Transcript Midway High School Science TAKS Review

Physics III
TAKS Review
OBJECTIVE 5 – MOTION FORCES AND
ENERGY (IPC 4)
INFORMATION ORIGINALLY FOUND FROM MIDWAY ISD
WEBPAGE
IPC (4) (A) – Calculate speed, momentum,
acceleration, work and power
One way to describe the motion of
an object is by its speed. Speed is
the distance an object moves in a
given time interval. As an equation
it is
d
v
t
The equation is given on the formula chart
and the units for speed are m/s or km/h.
This equation is for constant or average
speed.
Ex. An alien being driving a VW
bug travels 150 km in 2 hours.
What is the speed?
Note, km is a measure of distance
and hours is a measure of time.
Use the units to help you make
substitutions into the formula.
Now, you try this one (it is a little different)
An orangutan runs with a speed of 5 m/s
for 10 s. How far does the orangutan
travel?
Another way to describe motion is
acceleration. Acceleration is the
change in speed during a given
time period. As a formula it is
a
v f  vi
t
vf is the variable for final speed and vi is
the variable for initial speed (the speed
changes – ya dig?) The units are m/s2. The
formula is on the formula chart.
A Cro-Magnon man jogs at a speed of 4
m/s. He spots a saber toothed tiger and
increases his speed to 6 m/s in a time
period of 2 s. What is his acceleration?
Try this ‘un. A track sprinter starts
from the blocks and reaches a
maximum speed of 12 m/s in 3 s.
What is her acceleration?
Notes on graphs. Which of the
following shows an object moving
at a constant speed and which one
shows an object that is
accelerating?
d
d
t
t
New topic. A Ford Focus and a
Hummer collide with separate, yet
identical trees. Which tree receives
the most damage?
Mass and speed can be combined to
describe momentum. Momentum is
the product of mass and speed. The
form-you-lah is
p  mv
p is momentum, m is mass and v is
speed. The unit is kg·m/s and the
formula is on the chart.
So try this – A 0.5 kg baseball is
thrown with a speed of 30 m/s.
What is the momentum of the
baseball?
Conservation of Linear Momentum is one
of four conservation laws in physics.
It simply means that the
momentum a system of objects has
after a collision must be the same
momentum a system has before the
collision.
For example, let’s say a bowling ball
strikes a stationary bowling pin. The
ball has 120 kg·m/s of momentum
before the collision. If the ball has 100
kg·m/s of momentum after the
collision, what is the momentum
transferred to the pin?
Nuthernewidea (“Heigh-ho, heigh-ho,
it’s off to ? we go”)
Work means many things in everyday
usage but has a specific meaning in
physics. Work is a force applied on an
object over a given distance.
Symbolically
W  Fd
On the chart again. The unit for
work is a N·m which is also known
as a Joule (J)
Is work done in the following cases?
‘Splain.
 A student presses against a stationary chalkboard
with her hand.
 An intergalactic spacecraft coasts through space.
 Three students push a disabled car across a
parking lot.
Example – An extremely buff toddler
applies a force of 550 N to drag a 220 kg
comatose alligator 3 m across the
kitchen floor. How much work does
superchild do on the alligator?
A concept that relates work and
time is power. If the child pulls the
alligator across the floor in 2 s or 20
s, the work done is the same in
each case. However, doing the work
in 2 s requires more power than the
same work done in 20 s.
Power is the rate at which work is
done. The units are Joules per
second which is also known as a
Watt. And the formula is . . .
W
P
t
Ex. Find the power required for the
child to do the work describe on the
alligator (a few screens ago) in 2 s
and 20 s.
IPC (4) (B) – The student
investigates and describes
applications of Newton’s Laws
The First Law. Suppose a hockey puck
slides across frictionless ice (you can
buy this at Sam’s if you know where to
look). What are the horizontal forces
acting on the puck? And the answer is .
..
None! The puck will continue to
slide indefinitely at its current
velocity until some force (like
friction, a hockey stick, a dead cat
in the way) acts upon it.
The same is true if the puck is initially
at rest. It won’t move unless some net
external force changes it velocity (from
zero to some value that is not zero).
This describes Newton’s First Law. How
does the First Law explain how spacecraft
in our solar system (like the Voyager and
Pioneer probes) can travel through space
for decades using little or no fuel?
