Energy is defined as the ability to do work and is a scalar quantity. Energy has no direction, only magnitude. Mechanical energy comes in two varieties: kinetic energy (KE) and potential energy (PE).
Kinetic Energy
Kinetic energy represents the energy caused by an object’s motion
KE = ½mv2
where v is the object’s actual speed, that is, the magnitude of the object’s instantaneous resultant velocity. In this formula, m must be measured in kg and v must be measured in m/sec. Note that this collection of units
kg (m/sec)2 = kg m2/sec2 is called a joule (J).
Refer to the following information for the next three questions.
Suppose the skater shown below has a mass of 25 kg and is moving at a speed of 8 m/sec along a level surface.
1. How much KE would he possess?
2. How would his kinetic energy change if his speed doubled?
3. How would the kinetic energy of a second skater having twice as much mass but still moving at 8 m/sec compare with the kinetic energy of our original skater?
This type of potential energy represents the energy an object possesses by virtue of its location in a gravitational field.
PE = mgh
where h is the height above an arbitrary zero level that is convenient for the solution of the problem. For instance, the zero level might be assigned as the base of a cliff for a projectile being thrown from the top of the cliff or the zero level might be the floor for a marble rolling off a table onto the ground.
In this formula, m must be measured in kg, g equals 9.8 m/sec2, and h is in meters. Note that this collection of units
kg (m/sec2)(m) = kg (m2/sec2) is also a joule (J).
The expression mg represents the objects weight or the force of gravitational attraction between the earth and the object. Forces are measured in newtons. Thus the expression
Nm also equals a joule (J).
Refer to the following information for the next two questions.
The table is 1-meter tall and the apple has a mass of 100 grams.
Complete Part 2 to your Tilted Scale Investiagtion making sure it is shared to quicktime and uploaded to youtube. Message ma the url through Studywiz.
Choose 2 of the podcasts you have created this year and prepare an entry for the AIP Competition. You will need to read the information above and prepare your 250 word supporting statement.
Finish any outstanding activities from the Work Program.
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. This is because a force is defined as the interaction between two objects. In the metric system, forces are measured in a unit called a newton. Forces occur only in pairs, one action and the other reaction, both of which constitute the interaction between one thing and the other. Neither force exists without the other. Since action and reaction forces act on different objects, action and reaction forces can never cancel each other.
Law of Action-Reaction
If object A exerts a force on object B then object B exerts an equal but opposite force on object A.
FAB = – FBA
Julius Sumner Miller – Physics – Newton’s Third Law Part 1
Complete the following review on important terminology concerning Newtons Laws.
1.
While driving down the road, a firefly strikes the
windshield of a bus and makes a quite obvious mess in front
of the face of the driver. This is a clear case of Newton’s
third law of motion. The firefly hit the bus and the bus
hits the firefly. Which of the two forces is greater: the
force on the firefly or the force on the bus?
2. For years, space travel was believed to be impossible
because there was nothing which rockets could push off of in
space in order to provide the propulsion necessary to
accelerate. This inability of a rocket to provide propulsion
is because …
a. … space is void of air so the rockets have
nothing to push off of.
b. … gravity is absent in space.
c. … space is void of air and so there is no air
resistance in space.
d. … nonsense! Rockets do accelerate in space and
have been able to do so for a long time.
3.
Many people are familiar with the fact that a rifle recoils
when fired. This recoil is the result of action-reaction
force pairs. A gunpowder explosion creates hot gases which
expand outward allowing the rifle to push forward on the
bullet. Consistent with Newton’s third law of motion, the
bullet pushes backwards upon the rifle. The acceleration of
the recoiling rifle is …
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the
bullet.
4. In the top picture (below), Kent Budgett is pulling
upon a rope which is attached to a wall. In the bottom
picture, the Kent is pulling upon a rope which is attached
to an elephant. In each case, the force scale reads 500
Newtons. Kent is pulling …
a. with more force when the rope is attached to
the wall.
b. with more force when the rope is attached to the
elephant.
According to Newton’s third
law, for every action force there is an equal (in size)
and opposite (in direction) reaction force. Forces always
come in pairs – known as “action-reaction force pairs.”
Identifying and describing action-reaction force pairs is a
simple matter of identifying the two interacting objects and
making two statements describing who is pushing on
who and in what direction. For example, consider the
interaction between a baseball bat and a baseball.
The baseball forces the bat to the left; the bat forces
the ball to the right. Together, these two forces exerted
upon two different objects form the action-reaction force
pair. Note that in the description of the two forces, the
nouns in the sentence describing the forces simply switch
places.
Consider the following three examples. One
of the forces in the mutual interaction is described;
describe the other force in the action-reaction force pair.
Click the button to view the answer.
Baseball pushes glove leftwards.
Bowling ball pushes pin leftwards.
Enclosed air particles push balloon wall outwards.
Check
Your Understanding
1. Consider the interaction depicted below between foot
A, ball B, and foot C. The three objects interact
simultaneously (at the same time). Identify the two
pairs of action-reaction forces. Use the notation “foot
A”, “foot C”, and “ball B” in your statements. Click the
button to view the answer.
