Transcript Force and Motion

TAKS Objective 5
Force and Motion
Day 13
Forces and Motion

Forces can create
changes in motion.
Acceleration
 Deceleration

What happens if I put
force on my gas
peddle?
 What happens if I put
force on my Breaks?

Definition of a Force
A
force is a
push or a pull.
Balanced Force


A force that produces
no change in an
object’s motion
because it is
balanced by an equal
yet opposite force.
If I were to add
these two forces they
would equal zero
Unbalanced Forces
Are forces
that result in
an object’s
motion being
changed.
 Add together
to equal
greater force

+
Motion can be described
as:
A
change in an
object’s position.
 Average velocity
(speed) is the
change of position
of an object over
time.
Newton’s 1ST Law of
Motion

1st Laws States that an
object at rest will not move
unless an outside force acts
on it (such as friction). This
law is also called the LAW
OF INERTIA.

Ex. This law explains why
you fly forward in a car
when someone slams on
the brakes. Because of
Inertia, your body wants to
keep moving at the same
speed as the car.
Newton’s 2nd Law of
Motion

2nd Law States that a
force on an object will
move the object in the
direction of the force.
The relationship
between force, mass
and acceleration is
summarized by the
formula:
 f = ma

Ex. This law
explains why a golf
ball will roll in the
direction of a force
applied to it.
Q: The frog leaps from its resting position at the
lake’s bank onto a lily pad. If the frog has a mass
of 0.5 kg and the acceleration of the leap is 3 m/s2,
what is the force the frog exerts on the lake’s
bank when leaping?
(A) 0.2 N
(B) 0.8 N
(C) 1.5 N
(D) 6.0 N
Formula chart says F=ma, m is mass
in kg, a is acceleration in m/s2.
So, .5 kg x 3 m/s2= 1.5 N
Newton’s 3rd Law of
Motion
3rd Law States that
for every action
there is an equal but
opposite action.
 Ex. A skater pushes
back on the skates
but the skater moves
forward.


THESE LAWS EXPLAIN ALL MOTION
Q: The hands of a swimmer
pushing backward against
water represent an action
force. What is the reaction
force?
A. The swimmer’s body moving forward?
B. The water pushing against the swimmer’s hands
C. The swimmer’s body pushing against the water.
D. The water moving backward from the swimmer.
Momentum
•
The product of an object’s mass and
its speed. A force applied to an
object causes a change in its
momentum.
•
p(momentum)= m(mass) x v(velocity)
p = mv
•
common unit for momentum (kg x m/s)
Q: A ball moving at 30 m/s has a
momentum of 15 kg·m/s. The
mass of the ball is —
A. 45 kg
B. 15 kg
C. 2.0 kg
D. 0.5 kg
Formula Page says that
Momentum = Mass x Velocity
So, 15 kg.m/s = M x 30 m/s
solving for M it is:
Velocity Graphs
V = distance
time
 Velocity
60
Distance(m)
(v) is
the slope (rise
over run) of a
position (d) vs.
time (t) graph
Velocity
40
Series1
20
Series2
0
1 3 5 7 9 111315
Time(sec)
Q: The diagram represents the total travel
of a teacher on a Saturday. Which part of
the trip is made at the greatest average
speed?
How do we work this one?
A. Q
B. R
Calculate v = d/t for each segment.
C. S
D. T
Q: The picture shows the position of
a ball every 0.25 second on a
photogram. Using a ruler, determine
the velocity of the ball.
A. 3.5 cm/s
B. 10.5 cm/s
C. 14.0 cm/s
D. 28.0 cm/s
Use the ruler on the side of the
chart and the equation for velocity.
The answer was H.
Measure from the center of ball 1 to the
center of ball 2 and multiply by 4.
Acceleration
When an object’s speed
changes over time it is
accelerating (or
decelerating)
 A = vfinal – vinitial / time


Units for acceleration
m/s/s or m/s2
Acceleration Graphs

Acceleration (a) is the slope of a velocity (v)
vs. time (t) graph
Positive
Acceleration
Negative
Acceleration
Velocity (m/s)
Velocity (m/s)
Velocity (m/s)
Time (s)
NO
Acceleration
Time (s)
Time (s)
Teresa runs in one direction at 1.5
meters per second (m/s). She hen
turns around and runs in the
opposite direction at 2.0 m/s. The
entire trip takes 5.0 seconds (s).
What is Teresa’s average
acceleration, in meters per second
squared (m/s2)?
A. -0.7 m/s2
B. -0.1 m/s2
C. +0.1 m/s2
D. +0.7 m/s2
Work
Work: application of a force to an
object that results in the movement
of the object over a certain distance.
W = F x d
 The work done by forces on an object
= changes in energy for that object.
 Work and Energy are measured in
Joules
 1 Joule = 1 Newton • meter

