FRICTION AND GRAVITY

EQ: What changes an object’s motion?

Friction and Gravity What happens when

you jump on a sled on the side of a snow-covered hill?

What happens at the bottom of the hill?

Friction and Gravity Why does the sled’s

motion change on the side of the hill and then again at the bottom?

Friction and Gravity The answer is the forces

of friction and gravity! We experience these two

unbalanced forces every day.

The force of gravity causes the sled to accelerate down the hill.

The force of friction eventually cause the sled to stop.

These two forces affect many motions on Earth.

Friction - -- A Definition Friction is an

unbalanced force caused by two objects rubbing together

Friction --- Examples

Chair on floor Sledding Skateboarding Surfing

Friction --- Characteristics Unbalanced force–

one force is stronger than the other

Acts in the opposite direction of motion

Friction – Characteristics The amount depends

on two things: How smooth the

objects are In general, smooth

surfaces produce less friction than rough surfaces.

How hard the objects push together Force applied More force = more friction

○ Rub hands together

Friction

There are four types of friction Static Sliding Rolling Fluid

Types of Friction---Static Friction

The friction that acts upon objects that are not moving.

You must use extra force to start the motion of the stationary object if you want to overcome the static friction.

Acts in the opposite direction of the intended motion

Example: (Pushing a chair)

Types of Friction ---Sliding Friction

When two solid surfaces slide over each other.

Works in the opposite direction of the object’s motion

Example: A bike’s brakes (Sledding)

Types of Friction---Rolling Friction When an object rolls

over a surface Works in the

opposite direction of the object’s motion

Easier to overcome than sliding friction for similar objects.

Examples: (Wheels ) Ball bearings

Types of Friction ---Fluid Friction

Occurs when a solid object moves through a liquid or a gas.

Easier to overcome than sliding friction.

Examples: Motor oil in an

engine (Surfing)

Quick Check --- Friction A force caused by two objects rubbing together is

__. Friction is an unbalanced/balanced force What is the relationship between direction and

friction? What is the relationship between force and friction? What is the relationship between surface material

and friction? Which type of friction occurs when you push in a

chair? What makes sliding friction different from rolling

friction

Gravity – A Definition Gravity is an

unbalanced force that pulls objects towards each other

Newton realized that gravity works everywhere in the universe, not just on Earth.

Law of Universal Gravitation

Gravity --- Characteristics

Gravity depends on mass.

Mass is how much matter is in an object.

Objects with a large mass have a greater force of gravity than objects with a small mass.

Gravity --- Characteristics The force of gravity

acts between all objects

Gravity depends on distance

As the distance between objects increases, the force of gravity decreases.

Gravity --- Characteristics

Weight measures the force of gravity on an object.

An object’s weight can change if the force of gravity changes.

An object’s mass always stays the same.

Mass vs. Weight

Mass Weight

The amount of matter in an object

Matter --- four states: solid, liquid, gas, plasma

An object’s mass always stays the same

Measured in kg/pounds Astronaut in a spacesuit: 165 kg (120lbs) on Earth,

165 kg (120lbs) on the Moon

Measures the force of gravity on an object.

Can change if the force of gravity changes

Measured in newtons (N)

Astronaut in a spacesuit: Weight on Earth: 1,617 N Weight on the Moon:

270N

Gravity and Motion Gravity is the downward

force that pulls objects towards the Earth.

When you hang on monkey bars, you exert a force that balances the force that balances the force of gravity, keeping you still. When you let go, the gravity becomes unbalanced and the you fall.

Gravity and Motion If gravity is the only

force pulling on a falling object, the object is in free fall,

In a free fall, the force of gravity is an unbalanced force, which causes an object to accelerate.

Acceleration: increasing speed, decreasing speed, or changing direction

Gravity and Motion Most objects move

through the air. Friction caused by

air is called air resistance.

Air resistance is a force that pushes upward on falling objects.

Gravity and Motion As an object falls to

Earth, its velocity increases.

Velocity: the speed of an object in a given direction

Ex: 20 m/s traveling downward

The greatest velocity it reaches is called its terminal velocity

Gravity --- Quick Check

A force that pulls objects toward each other is called__.

If two objects move further apart, the gravity between them increases/decreases.

An object’s mass __if less gravity acts on the object

What makes mass different from weight?

Gravity --- Quick Check

Friction and Gravity

How are friction and gravity similar? How are friction and gravity different? What are the effects of these similarities

and differences? What might be someone’s reason for

preferring friction to gravity? What might be someone’s reason for

preferring gravity to friction?

Paper Airplane Lab

Background A force is a push or

pull on an object Depending on if

these forces are balanced or unbalanced, they can cause an object to move.

Background: Two unbalanced

forces that we experience every day are the forces of friction and gravity.

Friction vs. Gravity

Friction Gravity

Friction is a force that opposes motion as one object rubs against another

Friction depends on two things: The surface of the material The force applied

There are four types: Static Sliding Rolling Fluid

The force of friction always acts in the opposite direction of the object’s motion

Gravity is the force that pulls objects toward each other.

Gravity depends on two things: The mass of the object The distance between the objects

Due to Earth’s large mass, all objects close to the Earth are attracted to it, like a magnet

Gravity acts on objects in a downward direction toward the center of the Earth

Today’s Lab

Purpose: To understand two or more forces acting on an object, including gravity and friction

Problem: Why don’t paper airplanes stay in the air forever?

Materials and Procedures Materials:

Paper * Meter stick * Paper clips*Binder clips

Procedure:You will be making and altering a paper

airplane per the directions on the lab sheet.You will then answer questions to narrow

your focus on to the forces of friction and gravity.