Dynamics – Newton’s 1st Law
http://www.aplusphysics.com/courses/honors/dynamics/N1Law.html
Unit #3 Dynamics
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Objectives and Learning Targets
 Define mass and inertia and explain the meaning of
Newton’s 1st Law.
 Define a force and distinguish between contact
forces and field forces.
Unit #3 Dynamics
Newton’s 1st Law of Motion
 Newton’s 1st Law of Motion, also known as the law of inertia, can
be summarized as follows:
 “An object at rest will remain at rest, and an object in
motion will remain in motion, at constant velocity and in a
straight line, unless acted upon by a net force.”
 This means that unless there is a net (unbalanced) force on an
object, an object will continue in its current state of motion with
a constant velocity. If this velocity is zero (the object is at rest),
the object will continue to remain at rest. If this velocity is not
zero, the object will continue to move in a straight line at the same
speed. However, if a net (unbalanced) force does act on an object,
that object’s velocity will be changed (it will accelerate).
Unit #3 Dynamics
Newton’s 1st Law
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The second part of the law, “an object in motion will continue in its current
state of motion with constant velocity unless acted upon by a net force” isn’t
easy to observe here on Earth, making this law rather tricky. Almost all
objects observed in everyday life that are in motion are being acted upon
by a net force - friction.
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Try this example: take your physics book and give it a good push along the
floor. As expected, the book moves for some distance, but rather rapidly
slides to a halt. An outside force, friction, has acted upon it. Therefore, from
typical observations, it would be easy to think that an object must have a
force continually applied upon it to remain in motion. However, this isn’t so.
If you took the same book out into the far reaches of space, away from any
gravitational or frictional forces, and pushed it away, it would continue
moving in a straight line at a constant velocity forever and ever, as there are
no external forces to change its motion. When the net force on an object is
0, the object is in static equilibrium.
Unit #3 Dynamics
Inertia (1st Law)
 The tendency of an object to resist a change in
velocity is known as the object’s inertia. For example,
a train has significantly more inertia than a skateboard.
It is much harder to change the train’s velocity than it
is the skateboard’s. The measure of an object’s inertia
is its mass. For the purposes of this course, inertia and
mass mean the same thing - they are synonymous.
Unit #3 Dynamics
Inertia (1st Law)
Unit #3 Dynamics
Sample Problem #1
 Question: A 0.50-kilogram cart is rolling at a speed of 0.40
meter per second. If the speed of the cart is doubled, what
happens to the inertia of the cart?
Answer: The inertia (mass) of
the cart remains unchanged.
Unit #3 Dynamics
Sample Problem #2
 Question: Which object has the greatest inertia?
 a falling leaf
 a softball in flight
 a seated high school student
 a rising helium-filled toy balloon
Answer: (4) a 20.0-kg mass
has the greatest inertia.
Unit #3 Dynamics
What is a Force?
 If you recall from the kinematics unit, a change in velocity is
known as an acceleration. Therefore, the second part of this law
could be re-written to state that an object acted upon by a net
force will be accelerated.
 But, what exactly is a force? A force is a vector quantity
describing the push or a pull on an object. Forces are measured
in Newtons (N), named after Sir Isaac Newton, of course.
 A Newton is not a base unit, but is instead a derived unit,
equivalent to 1 kgm/s2. Interestingly, the gravitational force on a
medium-sized apple is approximately 1 Newton.
Unit #3 Dynamics
2 Types of Forces
 Two basic types of forces: contact forces and field
forces.
 Contact forces occur when objects touch each other. Examples of
contact forces include pushing a crate (applied force), pulling a
wagon (tension force), a frictional force slowing down your sled,
or even the force of air accelerating a spitwad through a straw.
 Field forces, also known as non-contact forces, occur at a
distance. Examples of field forces include the gravitational force,
the magnetic force, and the electrical force between two charged
objects.
Unit #3 Dynamics
Net Force
 What is a net force? A net force is just the vector sum of all
the forces acting on an object.
 Imagine you and your sister are fighting over the last
Christmas gift. You are pulling one end of the gift toward you
with a force of 5N. Your sister is pulling the other end toward
her (in the opposite direction) with a force of 5N. The net
force on the gift, then, would be 0N, therefore there would
be no net force. As it turns out, though, you have a passion
for Christmas gifts, and now increase your pulling force to
6N. The net force on the gift now is 1N in your direction,
therefore the gift would begin to accelerate toward you
(yippee!) It can be difficult to keep track of all the forces
acting on an object.
Unit #3 Dynamics
Net Force
Unit #3 Dynamics