Physics 1010 Review

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Physics 1010 Review

• Dr. Don Franceschetti
• September 21, 2007

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Physics 1010 first hour test

• On September 25 during normal class hour.
• A few minutes devoted to last minute questions.
  You will have at least an hour for the test,
  which is more than most of you will need.
• 40 questions, mostly multiple choice.
• Simple arithmetic only. You may bring a
  calculator but you wont need one.
• The few equations you might need will be on the
  test or on the board.

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Be sure you understand

• Velocity m/s
• Acceleration m/s2
• Mass kg
• Force N kg m/s2
• Net force
• Weight W mg
• Newtons three laws of motion.

4
What do I need to remember?

• The difference between velocity and acceleration.
• Can you name a situation in which an object has
  high velocity but zero acceleration?
• Can you name a situation in which an object has a
  definite acceleration but zero velocity?

5
What do I need to remember?

• The difference between mass and weight.
• Mass is the amount of stuff and is measured in
  kg.
• Weight is the force with which the Earth (or
  other planet) attracts an object. It is measured
  in newtons and varies from place to place.

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What do I need to remember?

• What goes on in free fall?
• An object is in free fall if the only force
  acting on it is gravity.
• On earth objects in free fall change their
  (vertical component of) velocity by 10 m/s every
  second. Use your common sense to figure if the
  velocity goes up or down.

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Newtons first law

• If there is zero net force on an object it is
  moving in a straight line with constant speed
  (maybe zero).
• If an object is moving in a straight line with
  constant speed the net force on it must be zero.
• If a object is moving with changing speed or
  direction there must be a non-zero net force
  acting on it.
• If there is non-zero net force acting on an
  object it is changing its speed or direction of
  motion or both.

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Newtons second law.

• Net force on any object
• mass x acceleration.
• Knowing any two of the three quantities lets you
  determine the third.

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Newtons Third law

• For any two objects A and B, if A exerts a force
  on B, B exerts a force equal in size and in the
  opposite direction on A.
• It doesnt matter if the objects are moving or
  not at the time or whether one object is much
  bigger than the other.

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In free fall

• The gravitational force on a more massive body is
  greater than that on a less massive body.
• But then so is the more massive bodys resistance
  to acceleration.
• So, the two bodies have the same acceleration and
  hit the ground at the same time

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What is a conservation law?

• A conservation law is a statement that under
  certain conditions, the value of a definite
  quantity for a physical system does not change in
  time.
• The conservation laws are consequences of
  Newtons laws and often make it easier to explain
  what is going on in a physical system.

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How many conservation laws are there, and what
are they?

• There are just three in Newtonian mechanics
• Momentum is conserved in the absence of an
  external force.
• Total Energy is conserved.
• Angular momentum is concerned in the absence of
  an external torque. (Dont worry about this one
  until after the test.)

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Momentum

• Is mass x velocity (kg m/s). Adds up over the
  parts of a system.
• Forces between the parts of a system do not
  change total momentum because of Newtons third
  law.

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A momentum example

• A car of mass 1000 kg traveling at 15 m/s
  rear-ends a pickup of mass 2000 kg initially at
  rest on a patch of ice. Assuming they lock
  bumpers at what speed do they move off?
• 3000kg v 1000kg x 15 m/s
• V (1000/3000) x 15 m/s 5 m/s

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Impulse

• Is the change in momentum produced by the force
  acting on an object and is equal to the force
  applied times the time it acts.
• Ft?(mv)
• Impulse can be calculated either from the force
  and time or from ?(mv).

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An impulse question

• In the car collision just considered, how much
  impulse did the first car deliver to the second.
• For the 2000 kg car the change in mv is 2000kg x
  5 m/s 10,000 kg m/s

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An impulse question

• And how much impulse did the second car deliver
  to the first?
• Change in mv is 1000kg x (5 - 15) m/s
• -10,000 kg m/s
• Exactly the opposite of that delivered by the
  first car.

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Impulse-Momentum Theorem

• The impulse delivered by the net force acting on
  a system is equal to the change in its momentum.

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Energy

• First define work
• Work F// d, no movement, no work no F// , no
  work

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Kinetic Energy and the Work-Energy Theorem

• Kinetic Energy (1/2) m v2
• Work done by net force ?(KE)

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Sometime you can get work back

• W(cons) ?(KE) W (noncons)
• Loss of PE ?(KE) gain in heat
• PE is potential energy
• ?(KE) ?(PE)gain in heat energy0
• Total Energy KEPEheat is conserved

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If an object is not moving

• It has zero momentum
• It has zero kinetic energy

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Gravitational Potential Energy

• PEmgh
• Is converted into KE as the body falls.
• Is converted into heat energy when the body goes
  splat

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A rock is tossed upward at 30 m/s. How high up
does it go?

• Old approach.
• It loses 10m/s in speed every second so it will
  go up for 3 seconds.
• Its average speed will be (300)/215m/s so it
  will rise 45 m.

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A rock is tossed upward at 30 m/s. How high up
does it go?

• Solve by conservation of energy
• Initial PE 0,
• Initial KE0.5m(30 m/s)2 is total energy.
• At max height KE0 PE KE const
• m10m/s2 h 0.5m(30 m/s)2
• h 45 m

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As an object falls freely downward, itsA)
velocity increasesB) acceleration increasesC)
both of theseD) none of theseFor an object in
free-fall, the acceleration is constant and not
zero, so the velocity must be changing.
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Important point

• Acceleration and velocity are different.
• Acceleration has units of m/s2.
• Velocity has units of in m/s.

