Force & Motion Part 2

Mrs. Wright

LET’S REVIEW…

Force: A push of pull applied to an object

Friction: The force that one surface exerts on another when the two surfaces rub against each other.

FRICTION

Friction is caused by microscopic bumps on surfaces called: MICROWELDS

The picture on the right is of Aluminum Foil at 10,000X magnification.

Friction Is Very Useful…

Friction keeps our feet from sliding out from under us when we are walking

Friction keeps our shoelaces tied

Friction gives us the ability to open a round door knob.

Can you think of others things it would be hard to do without friction?

FRICTION

Depends On 3 Things

How hard the surfaces push together

The surface areas in contact with each other

Types of surfaces involved

Example of Friction

Sled moving across the snow

The bottom of the sled rubs against the snow

Fireman sliding down the pole

The skin of his hands rub against the polished metal

4 Types of Friction

Static Friction

The friction that acts on objects that are not moving

Initial friction when moving an object

Example

4 Types of Friction

Sliding Friction

Occurs when two solid surfaces slider over each other.

Example

4 Types of Friction

Sliding Friction

Occurs when two solid surfaces slider over each other.

Example

Rolling Friction

When an object rolls across a surface

4 Types of Friction

Fluid Friction

Occurs when a solid object moves through a fluid

Fluid – a material that FLOWS

Water

Oil

AIR

Example

How well were you listening?

STAND UP…

What are the four types

of friction?

Can you show me an

example of sliding friction?

Give me a new example of

rolling friction.

Give me a new example of

fluid friction

Another Type of Force…

GRAVITY

A force that PULLS objects toward each other

People all around the planet are being pulled toward the surface of the Earth.

Gravity pulls everything toward the Earth’s surface at a rate of 9.8m/s2

WEIGHT VS. MASS

Mass: the measure of the amount of matter in an object, measured in grams on a balance

Weight: A measure of the gravitational force exerted on an object BECAUSE of its mass

WEIGHT = Mass (kg) X Gravity (9.8m/s2)

100kg X 9.8m/s2

WEIGHT = 980N

WEIGHT VS. MASS

FORCES

The strength of a force is measured in the SI unit called the NEWTON (N).

You exert about 1N of force when you lift a small lemon

The direction AND strength of a force can be represented by an arrow.

The arrow points in the direction of the force. The length of the arrow tells you the strength of the force – the longer the arrow, the greater the force.

FORCES

NET FORCES

The combination of ALL forces acting on an object

The net force determines whether

An object moves & what direction it moves in

Combining Forces

When forces act in the same direction, the net force can be found by ADDING the strengths of the individual forces

Combining Forces

When forces act in opposite directions, they also combine to produce a net force, HOWEVER, you must pay attention to the direction of the each force.

When two forces act in opposite directions, they combine by SUBTRACTION

UNBALANCED FORCES

Unbalanced forces can cause an object to start moving, stop moving, OR change direction.

Unbalanced forces acting on an object result in a net force and cause a change in the object’s direction

Tug of War

Balanced Forces

Equal forces acting on one object in opposite directions are called balanced forces

Balanced forces acting on an object DO NOT change the object’s motion

Time To Stand Up

Again…

Explain Unbalanced

Forces

Give a new example of

an unbalanced force

Explain Balanced Forces

Give a new example of a

balanced force