Think, pair and share…

1. Draw the diagram below in your journal.2. What would happen if both people pushed in the same direction?

Why?3. What happens when the people are pushing in the opposite direction?

Why?4. Write your answers in your journal and then share with your partner.

FORCES

A FORCE IS A

OR

Forces are measured in Newtons (N)

In a force diagram, a force may be represented by an arrow (shows the direction in which the force is acting)

A

B

CD

A force may be represented by an arrow. Since forces have both direction and magnitude (size), the length or the thickness of the arrow will give you a relative idea of the size of the force.

A

B

CD

Force Diagrams

Forces cause changes in motion

• In order for a motion to start, stop, speed up, slow down, or change direction you must have a force.

Forces cause changes in motion

• Describe how you used force to create changes in the car’s motion

Forces cause changes in motion

• What did you do to start your car moving?• Pushed it from the back• What did you do to stop your car?• Pushed it from the front• What did you do to change your car’s

direction?• Pushed it from the side• What did you do to speed your car up?• Pushed it in the direction it was already

going

Types of Forces1. Gravitational force (Gravity)2. Friction3. Applied force4. Net force5. Balanced force6. Unbalanced force

What are some common forces?

Gravitational Force (Gravity)

• The force of attraction between objects that is due to their mass is called gravity.

• It is what makes things fall to the ground on Earth.

Gravity also holds Earth and the other planets in their orbits around the Sun.

Gravitational Force (Gravity)

• The force of gravity also exists on the Moon but it is not as strong as it is on the Earth.

• This is because the Moon is much smaller than the Earth.

Planet A has two times as much mass as Planet B. How will the gravity on the planets compare?

• The gravity on Planet A will be twice that of Planet B.

• An astronaut could jump half as high on Planet A.

• On Planet A it takes twice the amount of force to jump the same height as on Planet B.

Planet A

Planet B

Mass vs. Weight

• The mass of an object is how much matter it contains. It is measured in grams (g) or kilograms (kg) but is NOT a force.

• The weight of an object is the force caused by gravity pulling down on the mass of an object. It is measured in Newtons (N).

Spring scale, spring balance, Newtonmeter or forcemeter

• Used to measure forces in Newtons (N)

• Weight is measured using a springscale.

• The bigger the weight attached to the springscale, the more the spring inside the springscale stretches.

Force on an inclined plane• How does the angle of the inclined plane

affect the amount of force needed to move the car up the ramp?

• Independent (manipulated) Variable:

• Dependent (responding) Variable:

• What should you keep constant?

Force needed to move the car

Angle of the inclined plane

Distance Mass Acceleration

Force on an inclined plane• How does the mass of the car affect the

amount of force needed to move the car up the ramp?

• Independent (manipulated) Variable:

• Dependent (responding) Variable:

• What should you keep constant?

Force needed to move the car

Mass of the car

Distance Angle of inclined plane

Acceleration

Force on an Inclined Plane

• You have a car on an inclined plane.• Attach the spring scale to the front of the car.• Attach different amounts of mass to the car and

measure the force it takes to move the car 10 cm at different angles

Force on an Inclined Plane

• How did the angle of the inclined plane affect the amount of force needed to pull the car?

• How did the mass of the car affect the amount of force needed to pull the car?

The larger the angle/steeper the incline, the more force needed to pull the car

The larger the mass of the car, the more force needed to pull it

• When two forces acting on an object are 1. equal in size 2. but act opposite in direction,

• we say that they are balanced forces. • Balanced forces do not change motion.

Balanced Forces

Balanced Forces• If the forces on an object are balanced (or if there are

no forces acting on it) this is what happens:– an object that is not moving stays at rest– an object that is moving continues to move at the same

speed and in the same direction

The forces on this hanging crate are equal in size but act in opposite directions. The weight pulls down and the tension in the rope pulls up.

The forces on this hanging crate are balanced.

Hanging Objects

Floating in WaterObjects float in water when their weight is balanced by the upthrust, or buoyancy, from the water. The object will sink until the weight of the water it pushes out of the way is the same as the weight of the object.

A boat floats because its weight is balanced by the upthrust from the water.

Buoyancy – a force that pushes up objects immersed in fluids.

Float Hover

Sink

Balanced Forces

QUESTION: How is the ruler balanced on the edge of the table?The forces on the ruler are balanced.The force from the handle of the hammer (its weight) is pushing up with equal and opposite strength as the force of gravity is pulling down on the ruler.

Hammer Balance

Unbalanced Forces

• Unbalanced forces change the way something is moving. Unbalanced forces can make objects start to move, speed up, slow down, or change direction.

• When two forces acting on an object are not equal in size, we say that they are unbalanced forces.

