# Chapter 12: Gravity, Friction, and Pressure

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Chapter 12: Gravity, Friction, and Pressure
12.1 Gravity is a force exerted by masses 12.2 Friction is a force that opposes motion 12.3 Pressure depends on force and area 12.4 Fluids can exert a force on objects

Sideways-Stopping Friction (N) Forward-Stopping Friction (N)
Sticky Sneakers Lab: Friction depends on: the kinds of surfaces involved and how hard the surfaces push together Sneaker Starting Friction (N) Sideways-Stopping Friction (N) Forward-Stopping Friction (N) A B C…

Challenge! A jet engine generates 160 kN of force as it propels a 20,000kg plane down a runway. If 40 kN of friction opposes the plane, how much time it will take the plane to reach a speed of 33m/s from rest?

Forces and Surfaces A flat surface may look and feel smooth, but it has tiny bumps and ridges which lead to friction Types of surfaces: Depends on the materials that make up the surfaces: puck on ice vs floor Motion of the Surfaces: Apply a force to start an object moving: friction increases to keep it from sliding Frictional force has a limit to how large it can be, and with enough force you can make the object move

12.2 Friction is a force that opposes motion
Friction occurs when surfaces slide against each other Easier to push a box over tile than carpet Friction: force that resists the motion between two surfaces in contact Friction between your feet and ground Easier to exert a backward force on rougher surfaces, with the reaction force moving you forward more than a slick surface Provides the action and reaction forces that enable you to walk

Forces and Surfaces Force pressing the surfaces together:
The harder two surfaces are pushed together, the more difficult it is for the surfaces to slide over each other An object has weight, and the surface exerts an equal and opposite reaction force on the object – this increases friction force Friction depends on the force pressing the surfaces together, NOT the surface area over which the forces act

Friction and Heat Friction between surfaces produces heat
Rub your hands together Your energy is transferred to the individual molecules on the surface of your hands, causing them to move faster, therefore increasing the temperature Strike a match Brakes

Motion through fluids produces friction
Recall two objects falling in a vacuum fall with the same acceleration In air this is different – air is a fluid, a substance that can flow easily An object moving through a fluid pushes the molecules of the fluid out of the way The molecules of the fluid exert an equal and opposite force on the object, slowing it down “drag” Depends on the shape of the moving object “Air resistance” depends on the surface area and speed of the object (different than friction) Larger surface area comes into contact with more molecules than a smaller surface = increase surface are, increase air resistance Faster moving object comes into contact with more molecules in a given amount of time = increase speed, increase air resistance

Terminal Velocity Skydiver: eventually the air resistance balances gravity = terminal velocity

Friction Simulation Projects
_MS-Science PS-8th forces-and-motion_en.jar ramp-forces-and-motion_en.jar