Unit 2

Force

Force – push or pull

• a force always acts in a certain direction• ex. if you push something, the force is

in the direction of the push

Forces in Nature

• Electric force• ex. lightning

• Magnetic force• ex. magnetic poles

• Gravity

Weight is a force

Newton

• metric unit of force

What is gravity?

• Sir Isaac Newton• 1642 – 1727• The Principia

Gravity

• force of attraction between all objects in the universe

Gravity

• the larger an object’s mass, the greater the gravitational force

Gravity

• the greater the distance between two objects, the smaller the gravitational force• the force of gravity decreases by the

amount equal to one divided by the distance (d) squared• 1/d2

What is air resistance?

• air resistance• force that opposes the movement of

an object in air

Air Resistance

• a larger surface area usually results in greater air resistance

• lighter objects feel more air resistance than heavier objects

Terminal Velocity

• speed at which air resistance and gravity acting on an object are equal• as an object falls, it’s speed increases

at a steady rate until it reaches it’s terminal velocity

Vacuum – empty space

• in a vacuum, all objects fall at the same speed

What is friction?

• Friction• force that opposes the motion of an

object• the force of friction works in the opposite

direction of the force of motion

Types of Friction

• Sliding Friction• the source of friction is the contact

between two surfaces, at least one of which is in motion

Types of Friction

• Air Resistance• friction results from air pushing on an

object as it is moving

Types of Friction

• Rolling Friction• friction between two surfaces that are

not in constant contact• ex. wheels

Friction

• friction makes motion possible

• friction also makes it hard to move objects

• reducing friction makes it easier to move objects

How can friction be reduced?

• by changing sliding friction into rolling friction

• by using lubricants• Lubricants – substances that reduce

friction

What is pressure?

• Pressure• force per unit area

Pressure

• pressure is the amount of force acting on a surface

• pressure equals force divided by area• p = f/a

Pressure

• pressure can be changed by changing the amount of force acting on an area

• pressure can be changed by changing the area on which a force acts

Fluid Pressure

• pressure in gases and liquids

Air Pressure

• air pressure is caused by the motion of particles in the air

• you do not feel the weight of the air because the pressure inside your body is equal to the air pressure around you

Air Pressure

• air pressure changes with altitude• the higher you are, the lower the air

pressure

Air Pressure

• Barometer• instrument used to measure air

pressure

Water Pressure

• the force of water pressure on an object is the same in all directions

• water pressure changes with depth• the deeper you are, the greater the

water pressure

Water Pressure

• Manometer• instrument used to measure pressure

in a liquid

What is Bernoulli’s principle?

• Bernoulli’s Principle• principle that states that as the speed

of a fluid increases, its pressure decreases

Bernoulli’s Principle

• airplane wings are designed to use Bernoulli’s principle• air traveling over the wings moves

faster than air underneath• this reduces the pressure on top of

the wing, allowing it to be pushed upward

Bernoulli’s Principle

• three forces combine to help an airplane fly• the upward force on a plane’s wing is

called lift• a forward force, or thrust, helps the

plane take off and maintain air speed• the air resistance on a plane is called drag