Download NEWTON LESSON 2

NEWTON LESSON 2
State of Motion
Inertia is the tendency of an object to resist changes in its state of motion.
But what is meant by the phrase state of motion? The state of motion of an
object is defined by its velocity - the speed with a direction. Thus, inertia
could be redefined as follows:
Inertia: tendency of an object to resist changes
in its velocity.
An object at rest has zero velocity - and (in the absence of an unbalanced
force) will remain with a zero velocity. Such an object will not change its
state of motion (i.e., velocity) unless acted upon by an unbalanced force. An
object in motion with a velocity of 2 m/s, East will (in the absence of an
unbalanced force) remain in motion with a velocity of 2 m/s, East. Such an
object will not change its state of motion (i.e., velocity) unless acted upon by
an unbalanced force. Objects resist changes in their velocity.
As learned in an earlier unit, an object which is not changing its velocity is
said to have an acceleration of 0 m/s/s. Thus, we could provide an alternative
means of defining inertia:
Inertia: tendency of an object to resist
accelerations.
DEMO/EXAMPLE: Trying to evade a large animal (bull
moose) in the woods, how should you run? Why?
(INERTIA)
Check Your Understanding
1. The group of physics teachers are taking some time off
for a little putt-putt golf. The 15th hole at the Hole-In-One
Putt-Putt Golf Course has a large metal rim which putters
must use to guide their ball towards the hole. Mr. S guides a
golf ball around the metal rim When the ball leaves the rim, which path (1,
2, or 3) will the golf ball follow?
The answer is 2. Once leaving the rim, the ball will follow an "inertial path"
(i.e., a straight line). At the instant shown in the diagram, the ball is moving
to the right; once leaving the rim, there is no more unbalanced forces to
change its state of motion. Paths 1 and 3 both show the ball continually
changing its direction once leaving the rim.
2. A 4.0-kg object is moving across a friction-free surface with a constant
velocity of 2 m/s. Which one of the following horizontal forces is necessary
to maintain this state of motion?
a. 0 N
b. 0.5 N
e. depends on the speed.
c. 2.0 N
d. 8.0 N
If an object is in motion, then it will stay in motion with those very same
motion characteristics. It doesn't take any force to maintain that same state
of motion. In fact, the presence of a force would "ruin" such a state of
motion and cause an acceleration.
Balanced and Unbalanced Forces
Newton's first law of motion has been frequently stated throughout this
lesson.
An object at rest tends to stay at rest and an object in motion
tends to stay in motion with the same speed and in the same
direction unless acted upon by an unbalanced force.
But what exactly is meant by the phrase unbalanced force? What is an
unbalanced force? In pursuit of an answer, we will first consider a physics
book at rest on a table top. There are two forces acting upon the book.
One force - the Earth's gravitational pull - exerts a downward force.
The other force - the push of the table on the book (sometimes
referred to as a normal force) - pushes upward on the book.
Since these two forces are of equal magnitude and in opposite directions,
they balance each other. The book is said to be at equilibrium.
There is no unbalanced force acting upon the book and thus the book
maintains its state of motion. When all the forces acting upon an object
balance each other, the object will be at equilibrium; it will not accelerate.
(Note: diagrams such as the one above are known as free-body diagrams and
will be discussed in detail in LATER)
Consider another example involving balanced forces - a person standing
upon the ground. There are two forces acting upon the person. The force of
gravity exerts a downward force. The floor of the floor exerts an upward
force.
Since these two forces are of equal magnitude and in opposite directions,
they balance each other. The person is at equilibrium. There is no
unbalanced force acting upon the person and thus the person maintains its
state of motion.
Now consider a book sliding from left to right across a table top. Sometime
in the prior history of the book, it may have been given a shove and set in
motion from a rest position. Or perhaps it acquired its motion by sliding
down an incline from an elevated position. Whatever the case, our focus is
not upon the history of the book but rather upon the current situation of a
book sliding to the right across a table top.
The book is in motion and at the moment there is no one pushing it to
the right. (Remember: a force is not needed to keep a moving object
moving to the right.) The forces acting upon the book are shown
below.
The force of gravity pulling downward and the force of the table pushing
upwards on the book are of equal magnitude and opposite directions.
These two forces balance each other.
Yet there is no force present to balance the force of friction. As the
book moves to the right, friction acts to the left to slow the book
down. There is an unbalanced force; and as such, the book changes its
state of motion.
The book is not at equilibrium and subsequently accelerate. Unbalanced
forces cause accelerations. In this case, the unbalanced force is directed
opposite the book's motion and will cause it to slow down.
To determine if the forces acting upon an object are balanced or unbalanced,
an analysis must first be conducted to determine what forces are acting upon
the object and in what direction. If two individual forces are of equal
magnitude and opposite direction, then the forces are said to be balanced. An
object is said to be acted upon by an unbalanced force only when there is an
individual force which is not being balanced by a force of equal magnitude
and in the opposite direction .
Check Your Understanding
Luke Autbeloe drops an approximately 5.0 kg fat cat (weight = 50.0 N) off
the roof of his house into the swimming pool below. Upon encountering the
pool, the cat encounters a 50.0 N upward resistance force (assumed to be
constant). Use this description to answer the following questions. Click the
button to view the correct answers.
1. Which one of the velocity-time graphs best describes the motion of the
cat? Support your answer with sound reasoning.
