Force & Motion Content from State Frameworks and Other State Documents
Forces
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Forces are the pushes or pulls in nature produced by interactions between objects.
Forces can cause objects to start moving, stop moving, or change direction. Objects tend
to keep on doing whatever they’re doing (Law of Inertia). An object at rest stays at rest
and an object in motion stays in motion unless something else exerts a force on it. An
object’s size (mass) and the amount of force exerted on it affect its speed and direction.
When forces on an object are balanced, the object will either be at rest or move at a
constant velocity. When forces on an object are unbalanced, the object will change its
velocity.
Objects change their motion only when a net force is applied
Changes in motion always involve forces which push or pull on objects in a variety of
ways
Balanced forces result in an object remaining at rest or moving at constant velocity
When an unbalanced force acts on an object, the greater the mass of the object, the
smaller its acceleration
Energy is the ability to do work. Without energy, forces can’t be generated to make things
move or change.
While there are multiple definitions of the word work, the term in science means to exert a
force on an object in order to move it.
Misconception: The terms "energy" and "force" are interchangeable. Proper Conception:
Force is an action that causes an object to change its state of motion. Mechanical energy
is the ability of an object to do work. Energy in general is something that is inherent in all
matter and that is present in many different forms.
Misconception: An object at rest has no energy. Proper Conception: Objects at rest have
some form of potential energy and rest mass energy.
Misconception: Force acting on an object does work even if the object does not move.
Proper Conception: For work to happen an object must suffer a displacement due to the
action of a force that is applied to it.
Misconception: If an object is at rest, no forces are acting on the object. Proper
Conception: An object is at rest only when all forces acting on it are balanced.
Misconception: Force is a property of an object. An object has force and when it runs out
of force it stops moving. Proper Conception: Force is an action (external to the object) that
acts on an object.
Misconception: Equilibrium means that all the forces on an object are equal. Proper
Conception: Forces acting on an object in equilibrium might be of different magnitudes.
However, if the object is in equilibrium then the sum of all the forces acting on the object is
zero. In other words there is not a net force present.
Misconception: Only animate things (people, animals) exert forces; passive ones (tables,
floors) do not exert forces. Proper Conception: All objects animated or not exert forces on
each other. One force that is always present is the gravitational attraction between objects
due to the fact that they possess mass. Another type of force that is present between
objects are contact forces, normal and friction.
Misconception: The only ― ”natural motion” is for an object to be at rest and, if an object
is at rest, there are no forces acting on the object. Proper Conception: Forces are pushes
or pulls on objects that are needed to make an object change its motion. An object at rest
will remain at rest unless a force acts on it and an object in motion will remain in motion
unless a force acts on it.
Force & Motion Content from State Frameworks and Other State Documents
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Misconception: All objects eventually stop moving when the force is removed. Proper
Conception: An object will continue to move with the same velocity forever if no force acts
on it.
Misconception: The term "work" is synonymous with "labor". Proper Conception: The term
work in science is used to express the result of a force that is applied through a distance.
Misconception: To ― ”work” means to exert oneself by doing mental or physical activity for
a purpose
Acceleration/Velocity/Speed
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There are mathematical relationships between speed, velocity, acceleration, and time that
can be used to help understand motion.
Acceleration occurs when there is a change in velocity
Acceleration is how fast velocity changes and can be written as a positive or negative
number.
Acceleration occurs whenever there is a change in motion of an object. Acceleration can
involve a change speed and/or direction and accelerations are caused by net forces.
There are mathematical relationships between speed, velocity, acceleration, and time that
can be used to help understand motion.
Any object that is changing speeds or direction is accelerating. An object that is slowing
down has a negative acceleration which is commonly referred to as deceleration.
All objects fall at the same rate of acceleration independent of its mass if there are not
frictional forces acting upon them.
Velocity is a similar concept to speed.
An object will continue to move with the same velocity forever if no force acts on it.
An object moving at a constant velocity has balanced forces acting on it.
The velocity of an object describes both the speed of an object and the direction that the
object is traveling.
