals the
total momentum of the two
objects after the collision.
• If the mass of each object stays
the same, the velocity of the
objects must change. This
explains why the speed and/or
direction of movement changes
for one or both objects during a
collision.
© Stephanie Elkowitz 51 Forces & Motion
Remember...
The momentum of an object
depends on mass and velocity.
Car Accidents (Inertia)
• During a car accident, the vehicle
(and passengers in the vehicle)
have inertia. When a car comes
to an abrupt stop, the vehicle and
passengers in the vehicle want to
continue moving forward. The
vehicle will crumple against the
object(s) it crashes into to, forcing
it to come to a stop. However,
passengers will continue to move
forward. Seatbelts help keep
passengers from being ejected
from the vehicle. Seatbelts apply
a force against passengers so
they stay within the vehicle. © Stephanie Elkowitz 52 Forces & Motion
Airplanes (Unbalanced Forces)
• Airplanes are able to fly because
of the shape of their wings.
When a plane propels forward,
the wings move through the air.
Air that moves under the wing
creates an upward force called
lift. The faster the plane moves,
the greater the upward force
(lift). When lift is greater than
the force of gravity acting on the
plane (weight), the plane
elevates in the sky. Airplanes
adjust their speed and the
shape of the wing to rise, stay
steady or lower in the sky. © Stephanie Elkowitz 53 Forces & Motion
Gravity
(weight)
Lift
Space Rockets (Newton’s 3rd Law)
• Rockets provide an excellent example
of Newton’s 3rd Law of Motion. The
engine of a rocket creates a strong,
downward force toward the surface of
Earth. This action is counteracted by a
reaction. The ground exerts an equally
strong but opposite force on the
rocket. This reaction causes the
rockets to move upward, through the
atmosphere and into space.
© Stephanie Elkowitz 54 Forces & Motion
Life Jackets (Buoyancy)
• Life jackets or vests help keep a
person afloat in water. A life
jacket is filled with trapped air.
When the jacket is submerged in
water, the jacket displaces some
water. The trapped air in the life
jacket weighs much less than the
water it displaces. So, water
pushes up harder than the life
jacket pushes down. This creates
buoyancy. When worn by a
person, the life jacket essentially
decreases the weight of a person.
It provides buoyancy (buoyant
force) to keep the person afloat. © Stephanie Elkowitz 55 Forces & Motion
© Stephanie Elkowitz 56 Forces & Motion
G-Force (Gravity)
• Some amusement rides, including
rollercoasters, accelerate
extremely fast, giving riders the
feeling of heaviness. This feeling
created by high acceleration is due
to a force called g-force. One
g-force is equal to the force of
gravity at Earth’s surface. An
amusement ride that produces a
force twice that of gravity is said to
produce 2 g-forces. Some rides,
including the Gravitron, produce
more than 3 g-forces. Flights into
space are known to produce more
than 7 g-forces!
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