• If the forces on an object are unbalanced this is what happens:– an object that is at rest starts to move– an object that is moving changes speed or direction

Unbalanced Forces• Unbalanced forces always cause a change in

motion. • They are not equal

• OR they are not opposite.

Unbalanced Forces

Two important things to remember:1) forces in the same direction combine by

addition2) forces in opposite directions combine by subtraction.

Balloon Rocket

As more air escapes from a balloon, it will travel a greater distance on a string.

Independent (manipulated) Variable:-Amount of propellant (air) in the balloonDependent (responding) Variable:-Distance the balloon travels

Balloon RocketAs more air was added to the balloon, what happened to the distance it traveled on the string?

As more air was added to the balloon, it traveled a greater distance on the string.

Balloon Rocket

The forces acting on the balloon when it is at rest are balanced.The forces acting on the balloon when it is moving are unbalanced.The air inside the balloon is the source of the force making the balloon move. The more air that is added, the more the balloon will move on the string.

Explanation:

Are there different forces acting on the figure skaters?

What direction are the forces acting?Are the forces balanced or unbalanced?

How do you know?

When a satellite orbits earth are the forces balanced or unbalanced?

Applied ForceForce applied to an object by a person or another object

Applied ForceThe applied forces on the box are: 20 N to the right20 N to the left

The forces on the box are balanced.

QUESTION: Is the box in motion?

• The image below shows a force of 10 N being applied to an object.

APPLIED FORCE

• The force is being applied toward the right, so the object is accelerating to the right.

• No other force is acting on the object.

Net ForceThe size of the overall force acting on an object is called the net force.

If the forces are balanced, the net force is zero.

Net Force = 0 N

If the forces are unbalanced, the net force is the difference between the two forces.

Net Force is the combined effect of all forces acting on an object.

Net Force

The applied forces are 20 N to the right and 40 N to the leftThe net force on the box is 20 N to the left.

QUESTION:Is the box in motion?In which direction?

Net Force is the combined effect of all forces acting on an object.

Net Force is the combined effect of all forces acting on an object.

If the force that is exerted is equal and opposite, the two forces cancel each other out and the resulting force is zero newtons.

NO

BALANCEDAre the forces balanced or unbalanced?

Will there be a change in motion?

Tug of War

QUESTIONS:Who is applying more force?Which direction is the rope going in?

In a tug of war the forces are moving away from each other. If the two teams have equal strength, or force, the rope will stay pretty much in the same place. It would look like this.

Would there be a change in motion?

What is the net force? 0 N

NO

Does the diagram represent balanced or unbalanced forces?

• The arrows are the same size, therefore the forces are the same size

• The arrows are pointing in directly opposite directions, therefore the forces are directly opposite each other

Since the forces are equal and opposite, they are BALANCED

Does the diagram represent balanced or unbalanced forces?

There is a net force to the left. The forces are UNBALANCED.

• The arrows are different sizes, therefore the forces are different sizes

• The arrows are pointing in directly opposite directions, therefore the forces are directly opposite each other

Question to think about: What would happen to the coin if you pulled the card very slowly?

Coin DropWhat do you think will happen to the coin?

Describe the forces acting on the coin and motion of the coin

Why did the coin move the way it did?

FrictionWhenever an object moves against another object, it feels frictional forces.

These forces act in the opposite direction to the movement.

Friction makes it harder for things to move (e.g. easier to move on ice than on sandpaper).

Friction Friction can be useful:Friction between our shoes and the floor stop us from slipping

Friction between tires and the road stop cars from skidding

Air Resistance is a Type of Friction

Air resistance is caused by the frictional forces of the air against an object.

Paper DropAs the surface area of the paper decreases, the speed with which it falls increases.

Independent (manipulated) Variable:

Dependent (responding) Variable:Air resistance on paper

Surface area of paper

Paper DropExplanation:Since the surface area of the crumpled paper is less that the uncrumpled piece of paper, it experienced less air resistance, so it fell faster.

QUESTION: What would happen if you placed a book under the uncrumpled piece of paper and then dropped it? Why?

What do you think will happen to the coin when you pull the card slowly from the side?

Coin Drop Revisited

What happened to the coin?

Describe the forces acting on the coin and motion of the coin

Why did the coin move the way it did?

Coin Drop Revisited

The force of friction between the card and the coin made the coin move with the card

It did not fall in the cup

Gravity acting downward and Friction acting in the direction of the card’s motion the coin moved with the card

Frictional Forces of Different Surfaces

As the friction between an object and the surface increases, the force it takes to move the object across the surface increases.Independent (manipulated) Variable:

Dependent (responding) Variable:Amount of friction between the block and the surface

Amount of force needed to pull the block

Frictional Forces of Different Surfaces

On which surface did you have to apply more force to move the block?