A FORCE is a push or a pull on an object. A force results

• 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


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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


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.


• 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).


• 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).


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?


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.


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


TRY IT:

What is the weight of a 50 kg person on Earth? On the moon?


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


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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


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.


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?


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


• 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.


• 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)


• 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


• 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


• 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?



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)


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



• 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.


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.


• 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.



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.



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.



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.


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.


• 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.


• 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.


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.


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


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!

Documents

A FORCE is a push or a pull on an object. A force results