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Science: FORCE and MOTION

Key Concepts:

Terms in this set (21)

-is a force that opposes motion
-if an object is already moving,friction can slow it down or make it stop
- objects on rough surfaces require force to move; objects on smooth surfaces require less force
Friction is a force that occurs when one object rubs against another object. Two factors determine the
amount of friction - (1) the kinds of surfaces, and (2) the force pressing the surfaces together.
● Friction is the force that acts to resist sliding between two surfaces that are touching. It can slow down or stop the motion of an object.
○ The slowing force of friction always acts in the direction opposite to the force causing the motion.
○ For example, friction slows or stops the motion of moving parts of machines.
○ Another example would be athletic shoes with tread grooves to increase friction have better traction
for starting or stopping motion than smooth-soled dress shoes.
● Friction can also be the force that makes it difficult to start an object moving. Enough force must be applied
to a nonmoving object to overcome the friction between the touching surfaces.
● The smoother the two surfaces are, the less friction there is between them; therefore, the moving object will not slow down as quickly.
○ Friction between surfaces can be reduced, in order for objects to move more easily, by smoothing the
surfaces, using wheels or rollers between the surfaces, or lubricating/oiling the surfaces.
○ If friction could be removed, an object would continue to move.
● The greater the force pushing the two surfaces together, the stronger friction prevents the surfaces from moving.
○ As an object gets heavier, the force of friction between the surfaces becomes greater.
○ To move a heavy object, a greater force must be applied to overcome the friction between the
surfaces.
"When one object exerts a force on a second object, the second one exerts a force on the first that is equal in magnitude and opposite in direction;" sometimes called the "Law of Action and Reaction"
○ Even though the forces are equal in magnitude and opposite in direction, they do not cancel each
other. This law addresses two objects, each with only one force exerted on it.
■ Each object is exerting one force on the other object.
■ Each object is experiencing only one force.
● The action and reaction forces are reciprocal on an object. Examples may include:
○ A swimmer swimming forward:
■ The swimmer pushes against the water (action force), the water pushes back on the swimmer
(reaction force) and pushes her forward.
○ A ball is thrown against a wall:
■ The ball puts a force on the wall (action force), and the wall puts a force on the ball (reaction
force) so the ball bounces off.
○ A person is diving off a raft:
■ The person puts a force on the raft (action force) pushing it, and the raft puts a force on the
diver (reaction force) pushing them in the opposite direction.
○ A person pushes against a wall (action force), and the wall exerts an equal and opposite force against
the person (reaction force).
○ The Space Shuttle engines push out hot gases (action force), and the hot gases put a force on the
shuttle engines (reaction force) so the shuttle lifts (there is no sling shot doing it!)
● Forces (including gravity and friction) can affect the speed and direction of an object.