Friction and Gravity

What is friction?

• The force that two surfaces exert on each other when they rub against each other.

The strength of the force of friction depends on two factors:

• 1st- how hard the surfaces push together.• 2nd – the type of surfaces involved.

In General:

• Smooth surfaces produce less friction than rough surfaces.

There are Four Types of Friction

• Static friction• Sliding friction• Rolling friction• Fluid friction

Static Friction

• The friction that acts on objects that are not moving.

• You must use extra force to get it to move.

Static Friction• Static friction prevents sliding of the feet.

• Static friction is occurring between the shoes and the floor and between the box and the floor.

Sliding Friction

• Occurs when two solid objects slide over each other.

• In this case the sliding is between the box and the floor.

Rolling Friction

• Occurs when an object rolls across a surface.• Example: wheels on the street or bowling ball

on the floor.

Fluid Friction

• Fluids, such as water, oil, or air, are materials that flow easily.

• Occurs when a solid object moves through a fluid.

• When an object pushes fluid aside, friction occurs.

Fluid Friction“AIR”

• The friction is when the air is hitting the object.

Friction

• Friction

What is Gravity?• Gravity is the force that pulls objects towards

each other.• Example: The Earth pulls objects to the ground.

Factors Affecting Gravity

• Two factors affect the gravitational attraction between objects: mass and distance.

• Gravity

Gravity

1. An attractive force between any two objects.2. It pulls objects together.3. All objects fall to Earth at the same rate (near Earth’s surface)4. On Earth, objects accelerate at 9.8m/s²

Which two objects do you think would have greater gravity between them?

Effect of Mass a. b.

B is the answerThat’s because greater mass increases gravity. They are directly proportional

Effect of Distance

• Which two objects do you think would have greater gravity between them?

• A.

• B.

B is the answer. That’s because greater distance decrease gravity. They are indirectly proportional

A.

B.

Quick Check1. Which objects would have greater gravity between them if they are the same distance apart: Two apples or Two planets2. What has a stronger force of gravity between them:

Two planets that are 1,000 kmOr

The same planets when they are 1,000,000 km apart?

Quick Check

• What is the acceleration due to gravity for a falling apple?

• A brick?• A Walrus?

Free Fall

• When gravity is the only force acting on an object, it is in free fall. It will accelerate at _________.

• Objects dropped at the same time will hit the ground at the same time regardless of mass.

• But sometimes we notice that this isn’t true!! WHY???

• This is due to air resistance!!• If you dropped your textbook and a sheet of

paper at the same time, they would not hit at the same time. The paper has more air resistance.

• Think of a person falling with a parachute and a person falling without one.

Air Resistance

• Is friction experienced by an object falling through the air.

• This friction opposes acceleration.• Air resistance resists the acceleration of some

objects more than others.• Air resistance will reduce the amount of

acceleration.

Air resistance depends on four things:

1. Mass.2. Area in contact with the air.3. How fast the object can push air out of the way.4. How fast the object is falling.

Weight and Mass

• Weight: a measure of the force of gravity on an object.

• Mass: a measure of how much is in an object.

• Weight and Mass

Newton’s Law’s Of Motion

• There are three Law’s

1st Law- The Law of Inertia

• An object at rest will stay at rest, and an object in motion will stay in motion at a constant speed unless it is acted upon by an unbalanced force..

• Ex. A car is traveling down the road as 45 mph with you as a passenger. The driver suddenly applies the brakes and you move forward towards the dashboard continuing in the direction in which you were moving.

• 1st Law

2nd Law- Force and Acceleration

• Acceleration depends on the object’s mass and on the net force acting on the object.

Force=mass X accelerationOr

F=ma2nd Law

3rd Law- Law of Action and Reaction

• For every action, there is an equal, but opposite reaction.

• Ex. You and your friend are on skates. The two of you are facing each other and push off from each other. The both of you would have pushed with the same amount of force.

• 3rd Law

HOORAY!!!!

Finally The End!!!!!!!!!

Dragon Racer LabFriction on different surfaces

• http://www.cape.k12.mo.us/blanchard/hicks/news%20pages/Force%20pdf%20files/Dragon%20Racer.pdf

Many labs

• http://www.cape.k12.mo.us/blanchard/hicks/news%20pages/scienceforce.htm#Friction