Part 1: Forces

Physics

Part 2: Motion

Part 3: Magnetism & Electricity

Part 1: Forces

Physics

WHAT IS FORCE?• OBJECTIVE• Analyze the resulting effect of balanced and

unbalanced forces on an object's motion in terms of magnitude and direction

WHAT IS FORCE?

• ANTICIPATORY ACTIVITY• Try to define the concept of force.

• A force is a push or pull that causes an object to move, stop, or change direction

• In physics a force is anything that makes an object accelerate

Example: Friction and gravity are two types of forces we encounter everyday.

WHAT IS FORCE?

HOW IS A FORCE REPRESENTED?

VECTORS represent forcesA vector is a physical quantity with direction and magnitude. Vectors are represented with arrows. The length of the arrow is the magnitude and its orientation or where it points is the direction.

Examples of vectors are: FORCE and VELOCITY.

Note: If a quantity has only MAGNITUDE and NO DIRECTION, is called SCALAR

Examples of scalar quantities are temperature, and time.

Skill Practice – Have student push a desk. Exerting a force on the desk causes it to move. Have two students push the desk in the same direction.

When two people exert a force on the object, in the same direction, the total force is the SUM of both forces exerted.

+ =

Skill Practice – Have one student push a desk one way and have another student push it the opposite way. (preferably same force in each direction)

When two people exert a force on the object, in the opposite direction, the total force is the DIFFERENCE of both forces exerted. If the force is the same, then they can cancel out each other.

- = 0

Net Force: The overall force on an object after all the forces are added together.

When there is a net force acting on an object, the force is said to be unbalanced.

An unbalanced force can cause an object to start moving, stop moving, or change direction. An unbalanced force acting on an object will change the object’s motion.

+ =

An unbalanced force will cause an object to accelerate. The object will accelerate in the direction of the GREATER force.

Equal forces acting on one object in opposite directions are called Balanced forces.

Balanced forces (equal) acting on an object will not change the object’s motion.

+ = 0

The net force is zero

• Consider an example of a balanced force - a person standing upon the ground.

• There are two forces acting upon the person.

• The force of gravity exerts a downward force.

• The force of the floor exerts an upward force.

• Since these two forces are of equal magnitude and in opposite directions, they balance each other.

• The person is at equilibrium. • There is no unbalanced force acting upon

the person and thus the person maintains its state of motion.

CHECK FOR UNDERSTANDINGWhat makes an arrow fly through the air to its

target?

What makes a long jumper thud to a stop?

What will make a soccer ball change direction?

If two kids are playing tug of war but the center is not moving they have ___________ force.

If one of the kids in the tug of war game pulls harder than the other then you have an ____________ force.

Which force will cause change in motion – balanced or unbalanced?

• Since these two forces are of equal magnitude and in opposite directions, they balance each other.

• The book is said to be at equilibrium. • There is no unbalanced force acting

upon the book and thus the book maintains its state of motion.

• When all the forces acting upon an object balance each other, the object will be at equilibrium;

• it will not accelerate.

Force

• Force Is measured in newton (N)– A newton is equal to the force of gravity

exerted in an object that weights 100 grams (about 1/5 of a pound).

Force• WORKING WITH FORCE VECTORS:

– The net force is the vector sum of all the forces. That is, the net force is the resultant of all the forces; it is the result of adding all the forces together as vectors.

Force VECTORS*Forces acting in the same

direction. (Net force is the sum of the forces).

*Forces acting in opposite directions (balanced if the net force is zero newton).

Combining forces

• Two men paddle their canoe on a river

Example #1• Two tugboats are moving a barge.

Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge?

Barge

Tugboat B

Tugboat A3000N

5000N

8000N = Total Force

Example #2• Now suppose that Tugboat A exerts a

force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge?

Barge

Tugboat B

Tugboat A2000N

4000N

2000N = Total Force

Friction and gravity: two types of forces we encounter everyday.

Gravity – is the force that acts to pull objects straight towards the center of the Earth.

– An objects speed increases as it falls.• Free Fall – When the only force acting on a falling object is gravity,

the object is said to be in free fall.

Friction – is the force that one surface exerts on another when the two rub against each other.

• Friction acts in a direction opposite to the object’s direction of motion.

• Without friction an object would continue to move at a constant speed forever.

Gravity• An object in free fall accelerates as it falls.

• In free fall, gravity is the only force acting on the object so it accelerates.– The rate at which objects accelerate is 9.8m/s/s.– This means that for every second an object is falling,

its velocity increases by 9.8m/s.

• ALL objects in free fall accelerate at the same rate.

• Despite the fact that all objects are supposed to fall at the same rate, we know that this is not always the case.

• For example, let’s think about the following example:– An maple leaf and an acorn falling.

Do they fall at the same speed?

What causes the speed to be different?

Friction

• The type of friction shown in the previous example is air resistance.

• The greater the surface area of an object, the greater the air resistance.

• Friction causes objects to slow down or stop.

Gravity pulling the acorn down.

