MOTION, FORCES

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MOTION, FORCES ENERGY

• TAKS REVIEW
• IPC (4)

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IPC (4) The student knows the concepts of force
and motion evidence in everyday life.

• (A) The student is expected to calculate speed,
  momentum, acceleration, work and power in systems
  such as in the human body, moving toys and
  machines.

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SPEED / VELOCITY

• To calculate speed/velocity (s), we need
• 1. Distance traveled (d) in meters (m)
• 2. Time (t) in seconds (s)
• Our formula is
• Speed distance traveled s d
• time t
• The units for speed are usually m/s.

THIS IS ON YOUR FORMULA SHEET!
4
Problem 1 speed / velocity

• A car traveled 150 km in 2.5 hours. What was its
  average speed in km per hour?
• Identify the information given (look at units)
  d150 km, t2.5 hrs
• Select the formula sd/t
• Plug in the numbers and solve
• s 150km / 2.5 hrs
• Use the calculator to obtain an answer with
  correct units s 60 km/hr

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Problem 2speed / velocity

• The diagram represents the total travel of a
  teacher on a Saturday. Which part of the trip is
  made at the greatest average speed?

This problem requires four steps. We need to
calculate the speed for each part of the trip. s
(Q) 14/12 1.2 km/min s (R) 12/8
1.5 km/min s (S) 15/9 1.7 km/min
s (T) 11/15 0.7 km/min The part with the
greatest average speed is S (1.7 km/min).
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Problem 3 speed GRAPHSThe slope of a
distance-time graph is the speed.
The graph shows the distance traveled by
a vehicle over a certain period of time.
Which segment of the graph shows the
vehicle moving with the greatest speed? On a
speed graph, the part of a line with the
steepest slope has the greatest speed. ANS
L Which segment represents the vehicle at
rest? At rest means the speed is zero. A line
with a slope of 0 is horizontal.
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MOMENTUM

• Momentum measures how much force is necessary to
  stop an object from moving.
• The heavier and/or faster an object is, the more
  momentum it has.

large mass large velocity large momentum
small mass small velocity small momentum
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MOMENTUM

• To calculate momentum (p), we need
• 1. Mass (m) in kilograms (kg)
• 2. Velocity (v) in meters per second (m/s)
• Our formula is
• Momentum mass x velocity
• p mv
• So the units for momentum are kg?m/s.

THIS IS ON YOUR FORMULA SHEET!
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Problem 4 momentum
Car velocity 6.3 m/s Driver velocity 6.3
m/s Driver mass 100 kg
Car velocity 0 m/s Driver velocity 6.3
m/s Driver mass 100 kg
Car velocity 0 m/s Driver velocity 0
m/s Driver mass 100 kg

• The pictures show how an air bag functions in a
  collision. How much momentum in kg?m/s does the
  airbag absorb from the crash-test dummy if all
  the crash-test dummys momentum is absorbed by
  the air bag?
• p mv 100 kg (6.3 m/s) 630 kg?m/s

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Problem 5 momentum

• Which bike rider has the greatest momentum?
• A. A 40 kg person riding at 45 km/h
• p mv 40 kg (45 km/h) 1800 kg?m/s
• B. A 50 kg person riding at 35 km/h
• p 50 kg (35 km/h) 1750 kg?m/s
• C. A 60 kg person riding at 25 km/h
• p 60 kg (25 km/h) 1500 kg?m/s
• D. A 70 kg person riding at 15 km/h
• p 70 kg (15 km/h) 1050 kg?m/s

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ACCELERATION

• To calculate acceleration (a), we need
• 1. Initial and final velocities (VF VI) in
  meters per second (m/s)
• 2. Change in time (?t) in seconds (s)
• Our formula is
• Acceleration Final v Initial v / change in
  time
• a vF-vI / ?t
• So the units for acceleration are m/s2.

THIS IS ON YOUR FORMULA SHEET!
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Problem 6 acceleration

• According to this graph, what was the bicycles
  acceleration between 10 and 12.5 seconds?
• At 10 s, vI is 6.5 m/s.
• At 12.5 s, vF is 8.5 m/s.
• ?t 12.5 s -10 s 2.5 s
• a vF vI / ?t
• 8.5 m/s 6.5 m/s / 2.5 s
• a 0.8 m/s2

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WORK

• To calculate work (W), we need
• 1. Force (f) in Newtons (N)
• 2. Distance (d) in meters (m)
• Our formula is
• Work force x distance
• W fd
• So the units for work are N?m or Joules (J).

