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 MassWeight  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?

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 FrictionGravity  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.