#separator:tab #html:false #tags column:4 What is a force? A push or a pull phys-forces-motion What unit measures force? The newton phys-forces-motion What force pulls objects toward Earth? Gravity phys-forces-motion What does friction do? Slows or stops objects that slide against each other phys-forces-motion Which surface has more friction, ice or sandpaper? Sandpaper phys-forces-motion How can you reduce friction? Use oil, wheels or a smooth surface phys-forces-motion Types of friction Static, sliding, rolling and fluid phys-forces-motion Static friction Friction that keeps a resting object from moving phys-forces-motion Sliding friction Friction between surfaces sliding over each other phys-forces-motion Rolling friction Friction when an object rolls; usually smaller than sliding phys-forces-motion Air resistance A force that slows things moving through air phys-forces-motion Magnetic force A push or pull between magnets phys-forces-motion Normal force The push of a surface upward on an object resting on it phys-forces-motion Tension A pulling force through a rope or string phys-forces-motion Applied force A force given to an object by a person or thing phys-forces-motion Buoyant force The upward push of water on an object phys-forces-motion Why does a feather fall slower than a rock in air? Air resistance affects it more phys-forces-motion Would a feather and a hammer fall together on the Moon? Yes, with no air they fall at the same rate; astronaut David Scott showed this in 1971 phys-forces-motion Mass vs weight Mass is the amount of matter; weight is the pull of gravity on it phys-forces-motion Is your mass different on the Moon? No; your weight is, because gravity is weaker phys-forces-motion How much is gravity on the Moon compared to Earth? About one-sixth phys-forces-motion Balanced forces Forces equal and opposite; no change in motion phys-forces-motion Unbalanced forces Forces that change an object's motion phys-forces-motion Inertia The tendency of objects to keep doing what they're doing phys-forces-motion Why do you lurch forward when a car brakes? Your body's inertia keeps it moving forward phys-forces-motion Newton's first law in short Objects at rest stay at rest and objects in motion stay in motion unless a force acts phys-forces-motion Newton's second law in short More force means more acceleration; more mass means less phys-forces-motion Newton's third law in short Every action has an equal and opposite reaction phys-forces-motion Example of Newton's third law A rocket pushes gas down and the gas pushes the rocket up phys-forces-motion Speed How fast something moves phys-forces-motion Velocity Speed with a direction phys-forces-motion Acceleration Changing speed or direction phys-forces-motion Is a car turning at constant speed accelerating? Yes, because its direction is changing phys-forces-motion Free fall Falling with only gravity acting phys-forces-motion How fast do falling objects accelerate on Earth? About 9.8 meters per second every second phys-forces-motion Terminal velocity The top speed of a falling object when air resistance balances weight phys-forces-motion Projectile An object launched and moving through the air phys-forces-motion Momentum How hard it is to stop a moving object: mass times velocity phys-forces-motion Collision When two objects hit each other phys-forces-motion Elastic collision Objects bounce and kinetic energy is kept phys-forces-motion Inelastic collision Objects crumple or stick together and some energy changes form phys-forces-motion Centripetal force A force pulling toward the center of a circular path phys-forces-motion Centrifugal force The feeling of being pushed outward in a turn; really inertia phys-forces-motion Orbit The curved path of one object around another due to gravity phys-forces-motion Why do planets stay in orbit? Gravity pulls them toward the Sun while their motion carries them forward phys-forces-motion Who described the law of gravity in the 1600s? Isaac Newton phys-forces-motion Who showed that objects of different mass fall at the same rate in a vacuum? Galileo's ideas led there; later experiments confirmed it phys-forces-motion Seat belts protect people by Slowing the body gradually and holding it in place phys-forces-motion Airbags work by Spreading force over a longer time phys-forces-motion Pressure Force spread over an area phys-forces-motion Why do snowshoes help you walk on snow? They spread weight over a larger area phys-forces-motion Torque A force that causes turning phys-forces-motion Equilibrium A state in which forces are balanced phys-forces-motion Speed formula Speed = distance divided by time phys-forces-motion If you travel 100 km in 2 hours, your average speed is 50 km per hour phys-forces-motion Reference point A place or object used to describe an object's position phys-forces-motion