The velocity of an object describes both the speed of an object and the direction the
object is traveling. Acceleration occurs when there is change in velocity of an object.
Acceleration measures the rate at which an object’s velocity changes with time.
The average speed (rate of motion) of an object is the distance traveled by the object
divided by the time it takes to travel that distance.
Speed is a measure of how fast something is moving. The average speed is measured by
finding the distance traveled by the object divided by the time it takes to travel that
distance.
Speed describes the rate of motion of an object. Velocity describes both the speed and
the direction of an object.
An object’s mass and the amount of force exerted on an object affect its speed and
changing motion.
Misconception: Acceleration always means that an object is speeding up. Proper
conception: Objects can have a negative acceleration (a deceleration), which occurs
when an object is slowing down.
Misconception: Acceleration is the same as speed. Proper Conception: Acceleration is the
change of velocity of an object. Speed is the magnitude of the velocity; it indicates how
fast the object is moving.
Misconception: The acceleration of a falling object depends upon its mass. Proper
Conception: The acceleration of a falling object is independent of its mass. The
acceleration of a free falling object is always equal to the acceleration of gravity.
Force & Motion Content from State Frameworks and Other State Documents
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Misconception: If an object is accelerating, it always means that it is speeding up. Proper
Conception: Any object that is changing speeds or direction is accelerating. An object that
is slowing down has a negative acceleration which is commonly referred to as
deceleration.
Misconception: Velocity and speed are the same. Proper Conception: Speed describes
the rate of motion of an object. Velocity describes both the speed and the direction of an
object.
Velocity is speed in a particular direction and can be written as a positive or negative
number.
Friction
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Friction is the force between two objects when they rub together. Friction makes motion
more difficult to move any object, you have to push or pull it with a certain amount of
force. In this lesson, students will explore how friction -a force that opposes or resists the
motion between any two objects – makes motion more difficult. Friction is caused
whenever two objects that are touching have to slide against each other.
Friction is the force that opposes motion when one object comes in contact with another
Misconception: Friction always hinders motion. Thus, you always want to eliminate
friction. Proper Conception: Frictional forces are sometime useful as they may help
objects move. Consider trying to walk if friction were not present.
Misconception: Since friction is a force that hinders motion, you always want to reduce or
eliminate it. Proper Conception: Friction is often useful and beneficial. For example, the
tread of a tire is designed to maximize friction (traction) between the tire and the road and
friction can be used to produce heat when needed.
When you push something across a surface, the force of friction makes it harder to move
the object. The little bumps from one surface catch against the little bumps of the other
surface and friction is produced.
The amount of friction produced depends on the kind of surfaces that are touching and on
how hard they are pressing against each other. Surfaces like sandpaper with lots of
bumps tend to cause more friction than smoother surfaces like the aluminum foil. But
even smooth surfaces will cause friction too because they have bumps too --just not as
large and obvious as the ones in sandpaper.
Friction can be reduced by using a lubricant. Lubricants are slippery or oily substances
that act to reduce friction between two objects.
Friction is often useful and beneficial. For example, the tread of a tire is designed to
maximize friction (traction) between the tire and the road and friction can be used to
produce heat when needed.
Force & Motion Content from State Frameworks and Other State Documents
Gravity
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Gravity refers to the force of attraction between any two objects in the universe that have
a mass. The strength of this force of gravity depends on the mass of the objects and the
distance between them. Since the earth is an object with a very large mass, things on or
near the earth are pulled toward it by the earth's gravity. Gravity is a constant force that
causes constant acceleration. The constant acceleration causes change in velocity.
Every object exerts a gravitational force on every other object
Gravity is the earth’s pull on things. Things on or near the earth are pulled toward it by the
earth's gravity. Gravity is always present.
The force of gravity between two objects depends on their mass and the distance
between them
The force of gravity is the result of the attraction of the Earth on the objects around it.
Therefore, the force of gravity is always present.
Misconception: Mass and volume, which both describe an "amount of matter”, are the
same property. Proper Conception: Mass is a measure of the amount of matter. Volume is
the amount of space that matter occupies.
