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