Air resistance working against gravity is greater for the leaf.

Air resistance working against the acorn is less than the leaf Gravity

pulling the leaf down.

In this situation, the acorn falls faster because it has less surface area and therefore, less air resistance.

Friction• Friction is increased by the amount of

surface area the object has in contact with the surface.– For example, tires with more surface area

have better traction on the road.

• Friction causes objects to slow down or stop.

How is Force Calculated? f = m x a (Newton’s second law of motion)

Force ( f ) is anything that makes an object accelerate

mass ( m ) is a measure for the amount of matter in an object

Acceleration is the rate of change in velocity

BEWARE: velocity is different to speed!!!

*Velocity is a vector that has as magnitude (the speed - how fast) and as orientation the direction (+ or -)

Newton’s 1st Law (Also called the law of inertia)

• Greeks thought that the natural state of an object was at rest.– ex. A ball stops rolling– without friction an

object would never stop!• Newton’s 1st an

object in motion tends to stay in motion; an object at rest tends to stay at rest

Newton’s 1st Law (Also called the law of inertia)

• Inertia property of matter that resists a change in motion– An object with great mass has high inertia

Newton’s 2nd Law (f = m x a ) 2nd Law states that if a net force

acts on an object, it will cause an acceleration of that object.

An object will only accelerate

if there is an unbalanced force.

An object in motion will remain in motion without changing its speed or direction until an unbalanced force acts on the object.

Force = mass x acceleration

Newton’s 2nd Law (f = m x a )

FA = 1000 kg x 0.05 m/s/s

FA =

FB = 2000 kg x 0.05 m/s/s

FB =

Newton’s 2nd Law (f = m x a )

Newton’s 3rd Law (Action and reaction)

• 3rd Law states that for every action there is an equal and opposite reaction– If an object exerts a

force in a second object, the second exerts an equal and oppositely directed force in the first one

Newton’s 3rd Law (ACtion and reaction)

• For every action there is an equal and opposite reaction– A bird pushes down

on the air and the air pushes up on the bird

Newton’s 3rd Law

Newton’s Laws

• 1st Law: (inertia: objects tend to do what they are doing)– cannon ball will rest until a force is put on it– ball will roll straight until ramp puts a force on it

• 2nd Law: (f = m x a) force equals mass times acceleration– greater force put on ball accelerates it more– greater mass of ball but greater force on water

• 3rd Law: (every action has an equal but opposite reaction)– ball moves right, cannon recoils left– ball move down, water splashes up

Newton’s Laws

Check the following examples to understand how Newton's first law occurs in everyday events: a) car suddenly stops and you strain against the seat belt b) when riding a horse, the horse suddenly stops and you fly over its head c) the magician pulls the tablecloth out from under a table full of dishes d) the difficulty of pushing a dead car e) lawn bowling on a cut and rolled lawn verses an uncut lawn f) car turns left and you appear to slide to the right

Demo: coin falls if force removed

Newton’s Laws

Check the following examples to understand how Newton's second law occurs in everyday events:

a) hitting a baseball, the harder the hit, the faster the ball goes b) accelerating or decelerating a car c) The positioning of football players - massive players on the line with lighter (faster to accelerate) players in the backfield d) a loaded versus an unloaded truck

Demo: spring scale and acceleration

Newton’s Laws

Check the following examples to understand how Newton's third law occurs in everyday events:a) rockets leaving earth--many physicists of the nineteen hundreds (Goddard's time) said that rockets could never leave the earth. How a spaceship flies in space?b) guns being fired- Why they kick in proportion to the size of the bullet. c) two cars hit head on d) astronauts in space e) pool or billiards f) jumping out of a boat onto the dock g) sprinklers rotating Demo: rocket balloon

Physics

Part 2: Motion

Position, Speed and Velocity

• Space and Position• Graphs of Speed and Velocity• Working with Equations

Objectives

• Calculate time, distance, or speed when given two of the three values.

• Solve an equation for any of its variables.

• Draw and interpret graphs of experimental data, including velocity versus position, and speed versus time.

Position, Speed, and Velocity

Key Question:How are position, speed, and velocity related?

Space and position

• In physics, the word position refers to the location of an object at one instant.

• A position is always specified relative to an origin. • The net change in position relative to the origin is

called displacement.

Position and distance• Distance is a scalar quantity that refers to "how much ground an

object has covered" during its motion.• Displacement is a vector quantity that refers to "how far out of

place an object is"; it is the object's overall change in position.

Speed and distance• Speed is the rate at which distance changes. • In physics, the word rate means the ratio of how

much something changes divided by how long the change takes.

• Constant speed means the same change in distance is traveled every second.

• Speed is a scalar quantity that refers to "how fast an object is moving."

Calculating speed• The change in position is a

distance traveled in a given amount of time.

• To calculate the speed of an object, you need to know two things:– the distance traveled by the

object– the time it took to travel the

distance

Calculating speed

• Since speed is a ratio of distance over time, the units for speed are a ratio of distance units over time units.