Misconception: Weight and mass is the same thing. Proper Conception: Mass is a
measure of the amount of matter that forms (or composes) an object. Weight is the result
of the force of gravity on the mass of an object.
Misconception: Heavier objects fall faster than light ones. Proper Conception: The weight
of an object does not affect how fast it will fall. Under frictionless conditions objects fall at
the same rate independently of their weight. All objects fall at the same rate of
acceleration independent of its mass if there are not frictional forces acting upon them.
Misconception: The gravitational force is the same on all falling bodies. Proper
Conception: The gravitational force acting on an object will change as the distance
between the objects change. The magnitude of a gravitational force increases as the
distance between the objects decreases.
Misconception: Gravitational force is not present in space. Proper Conception:
Gravitational forces are present everywhere where matter is present. Objects in space are
affected by the gravitational forces produced by other planets, stars, galaxies, etc.
Misconception: If there was no air there would be no gravity. For example, above the
earth's atmosphere there is no gravity, and you become "weightless". Proper Conception:
Gravitational forces are present everywhere where matter is present. Objects in space are
affected by the gravitational forces produced by other planets, stars, galaxies, etc.
Misconception: Gravity increases with height above the earth's surface. Proper
Conception: The magnitude of a gravitational force increases as the distance between the
objects decreases.
Misconception: It is natural for things to fall down toward the earth and there is no need to
consider gravity as a concept to help explain why objects in the air fall to the earth. Proper
Conception: Gravity is the earth’s pull on things. Things on or near the earth are pulled
toward it by the earth's gravity. Gravity is always present.
Misconception: Gravity only acts on things when they are falling. Proper Conception: The
force of gravity is the result of the attraction of the Earth on the objects around it.
Therefore, the force of gravity is always present.
With his Universal Law of Gravity, Newton showed us that any two objects in the universe
exert gravitational attraction on each other. The degree of this attraction depends on the
size (mass) of the objects and the distance between them. Since the earth is by far the
biggest object around us, it will exert the most pull on the other things around it (but the
other things will also pull back on the earth just a little).
Force & Motion Content from State Frameworks and Other State Documents
Simple Machines
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Simple machines are tools that help us do work. Simple machines make work easier for
us by changing the amount of force needed to do certain kinds of work.
Simple machines are all around us and can make work easier to perform.
Simple machines are all around us, they reduce the effort to do work.
Items that use multiple simple machines are known as complex machines. An example is
a door knob that is comprised of a wheel and axle system and a lever system.
Simple machines are tools that help us do work.
Simple machines make work easier for us changing the amount of force (pushing or
pulling) needed to do certain kinds of work.
Simple machines are tools that help us do work but they do not do it for us. Simple
machines make work easier for us changing the amount of force needed to do certain
kinds of work (i.e., provide a mechanical advantage).
lever – a rigid bar that pivots about a fixed point (fulcrum)
pulley – a wheel with a groove and a rope that is usually used to help lift an object by
changing the direction of a pull (force)
inclined plane – a slanted surface that is used to help raise an object
wheel and axel - a wheel is locked to a central rod (called the axel) so that when one
turns the other turns – a longer motion at the outside of the wheel is converted into a
shorter more powerful force at the axel, example: steering wheel – also works in reverse,
a short powerful force at the axel will move the wheels outer edge a long distance but
without much force, example, pedaling a bike
wedge – an inclined plane that moves – usually a wedge is a combination of two inclined
planes – wedges are used to split, cut, or divide other objects – an axe, knife, and our
canine teeth are examples of wedges
screw – a rod with a thread wrapped around it to form a spiral – it is used as a fastener to
hold objects together
Misconception: Machines put out more work than we put in. Proper Conception: Machines
don’t change the amount of work done just the amount of effort put in doing the work.
Misconception: Simple machines do work for us. Proper Conception: Simple machines are
tools that help us do work but they do not do it for us. Simple machines make work easier
for us changing the amount of force needed to do certain kinds of work (i.e., provide a
mechanical advantage).