EQ: What changes an object’s motion? Friction and Gravity

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)

EQ: What changes an object’s motion? Gravity

Quick Review --- 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. http://www.exploratorium.edu/ronh/weight/index.html

Mass Weight Mass vs. Weight 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 Mass Weight

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?

Paper Airplane Lab

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 Gravity Friction vs. Gravity 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 Friction Gravity

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 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.