Q: How much work is performed when a 50 kg crate is pushed 15 m
with a force of 20 N?
A. 300 J
Use the formula Work = Force x distance
B. 750 J
C. 1,000 J
Force of 20 N x 15 meters = 300 Joules
D. 15,000 J
Answer:
Q: If a force of 100 newtons was
exerted on an object and no work
was done, the object must have —
A. accelerated rapidly
B. remained motionless
C. decreased its velocity
D. gained momentum Work = Force x Distance
Work = 0
Force = 100 N so
0 J = 100 N x d
distance must be 0
It did not move!
Work

1.
2.
3.
4.
Example:
The teacher pushes
on the wall until she is
exhausted.
A book falls off the
table and hits the
floor.
The waiter carries a
tray of food.
A rocket accelerates
through space.

1.
2.
3.
4.
Is Work Being Done?
No. The wall did not
move.
Yes, gravity applied a
force and moved the book
in the direction of the floor.
No. The force to hold the
tray is not applied in the
direction of the motion.
Yes. The force of the
rocket thrust is causing the
rocket to move.
Friction
A force that opposes, or works
against, motion of two objects that
are touching.
Friction
•
•
Friction causes an
object to slow down
and stop.
Since the amount of
energy stays constant,
the energy becomes
heat.
Why Use a Machine?

In an ideal (perfect)
machine the work put
into the machine (Win)
= the work put out by
that machine (Wout)
Machines Make Work Easier
The ideal mechanical advantage
of a machine (IMA) of a machine
is the number of times the
output force is larger than the
input force IMA = Fout/Fin
 A machine can only make this
happen by moving the input
force through a farther distance
than the output force


Fin • din= Fout • dout
Q: The diagram shows an
electric motor lifting a 6
N block a distance of 3
m. The total amount of
electrical energy used
by the motor is 30 J.
How much energy does
the motor convert to
heat?
A. 9 J
B. 12 J
C. 18 J
D. 21 J
Work
Input =
30J done
by the
motor
Work Output =
Resistance Force x
Resistance Distance
Workout = 18J = 6N x
3m
The difference is lost as
heat due to friction, which
is 30J – 18J = 12J
Answer B
Real Machines use
Energy
No real machine is 100 %
efficient. i.e. none put out more
work than is put in
 Efficiency of a machine is work
output/work input X 100 %


%Efficiency = Woutput / W
input
X 100%
Machines use Power
 Power:
the rate at
which energy is used
(work is done)
 P=Work/time
 Power is measured in
H.P. or watts
 1 watt = 1 Joule
1 sec
Q: Shelby does 150 J of work to
move a cart 3 meters in 30
seconds. How much power did
Shelby use to do this work?
A. 4500 W
B. 450 W
C. 50 W
D. 5 W
6 Types of Simple
Machines
 Inclined
planes
 Screws
 Pulleys
 Wheel
and axle
 Levers
 Wedge
Universal Law of Gravity
All objects in
the universe
attract each
other by the
force of
gravity.
Universal Law of
Gravity
Gravity varies depending
on two factors:
1) the mass of the object
doing the pulling, and
2) the distance from the center
of that object
On Earth gravity = 9.8 m/s/s
For
every
second that an
object falls its
speed increases
by 9.8 m/s
Weight=
Mass (m) X gravity (g)
 Unit
of mass = kg
 Unit of acceleration = m/s/s
 Unit of weight = Newton
 1 Newton= about ¼ pound
USE THE FORMULA PAGE
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Some of the
problems
require you to
grid in an
answer. Make
sure you pay
attention to the
decimal point in
the square in
the middle.
Discussion Question
Lamont wants to move a 4,800 gram box
from the floor to a shelf directly above the
box. It takes Lamont 8 seconds to move the
box to a shelf that is 0.4 meters from the
ground. It takes 12 seconds to move the box
to a shelf that is 1.2 meters off the ground.
How much more work in joules is required to
put the box on the higher shelf?