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Science: FORCE and MOTION
Terms in this set (21)
a push or pull
-gravity is a force
-forces can come from different directions
-forces have a magnitude (strength) and a direction If an object is in motion and more force is applied to it, the object will begin moving faster.
● If two objects have the same mass and a greater force is applied to one of the objects, the object which
receives the greater force will change speeds more quickly. For example if a ball is hit harder, it will speed up faster.
● If an object must be slowed down quickly, the force applied to the object must be greater than what is
needed for a gradual slowing down. For example, the greater the force applied to the brakes of a bicycle, the
more quickly it will slow down or stop.
● Varying the amount of force applied to a moving object can also change the direction that the object is
moving more or less quickly. For example, a baseball pitched toward the batter may quickly change direction and speed if hit very hard, or may change direction and speed more slowly if hit softly as with a bunt.
-measured in newtons
-Sir Isaac Newton-founder of Newton's Laws of motion
-net force/ combination of all forces acting on an object + (SUM)
-determines motion of an object
-Magnitude and direction
balanced and unbalanced forces
-Balanced Force: Produce a net force that equals zero
Unbalanced Force: Produces a net force that is a non zero value
-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
A force is a:
push or pull
It can cause an object at rest to:
It can cause an object that is moving to change:
direction or speed or to stop moving
A force has both:
magnitude (strength) and direction
Forces can be represented as arrows with the length of the arrow representing the magnitude of the force and the head of the arrow pointing in the direction of the force.
Using such arrows, the resulting force (net force) and direction can be determined.
Forces acting on an object can be balanced or unbalanced.
An object with balanced forces:
Will NOT change its state of motion
Balanced forces will cause no change in the speed of an object.
• Balanced forces acting on an object in opposite directions and equal in strength do not cause a change in the speed of a moving object.
• Objects that are not moving will not start moving if acted on by balanced forces.
• In arm wrestling where there is no winner, the force exerted by each person is equal, but they are
pushing in opposite directions. The resulting force (net force) is zero.
• Or, in a tug of war, if there is no movement in the rope, the two teams are exerting equal, but opposite
forces that are balanced. Again, the resulting force (net force) is zero.
a measure of how fast something moves a particular distance (for example, meters) over a given
amount of time (for example, seconds). S= d/t
● Therefore, speed is the rate of change of the position of an object, or how far something will move in a given
period of time.
● Speed does not necessarily mean that something is moving fast.
The slope of the line can tell the relative speed of the object.
● When the slope of the line is steep, the speed is faster than if the slope were flatter.
● When the slope of the line is flatter, the speed is slower.
● When the slope of the line is horizontal to the x-axis, the speed is zero (the object is not moving).
Calculation of speed
• Average speed can be calculated by dividing the total distance the object travels by the total amount of time
it takes to travel that distance.
● While the speed of the object may vary during the total time it is moving, the average speed is the result of the total distance divided by the total time taken.
● Speed measurements contain a unit of distance divided by a unit of time. Examples of units of speed might include "meters per second" (m/s), "kilometers per hour" (km/h), or "miles per hour" (mph or mi/hr).
● Average speed can be calculated using the formula v=d/t where the variables are:
○ v is the average speed of the object with units of m/s
○ d is the total distance or length of the path of the object with units of m
○ t is the total time taken to cover the path with units of s
○ Speed cannot have a negative value.
● Velocity refers to both the speed of an object and the direction of its motion. (For the intent of this indicator, disregard the direction of the motion.)
● A velocity value should have both speed units and direction units, such as m/sec north, km/h south, cm/s left, or km/min down.
● If an object is moving forward, it has positive velocity. When an object is moving backwards, it has negative
○ When you throw a ball in the air, it has positive velocity. When it heads back towards you, it has
● If a heavy (more massive) object is in motion, more force must be applied to get the object moving faster.
● If the same force is applied to two objects, the object with the smaller mass will change speeds more quickly.
For example if a baseball and a bowling ball are thrown with the same force the baseball will speed up faster.
● In order to slow down or stop a heavier (more massive) object, the force on that object must be greater than
for a less massive object. For example, if the same braking force is applied to a small car and a large truck, the
car will slow down more quickly.
● It is more difficult to change the direction of a heavy moving object, than one that is lighter in mass.
Unbalanced forces are not equal, and they always cause the motion of an object to change the speed and/or direction that it is moving.
• When two unbalanced forces are exerted in opposite directions, their combined force is equal to the
difference between the two forces. (you will subtract)
• the magnitude and direction of the net force affects the resulting motion
• this combined force is exerted in the direction of the larger force
• if two students push on opposite sides of a box sitting on the floor, the student on the left pushes with
less force on the box than the student on the right side of the box. The box will move will be moved by
the student on the right.
Newton's First Law of Motion
"The velocity of an abject will remain constant unless a net force acts on it;" often call the Law of Inertia.
• If an object is moving, it will continue moving with a constant velocity (in a straight line and with a constant
speed) unless a net force acts on it.
• If an object is at rest, it will stay at rest unless a net force acts on it.
• the tendency of the motion of an object to remain constant in terms of both speed and direction.
• The amount of inertia that an object has is dependent on the object's mass. The more mass an object has the
more inertia it has.
• If an object has a large amount of inertia (due to a large mass)
o It will be hard to slow it down or speed it up of it is moving.
o It will be hard to make it start moving if it is at rest.
o It will be hard to make it change direction.
• Examples of the effects of inertia might include:
o Inertia causes a passenger in a car to continue to move forward even though the car stops.
This is the reason that seat belts are so important for the safety of passengers.
o Inertia is the reason that it is impossible for vehicles to stop instantaneously.
o Inertia is the reason that it is harder to start pushing a wheelbarrow full of bricks than to start pushing
an empty wheelbarrow. The filled wheelbarrow has more mass and therefore, more inertia.
o Inertia is also the reason that it is harder to stop a loaded truck going 55 miles per hour than to stop
a car going 55 miles per hour. The truck has more mass resisting the change of its motion and
therefore, more inertia.
Newton's Third Law of Motion
"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.
● a force that always attracts or pulls objects toward each other without direct contact or impact.
● Gravitational attraction depends on the mass of the two objects and the distance they are apart.
● Objects on Earth are pulled toward the center of Earth.
● The force of gravity, like all other forces, can cause changes in the speed of objects. As an object falls, its
speed will continually increase as Earth's gravity continually pulls it downward. When air resistance is ignored,
all objects will speed up at the same rate as they fall.
● Gravity can also cause an object that is thrown into the air to change its upward motion, slow down, and fall
back toward Earth's surface.
● The pull of Earth's gravity keeps the Moon in orbit; the moon is constantly changing direction because of
● Motion occurs when there is a change in position of an object with respect to a reference starting point.
● The final position of an object is determined by measuring the change in position and direction of the
segments along a trip. The following terms are used to describe and determine motion:
Position is the location of an object.
● An object changes position if it moves relative to a reference point.
● The change in position is determined by the distance and direction of an object's change in position from
the starting point (displacement).
Direction is the line, or path along which something is moving, pointing, or aiming.
● Direction is measured using a reference point with terms such as up, down, left, right, forward, backward,
toward, away from, north, south, east, or west.
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