2. Identify at least six pairs of action-reaction force
pairs in the following diagram.
When an object sits on a frictionless incline plane, the two forces acting on the object are the gravitational pull of the earth, its weight, mg, and the supporting force supplied by the surface of the incline, the normal, . These forces are shown in blue.
Since the object slides along the surface of the incline, we need to use the components of its weight, mg, when working problems. The formula used to calculate the component of the weight which acts parallel to the incline’s surface is
Fd = mg sin(θ)
In this formula, you can use the mnemonic that the “d” represents “down” which is parallel to the incline.
While the formula used to calculate the component of the weight that acts
perpendicular to the incline’s surface is
Fn = mg cos(θ)
In this formula, you can use the mnemonic that the “n” represents “normal” which is perpendicular to the incline.
Let’s take a moment and practice using these two preliminary formulas.
Refer to the following information for the next five questions.
A 5-kg mass is placed on an incline titled at 15º.
How much does the 5-kg mass weigh?
Calculate the magnitude of the component that is acting parallel to the incline’s surface?
Calculate the magnitude of the component that is acting perpendicular to the incline’s surface?
At what angle would these two components be equal in magnitude?
In which range of angles would Fd > Fn? (a) 0º<θ<45º (b) 45º<θ<90º
Usually our problems are more complicated than just calculating the components of the object’s weight. Here are some classic situations involving a mass moving along the surface of a frictionless incline.
Refer to the following information for the next four questions.
Suppose that you now want to drag a 5-kg mass up and down a frictionless 15º inclined plane.
How much force must you apply to a string acting parallel to the incline’s surface to slide the 5-kg mass up or down the incline at a constant velocity?
How much upward force would be needed to accelerate the 5-kg mass up this incline at 3 m/sec2?
How much upward force would be needed to restrict the 5-kg mass’ downward acceleration to 1 m/sec2?
If the 5-kg mass were allowed to slide down this incline without any additional applied forces acting upon it, what would be its acceleration down the incline?
1. Using a free body diagram and your knowledge of friction and vector components complete the following table in your workbook. Note: Fgparr=Force of gravity parallel to the incline and Fgperp=Force of gravity perpendicular to the incline.
2. Plot this data using Excel.
3. Compare this graph and the graph you created using your original measurements.
4. How are the 2 graphs from above related?
Questions
1. What would the component of gravity parallel to the incline be if the angle of inclination was 90 degrees.
2. What would the component of gravity perpendicular to the incline be if the angle of inclination was 90 degrees.
3. Create a table showing angle of inclination and force due to friction.
4 What would happen the the scale if friction did not exist?
Conclusion
Ceate a conclusion for this investigation.
Format: You will be required to prepare a podcast that includes all on this part of the investigation. You should use the bold headings in this entry as a guide for your report.
The acceleration of an object as
produced by a net force is directly proportional to
the magnitude of the net force, in the same direction
as the net force, and inversely proportional to the
mass of the object.
a = Fnet /m
Fnet = m *a
Check
Your Understanding
1. Determine the accelerations which result when a 12-N
net force is applied to a 3-kg object and then to a 6-kg
object.
2. A net force of 15 N is exerted on an encyclopedia to
cause it to accelerate at a rate of 5 m/s2.
Determine the mass of the encyclopedia.
3. Suppose that a sled is accelerating at a rate of 2
m/s2. If the net force is tripled and the mass is
doubled, then what is the new acceleration of the sled?
4. Suppose that a sled is accelerating at a rate of 2
m/s2. If the net force is tripled and the mass is
halved, then what is the new acceleration of the sled?
In conclusion, Newton’s second law
provides the explanation for the behavior of objects upon
which the forces do not balance. The law states that
unbalanced forces cause objects to accelerate with an
acceleration which is directly proportional to the net force
and inversely proportional to the mass.
Cognitive
scientists (scientists who study how people learn) have
shown that physics students come into physics class with a
set of beliefs which they are unwilling (or not easily
willing) to discard despite evidence to the contrary. These
beliefs about motion (known as misconceptions) hinder
further learning.
Newton’s laws declare loudly that a net
force (an unbalanced force) causes an acceleration; the
acceleration is in the same direction as the net force. To
test your own belief system, consider the following question
and its answer as seen by clicking the button.
Two
students are discussing their physics homework prior to
class. They are discussing an object which is being acted
upon by two individual forces (both in a vertical
direction); the free-body diagram for the particular object
is shown at the right. During the discussion, Anna Litical
suggests to Noah Formula that the object under discussion
could be moving. In fact, Anna suggests that if friction and
air resistance could be ignored (because of their negligible
size), the object could be moving in a horizontal direction.
According to Anna, an object experiencing forces as
described at the right could be experiencing a horizontal
motion as described below.
Noah Formula objects, arguing that the
object could not have any horizontal motion if there are
only vertical forces acting upon it. Noah claims that the
object must be at rest, perhaps on a table or floor. After
all, says Noah, an object experiencing a balance of forces
will be at rest. Who do you agree with?
Remember last winter when you went
sledding down the hill and across the level surface at the
local park? (Apologies are extended to those who live in
warmer winter climates.)
1.
3.
. These forces are shown in blue.
Two
