Newton’s Laws Review
THE SCIENCES OF MOTION
Prior to this unit, we had been studying ____________________, which is the science of
describing motion with words, numbers, pictures, and symbols, and no attention was
given to the __________ of the motion.
In this unit, we turned our attention to ________________, which is the study of the
causes of motion, and unbalanced forces in particular.
Both kinematics, dynamics, and statics (the study of objects being acted upon by
balanced forces) all combine to form the science of __________________, the interaction
between matter and forces acting upon it.
THE HISTORY OF NEWTON’S LAWS OF MOTION
__________________ (384-322 BC) developed the earliest theory of motion. He was
one of the first s__________________ because he actually made observations upon
which he drew conclusions (i_________________ or h__________________). Prior to
Aristotle, “science” was nothing more than p__________________ aimed at the outside
world. Aristotle did not run e____________________ as we think of them however.
Aristotelian Physics was developed on the premise that all things were made of f______
e______________, but these differed from our present day ones. Aristotle said all
earthly objects were made of a combination of _________, ________, ________, and
__________. A fifth element, e_________, existed too; it was the building block of all
objects in the heavens. As such it was perfect and incorruptible.
The earthly elements had a natural order, which went from the bottom up __________,
__________, __________, and __________. Aristotle thought that nature sought to
maintain this natural order. Aristotle said when an object found itself outside of its
naturally ordered place, it would desire to return to its natural home. This desire was so
great that given the chance to go home, and object would move there on its own. This
motion, which would seemingly happen all by itself, Aristotle called ______________
motion. The important thing about this motion was the “happening all by itself” part.
Aristotle said that no “agent” ( __________, as we would call it) was required to sustain
Aristotle went on to say that motion that was not natural was v____________ motion.
This motion was imposed motion by an “agent” (__________), like a person or animal,
which did not result in an object being back in its natural place after its completion. This
motion would not happen on its own.
In Aristotle’s physics, a third type of motion was the motion of the heavenly bodies in
perfect circles around the earth. This motion was named __________________ motion.
There were problems with Aristotelian physics that even occurred to Aristotle himself.
First, p____________ did not seem to obey the circular orbit idea. Next, two objects of
different mass, but similar shape will fall at the _______ ________, even though the
heavier one should want to reach the earth more. Finally, an object thrown or fired, like a
rock or an arrow, will continue to cover a great horizontal distance without the benefit of
an applied force, and do not directly return to the ground when given the chance. These
problems were largely ignored and not rectified.
Despite its problems, Aristotle’s physics was widely accepted through the 1600’s AD. It
was so popular for a few reasons: it supported the earth-centered (______-centric) view
of the universe that was so prevalent during the times; the idea that a __________ is
required to sustain horizontal motion seems to make sense in the real world and is
consistent with everyday observation; and Aristotle was Aristotle-the leading authority on
In the early 1500’s, Nicolaus C__________________ began to notice that the
mathematical model of the universe comes out a lot simpler if you assume the sun is the
center of the universe, rather than the Earth. This view of the universe is called a
h________-centric model. This theory contains a moving earth, which hindered its
acceptance since the prevailing thought is violent motion is caused by forces, and what
could exert such a force?
Copernicus’ work on motion was built upon my G____________, who faced much
persecution, even ten years __________ ____________ for his work in motion and the
universe. Galileo developed many strategies that scientists use as second nature today.
Galileo liked Aristotle’s stance of making observations and hypotheses, but Galileo was
the first to realize that ______________________ must also be done to test the
hypotheses. Galileo was also the first to take small details out of an experiment to see the
main idea. Today, we call this ____________________ an experiment. Galileo was
also one of the first to use q______________________ methods, which involves
associating numbers with measurements. These techniques make Galileo the father of
the s__________________ m__________.
Galileo ran a very important experiment. Galileo had a ramp with a downward incline
followed by an upward incline with equal slope (like the picture on the left). He noticed
that a ball would roll down the first incline and up the second incline to a height
ALMOST the same as it was released. Galileo attributed this difference in height to
________________. If friction could be eliminated, Galileo theorized that the ball would
roll to the same height. This would be true even if the upward incline were of lesser
slope (like the right picture); the ball would just have to cover more incline distance to
reach the original height.
Finally, Galileo understood that if the second incline had no slope at all, the ball would
never reach its original height. It would continue to roll ______________.
This understanding was monumental; for the first time in history, it was proposed that a
force was not necessary to keep something moving that was moving in the first place.
The object could continue to move on its own, if friction could somehow be eliminated.
About the same time, Rene Descartes made an extreme idealization: what if friction and
______________ could be eliminated?
In this friction-free, gravity-free environment, an object which was not moving would
remain not moving unless a force caused it to change its state of motion. Also, in
this environment, an object which was already moving would continue to move in a
straight line at constant speed, unless a force caused it to change its state of motion.
These ideas about the motion of objects became known as the Law of I____________.
Descartes said that ______________ is a property of an object, ________________
proportional to the object’s ________, that causes an object to ____________ changes in
its state of motion.
In the 17th century, __________ ____________ wrote the world’s first ______________
book, The P________________. In it, Newton outlined three laws of motion.
1. Law of ______________
Bodies at ________ tend to stay at ________.
Bodies in ____________ tend to stay in ____________.