Suppose you kick a soccer ball and a
bowling ball (ouch!) with equal forces.
Which one will experience the greatest
acceleration (change in velocity,
remember?)?
So, for a constant force, more mass means
____ acceleration.
Now, suppose you have soccer balls
of equal mass and you kick them
with different forces. Which one
will experience the greatest
acceleration (the one kicked with
the larger or smaller force?)?
Then we can say, for constant mass a larger
force produces a _____ acceleration.
Putting the two relationships together
F
a
m
Or more commonly written as
F  ma
Force is measured in kg·m/s2,
which is a Newton (N).
Keerful Podnahs - the force used in
Newton’s 2nd Law is the net force. The net
force is the is the sum of the forces acting
on an object.
For instance, a book resting on a table is
not accelerating, even though there are
two vertical forces acting on it (what are
they?).
The forces are equal, but in
opposite directions, therefore there
is no net force (the sum is 0) and
consequently no acceleration.
Ex. A 2 kg object starts from rest
and reaches a final velocity of 10
m/s in 5 s. What is the acceleration
of the object? What is the net force
acting on the object?
Now try this. A force of 20 N to the right
acts on a 2 kg object while simultaneously
a force of 8 N acts to the left. What is the
net force on the object? What is the
acceleration?
The Third Law. Try this at home without
adult supervision. Have a friend, neighbor
or total stranger hold a piece of paper
vertically. Punch it with your fist as hard
as you can.
Then, go outside and punch the
bricks on the side or your house,
again, as hard as you can. One of
these experiences is going to hurt
and one isn’t. Guess which one will
hurt. Why?
The force your fist experiences
when it hits the paper is only as
great as the force the paper can
exert on your fist (not much).
Likewise, the force the brick wall exerts
on your fist is the same (only in the
opposite direction) as the force your fist
exerts on the wall. So when you get mad
and punch the wall, it gets mad an
punches back at you. So be nice.
These examples describe Newton’s Third
Law which says: if one object exerts a force
on a second object, the second object will
exert an equal, but opposite force on the
first object. These are called “actionreaction” pairs
A little tricky – Newton’s Third law
describes two forces and two objects. A net
force (several forces) on one object is an
example of which law?
Which of the following are action
reaction pairs?
 The weight of a person pushing downward on a chair
and the chair pushing upward on the person.
 A student pushing a box across the floor while
friction acts against the motion of the box.
 A kangaroo doing pushups pushes down on the floor
while the floor pushes upwards on the kangaroo
IPC (4) (D) – The student
investigates and demonstrates
mechanical advantage and
efficiencies of various machines
We use simple machines everyday.
A couple of ways to describe the
usefulness of machines is their
efficiency and mechanical
advantage.
The ideal mechanical advantage
(IMA – it’s called ideal if we
assume the machine is frictionless)
is calculated from
Fout
IMA 
Fin
Fout is the output force (the force a
machine exerts on an object) and
Fin is the input force (the force
exerted on the machine
Ex. A prince uses a pulley system to raise
an ogre to a second floor castle window. If
the weight of the ogre is 1600 N and the
prince applies a force of 80 N on the ropes
of the pulley, what is the IMA?
This formula is used for real machines
(not frictionless ones). In the real world
we never get as much work out of a
machine as the work we put into it.
Another way to describe the
usefulness of a machine is its
efficiency. The efficiency of a
machine is found from
Wout
% efficiency 
 100
Win
Recall, work is found by multiplying the
force applied to an object by the distance.
Ex. A 62.5 N force is applied to one end of
a lever over a distance of 0.4 m. This raises
a 200 N rock at the other end of the lever
0.1 m. What is the efficiency of the lever?
Before substituting numbers into a
formulas, decide what each of the
values describe. A couple of hints
solving efficiency problems for
machines: the output force is always
greater than the input force and the
input distance is always greater than
the output distance.
So back to the example. 62.5 N is the
input force and 0.4 m is the input
distance. 200 N is the output force and
0.1 m is the output distance.
Calculating
(200 N)(0.1 m)
% eff. 
100
(62.5 N)(0.4) m
% eff. = 80%
Yerz: A force of 20 N is used to push a
30 N box 2 m along an inclined plane.
This raises the block 0.5 m above its
initial height. What is the efficiency of
the inclined plane?