THIS IS ON YOUR FORMULA SHEET!
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POWER

• To calculate power (P), we need
• 1. Work (W) in Joules (J)
• 2. Time (t) in seconds (s)
• Our formula is
• Power work / time
• P w / t
• So the units for power are J/s or Watts (W).

THIS IS ON YOUR FORMULA SHEET!
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Problem 7 work power

• The weight lifter used a force of 980 N to raise
  the barbell over her head in 5.21 seconds.
  Approximately how much work did she do in raising
  the barbell?
• W Fd 980 N (5.21 s) 5106 J
• How much power did she have?
• P W / t
• 5106 J / 5.21 s
• P 980 W

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Problem 8 work power

• 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

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Problem 9 work power

• A horizontal force of 600 N is used to push a box
  8 m across a room. Which of these variables must
  be known to determine the power used in moving
  the box?
• A. The weight of the box
• B. The potential energy of the box
• C. The time it takes to move the box
• D. The length of the box

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FORCES

• The overall force on an object is called the net
  force. An object will accelerate when there is a
  net force acting on it.
• Friction is the name given to the force that acts
  between materials that are in contact and moving
  past each other.
• Air resistance is friction between objects and
  the air.
• Gravity is the force that gives an object its
  weight, pulling it toward the center of the earth
  at an acceleration of 9.8 m/s2.

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Newtons 1ST Law

• An object at rest will stay at rest and an object
  in motion will continue that motion
• (this is called INERTIA)
• unless an unbalanced force acts on it.

If you are in an accident and arent wearing a
safety belt
you would continue your motion
Do not let this happen to you buckle up!
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Newtons 2ND Law

• The unbalanced force acting on an object equals
  its mass times its acceleration.
• Force mass x acceleration
• Ex A filled wheelbarrow requires more
  force to move than an empty one.
• The acceleration of a golf ball
  times its mass is equal to the force with
  which its hit.

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Newtons 3RD Law
Action / Reaction
For every action there is an equal and opposite
reaction.
What is the action? What is the reaction?
The gases push downward out of the rocket,
The rocket is pushed upward by gases.
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Problem 10 Forces

• A catapult was designed to project a small
  metal ball at a target. The resulting data
  are shown in the table. Which of
  these might explain the difference
  between the calculated and actual
  distances?
• A The ball landed short of the calculated
  distance because of an increase in
• momentum.
• B Air resistance caused the ball to land short
  of the calculated distance.
• C Initial mass of the ball changed with each
• trial.
• D The metal ball was too small for accurate
• measurements to be made.

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Problem 11 Forces

• After shooting a cannonball, a cannon
• recoils with a much lower velocity than
• the cannonball. This is primarily
• because, compared to the cannonball,
• the cannon has a
• A. much greater mass
• B. smaller amount of momentum
• C. greater kinetic energy
• D. smaller force applied to it

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Problem 12 Forces
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? According to
Newtons 2nd Law, F ma F (0.5 kg)(3
m/s2) F 1.5 N
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Problem 13 Forces

• The illustration to the right shows a student
  about to throw a ball while standing on a
  skateboard. Which illustration below correctly
  shows the skateboards direction of motion after
  the student releases the ball?

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Machines

• Pulley a rope around a grooved wheel. Used to
  change the direction of the force applied to it.
• Ex flagpole, block tackle
• Lever a bar that pivots around a fixed
  point (fulcrum).
• Generally, the farther the effort (input) force
  is away from the fulcrum, and the closer the load
  is to the fulcrum, the easier it is to lift an
  object.

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Machines

• Remember! Machines do not reduce the amount of
  work done they only change the amount of force,
  the distance, the direction or the speed.
• The efficiency of a machine tells us the of
  work put into the machine that is useful. You can
  never get more work out of a machine than you put
  into it! (Efficiency can never be gt 100)

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Problem 14 machines

• Which lever arrangement requires the least effort
  force to raise a 500 N resistance?

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Problem 15 machines

• What is the efficiency of an air conditioner if
  there is a work input of 320 J and a work output
  of 80 J?
• work output (WO) 80 J
• work input (WI) 320 J
• EFF WO x 100 80 J x 100 25
• WI 320 J