TAKS Objective 5 Force and Motion

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TAKS Objective 5 Force and Motion

• Day 13

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Forces and Motion

• Forces can create changes in motion (acceleration
  or deceleration).

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Definition of a Force

• A force is a push or a pull.

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Balanced Force

• A force that produces no change in an objects
  motion because it is balanced by an equal yet
  opposite force.

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Unbalanced Forces

• Are forces that result in an objects motion
  being changed.

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Motion can be described as

• A change in an objects position.
• Average velocity (speed) is the change of
  position of an object over time.

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Newtons 1ST Law of Motion

• 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.

• 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.

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Newtons 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.

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Q The frog leaps from its resting position at
the lakes 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 lakes bank when leaping?

• (A) 0.2 N
• (B) 0.8 N
• (C) 1.5 N
• (D) 6.0 N

Formula chart says Fma, m is mass in kg, a is
acceleration in m/s2. So, .5 kg x 3 m/s2 1.5 N
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Newtons 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

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Q The hands of a swimmer pushing backward
against water represent an action force. What is
the reaction force?

• A. The swimmers body moving forward?
• B. The water pushing against the swimmers hands
• C. The swimmers body pushing against the water.
• D. The water moving backward from the swimmer.

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Momentum

• The product of an objects 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)

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Q A ball moving at 30 m/s has a momentum of 15
kgm/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
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Velocity Graphs V distance
time

• Velocity (v) is the slope (rise over run) of a
  position (d) vs. time (t) graph

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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? A. Q B. R C.
S D. T
How do we work this one? Calculate v d/t for
each segment.
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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

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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.
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Acceleration

• When an objects speed changes over time it is
  accelerating (or decelerating)
• A vfinal vinitial / time
• Units for acceleration m/s/s or m/s2

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Acceleration Graphs

• Acceleration (a) is the slope of a velocity (v)
  vs. time (t) graph

Positive Acceleration
Negative Acceleration
NO Acceleration
Velocity (m/s)
Velocity (m/s)
Velocity (m/s)
Time (s)
Time (s)
Time (s)
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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 Teresas
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

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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

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• Q How much work is performed when a 50 kg crate
  is pushed 15 m with a force of 20 N?
• A. 300 J
• B. 750 J
• C. 1,000 J
• D. 15,000 J

Use the formula Work Force x distance Force
of 20 N x 15 meters 300 Joules Answer
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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!
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Work

• 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.

• 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.

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Friction

• A force that opposes, or works against, motion
  of two objects that are touching.

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Friction

• Friction causes an object to slow down and stop.
• Since the amount of energy stays constant, the
  energy becomes heat.

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Why Use a Machine?

• In an ideal (perfect) machine the work put into
  the machine (Win) the work put out by that
  machine (Wout)

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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

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• 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

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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
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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

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Machines use Power

• Power the rate at which energy is used (work is
  done)
• PWork/time
• Power is measured in H.P. or watts
• 1 watt 1 Joule
• 1 sec

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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

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6 Types of Simple Machines

• Inclined planes
• Screws
• Pulleys
• Wheel and axle
• Levers
• Wedge

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Universal Law of Gravity

• All objects in the universe attract each other
  by the force of gravity.

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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
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On Earth gravity 9.8 m/s/s

• For every second that an object falls its speed
  increases by 9.8 m/s

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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

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USE THE FORMULA PAGE

• 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.

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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?