These rules hold unless an ______________ __________ acts upon the
Basically, this says f____________ change v___________________ (cause
2. Law of ________________________
Acceleration is ____________________ proportional to mass and
____________________ proportional to __________.
This can be ______________ly stated as :
Basically, this says the ________ __________ as the Law of Inertia. The
Law of Acceleration is qu____________________ while the Law of Inertia is
3. Law of __________ __________
For every ____________, there is an __________ and ________________
This means, forces always come in __________.
Some non-physics types think that since action and reaction are equal and
opposite (like +5N and -5N), they will ___________ each other out. This is
incorrect however; action and reaction will always be on
__________________ ________________, and consequently, could never
An confusing example of action and reaction comes from the gravitational
force on an object, say a hot dog. If one asked, “What is the reaction to
gravity?”, a wrong answer is likely. However, gravity is really the force of
the __________ on the _____ _______. If this is the action, then the reaction
is obtained by simply switching the objects in the interaction: the force of the
hot dog on the earth. If the hot dog weights 2 N, then the eFh = -2 N, while the
hFe = +2 N.
One might ask, “If the forces are equal, why don’t we get equal
________________________?” This would imply the earth rising up toward
the hot dog the same as the hot dog drops to the earth. The answer lies in the
difference in ____________. While it is true the forces are the same strength,
the small mass of the hot dog allows this force to be very effective in
accelerating the hot dog. Conversely, this same force is rendered
PRACTICALLY useless by the gigantic mass of the earth. The word
practically is important since even though the effect of the small force on the
huge mass is slight, there still is an ___________________ force, and
consequently, there still is an ________________________, however
The key idea here is: there can be a big difference between a __________ and
A force = a ________ or a ________.
A force also = an ______________________ between two objects.
There are really two types of forces:
1. Contact Forces-forces in which the two interacting objects are
2. ____________-at-a-________________ Forces-forces in which the two
interacting objects are not actually touching.
*see your Types of Forces Handout for more on these forces.
Mass is a ______________________ property of an object. We say this because it does
not depend on the object’s ______________________.
Weight is different. Weight is the ____________ of ______________ on an object.
Weight depends on _____________, but ________ is independent of
Weight can be easily calculated given the mass and the acceleration due to gravity at a
If mass is given in kilograms, and “g” is given in m/s/s, then the unit on weight is kg
m/s/s. This cumbersome unit is nicknamed the ______________.
Volume is different still. Volume is the amount of __________ an object takes up.
An example of something with big mass but small volume is:
An example of something with small mass but big volume is:
The net force is the ______ force an object __________. It is the r__________________
of all the forces on the object.
A net force causes acceleration. Net force and acceleration will always be in the
To determine the net force, draw a ________-________ diagram (a picture of all the
forces acting on an object). From this picture, pick a direction to be positive and the
opposite direction to be negative. (In class, we usually pick the direction of the
________________________ to be positive, but this is only for convenience; you could
pick either direction and still get the correct answer.)
The net force will be the positive forces added together and then the negative forces
subtracted from this sum. For example, if right is considered positive in this picture, the
net horizontal force
For example, if up is considered positive in this picture, the net vertical force
Newton’s second law says that the net force (ΣF) = m a.
FRAME OF REFERENCE
Remember frame of reference is the set of objects in your surroundings that are not
moving relative to ______. These things are not getting ____________ or
______________ from you at the present time. ________________ frames of reference
are ones that are not _______________________. An inertial frame of reference could
be at rest, or it could be moving at ________________ speed. ________________ said
that these frames of reference are ____________________. That is, the Newton’s Laws
pertain to both reference frames equally. Moreover, if you were in an inertial frame of
reference on earth or in an identical inertial frame of reference aboard a spaceship
moving with constant velocity, there is no way to discern which is which without looking
outside the frame of reference. Moreover, there is no experiment which can be done to
detect the constant motion.
A non-inertial reference frame is different. A non-inertial reference frame is one that is
being ______________________. Examples might be an accelerating car or an
accelerating elevator. The Law of Inertia does not pertain to these reference frames. For
instance, when aboard an accelerating car, you will feel the force the seat applies to your
body. You can detect this unbalanced force. However, relative to things inside the car
(things in your frame of reference) you remain at rest. Here, an object being acted upon
by an unbalanced force should accelerate out of its frame of reference, but you do not
move relative to the other items in the car. You are in a non-inertial reference frame.
Another example is an accelerating elevator. Depending on whether an elevator is
accelerating upward or downward, the rider’s weight will seem to change. With upward
acceleration and upward velocity, the rider will experience a sensation of being
_____________ than normal. We say that the rider’s ________________ weight is more
than the rider’s actual weight. Similarly, with downward acceleration and upward
velocity, the rider will experience a sensation of being ______________ than normal.
We say that the rider’s apparent weight is less than the rider’s actual weight. Drawing a
free-body diagram of the rider in each case will show this.
All objects free-fall the same because they have equal f____________ to m_______
Air resistance changes the a_____________________ of a falling object.
When air resistance = ____________, we get _______________ ______________,
during which acceleration = ________
______________ is a force that ______________ motion.
The three kinds of friction are:
__________________ of _______________ is the number that describes the roughness
or smoothness of a surface.
The rougher the surface, the ____________ the µ.
Smooth surfaces have µ’s closer to __________.
There are two kinds of sliding friction:
Kinetic friction will be _______ ________ or equal to static friction, because it’s easier
to _________ something moving than to __________ something moving.
Sliding friction also depends on ______________ __________ , as seen in the equation: