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

Free flashcards from Peabody at peabody.kids/flashcards/phys-forces-motion