Graph B is correct. The cat first accelerates with a negative (downward)
acceleration until it hits the water. Upon hitting the water, the cat
experiences a balance of forces (50 N downwards due to gravity and 50 N
upwards due to the water). Thus, the cat will finish its motion moving with a
constant velocity. Graph B depicts both the initial negative acceleration and
the final constant velocity.
2. Which one of the following dot diagrams best describes the motion of the
falling cat from the time that they are dropped to the time that they hit the
bottom of the pool? The arrows on the diagram represent the point at which
the cat hits the water. Support your answer with sound reasoning.
Tape A is correct.
The cat first accelerates with a negative (downward) acceleration until it hits
the water. Upon hitting the water, the cat experience a balance of forces (50
N downwards due to gravity and 50 N upwards due to the water). Thus, the
cat will finish its motion moving with a constant velocity. Diagram A
depicts both the initial downward acceleration and the final constant
velocity.
3. Several of Luke's friends were watching the motion of the falling cat.
Being "physics types", they began discussing the motion and made the
following comments. Indicate whether each of the comments are correct or
incorrect? Support your answers.
a. Once the cat hits the water, the forces are balanced and the
cat will stop.
False.
Once the cat hits the water, the forces are balanced (50 N down and 50 N
up). However, an object in motion (such as the cat) will continue in motion
at the same speed and in the same direction. When the cat strikes the water,
it stops accelerating; yet it does not stop moving.
b. Upon hitting the water, the cat will accelerate upwards
because the water applies an upward force.
False.
Once the cat hit the water, the forces are balanced (50 N down and 50 N up).
The upward force of the water on the cat is balanced by the downward pull
of gravity. The cat will continue in motion at constant speed.
c. Upon hitting the water, the cat will bounce upwards due to
the upwards force.
False.
Once the cat hits the water, the forces are balanced (50 N down and 50 N
up). The cat would only bounce upwards if the water applied an upward
force greater than 50 N. As stated in the problem, the water applies only 50
N of upward force. Furthermore, the upward force would first contribute to
slowing the cat down (an upward acceleration) before it could begin to
actually move it upward.
4. If the forces acting upon an object are balanced, then the object
a. must not be moving.
b. must be moving with a constant velocity.
c. must not be accelerating.
d. none of these
The answer could be A (but does not have to be A) and it could be B (but
does not have to be B). An object having balanced forces definitely cannot
be accelerating. This means that it could be at rest and staying at rest (one
option) or could be in motion at constant velocity (a second option). Either
way, it definitely is not accelerating - choice C of your four choices.
NEWTONS LESSON 2 HOMEWORK
GROUP DISCUSSION:
This activity is designed to reveal your preconceptions about the causes of motion.
Instructions:
Discuss the following situations with the other members of your group. Try to come up
with specific and complete answers. ("Gravity", for instance, is not a complete answer.)
Each individual in the group should record their own answer.
1. Situation: A book is lying at rest on a table. Why? In other words, what causes the
book (or any object) to remain at rest?
2. Situation: The Voyager spacecraft is moving out of the Solar System in a straight
line with constant speed. Why is it doing that? What causes an object to move
with constant velocity?
3. Situation: A pencil rolls off the desk and falls to the floor. Why does it move the
way it does? We know a lot about this motion (free fall) - but what causes it to
happen? Why do objects accelerate?
4. Situation: A book sliding across a table gradually comes to a stop. Why? What
causes a moving object to gradually come to a stop?
INDIVIDUAL WORK
1. The velocities and masses of four objects are given below. Which one of the following
objects has the greatest inertia?
a.
b.
c.
d.
5 kg object moving at 12 m/s
12 kg object moving at 5 m/s
21 kg object moving at 3 m/s
55 kg object which is not moving (v = 0 m/s)
2. If the forces acting upon an object are balanced, then the object
a.
b.
c.
d.
must not be moving.
must be moving with a constant velocity.
must not be accelerating.
none of these
3. Mr. Smith guides a golf ball around the outside rim of the green at
the Hole-In-One Putt-Putt Golf Course. When the ball leaves the rim,
which path (A, B or C) will the golf ball follow? _________ Explain
why.
4. Ben Tooclose is being chased through the woods by a bull moose which he was
attempting to photograph. In the course of the chase, who (Ben, the moose, or neither)
would have the mass advantage? ____________________ Explain your answer (i.e.,
explain how).
5. Bronco Brown the skydiver pulls the chord of his parachute. The
forces acting upon Bronco and his parachute are shown in the diagram at
the right. From this diagram, one might infer that ____. Note all that
apply:
a. Bronco will be accelerating upwards.
b. Bronco will immediately stop.
c. Bronco will begin moving upwards.
d. Bronco will continue to fall, but now will slow down.
e. None of these will occur
6. If an object is at equilibrium, then the object must
a.
b.
c.
d.
e.
be at rest
be moving at constant speed
be moving in the same direction
have an acceleration of 0 m/s2.
none of the above
7. If an object is at equilibrium, then all the individual forces which act upon it must be
equal.
TRUE or FALSE
Answer Key
1d
2c
3b, ball continues in straight line once no unbalanced forces are acting apon it.
4 Ben has the mass advantage which means he has a smaller inertia and can change
direction with less force.
5d
6d
7T