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The gain in speed each second for a freely
falling object is about

• A) 0
• B) 5m/s
• C) 10 m/s
• D) 20 m/s

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If no external forces are acting on a moving
object it will

• A) continue moving at the same speed.-- ok
• B) continue moving with the same velocity.
• C) move slower and slower until it finally stops.
• This is counter to experience because there is
  always a force of friction and/or fluid
  resistance acting on objects in our environment.

30
When a rocket ship accelerating in outer space
runs out of fuel it

• A) accelerates for a short time, then slows down
  to a constant velocity.
• B) accelerates for a short time, and eventually
  stops.
• C) no longer accelerates.

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Galileos use of inclined planes allowed him to
effectively

• A) slow down the acceleration of free fall.
• B) increase the acceleration beyond that of free
  fall.
• C) eliminate the acceleration of free fall.
• D) eliminate friction.

32
A car maintains a constant velocity of 10 km/hr
for 10 seconds. During this interval, its
acceleration is

• A) zero
• B) 10 km/hr
• C) 110 km/hr
• D) 1000 km/hr

33
An object is released from rest near the surface
of a distant planet and begins to fall freely.If
the acceleration of gravity there is twice that
of the earth, its speed one second later would be

• A) 10 m/s
• B) 20 m/s
• C) 30 m/s
• D) 40 m/s

34
If an object moves with constant acceleration,
its velocity must

• A) be constant also.
• B) change by the same amount each second.
• C) change by varying amounts depending on its
  speed.
• D) always decrease.

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If a car accelerates from rest at 2 meters per
second each second, its speed three seconds later
will be

• A) 2 m/s
• B) 3 m/s
• C) 4 m/s
• D) 6 m/s

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Ten seconds after starting from rest, a car is
moving at 40 m/s. What is the cars acceleration
in meters per second per second?

• A) 0.25
• B) 2.8
• C) 4.0
• D) 10
• E) 40

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A kilogram is a measure of an objects

• A) weight
• B) force
• C) mass
• D) size
• Its important to be able to distinguish between
  weight and mass. Mass is the amount of substance
  and is measured in kg. The weight is the
  gravitational force acting on it and is properly
  measured in newtons. Weight will vary from place
  to place.

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Your weight is

• A) equal to your mass.
• B) the gravitational attraction between you and
  the earth.
• C) a property of mechanical equilibrium.
• D) all of these.

39
A rock weighs 30 N on Earth. A second rock
weighs 30 N on the moon. Which of the two rocks
has the greater mass?

• A) the one on Earth.
• B) the one on the moon.
• C) They have the same mass.
• D) not enough information to say

40
A 10-kg brick and a 1-kg book are dropped in a
vacuum. The force of gravity on the 10-kg brick
is

• A) the same as the force on the 1-kg book.
• B) 10 times as much as the force on the 1-kg
  book.
• C) one-tenth as much as the force on the 1-kg
  book.
• D) zero.

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Important Point

• Weight, mass and acceleration are different
  quantities. An objects weight is the
  gravitational force on the object. It is
  proportional to the objects mass. In free fall
  the acceleration is
• a net force / mass mg / m

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A block is dragged without acceleration in a
straight line path across a level surface by a
force of 6 N. What is the force between the
block and the surface?

• A) less than 6 N
• B) more than 6 N
• C) 6N
• D) need more information to say
• This is the force of friction.

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In each second, the distance a freely falling
object will fall is

• A) about 5 m.
• B) about 10 m.
• C) the same, but not 5 m or 7 m.
• D) increasing
• E) none of these

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When a rock thrown upward gets to the exact top
of its path, its

• A) velocity is zero and its acceleration is zero.
• B) velocity is zero and its acceleration is about
  10 meters per second per second.
• C) velocity is about 10 m/s and its acceleration
  is zero.
• D) velocity is about 10 m/s and its acceleration
  is about 10 meters per second per second.
• E). None of these.
• -again, you must distinguish between acceleration
  and velocity.

45
A bullet is fired straight down from the top of a
high cliff. Neglecting air resistance, the
acceleration of the bullet in meters per second
per second is

• A) less than 9.8.
• B) is 9.8
• C) is more than 9.8
• D) depends on the height of the cliff.

46
Compared to a 1-kg. block of solid iron a 2-kg
block of solid iron has twice as much

• A) inertia
• B) mass
• C) volume
• D) all of these
• E) none of these

47
Compared to the mass of a certain object on
Earth, the mass of the same object on the moon is

• A) one sixth as much.
• B) the same.
• C) six times as much
• D) zero
• Mass is the amount of stuff and does not depend
  on location.

48
The newton is a unit of

• A) force
• B) mass
• C) density
• D) inertia

49
An objects weight may properly be expressed in
units of

• A) meters
• B) kilograms
• C) newtons
• D) cubic centimeters
• It is a force, after all.

50
A 10-N falling object encounters 4 N of air
resistance. The net force on the object is

• A) 0 N
• B) 4 N
• C) 6 N
• D 10 N
• E none of these

51
An apple weighs 1 N. When held at rest above
your head , the net force on the apple is

• A) 0 N.
• B) 0.1 N.
• C) 1 N
• D) 9.8 N
• E) none of these
• Be sure to understand net force as
• The sum of all forces acting on an object
• Mass times acceleration

52
A heavy block at rest is suspended by a vertical
rope. When the block is accelerated upward by
the rope, the rope tension

• A) increases
• B) decreases
• C) remains at rest
• The net force is the amount by which the tension
  exceeds the weight of the block and this is equal
  to its mass times its acceleration.

53
A bag of groceries has a mass of 10 kilograms and
a weight of

• A) about 1 N.
• B) about 10 N.
• C) about 100 N.
• D) about 1000 N.
• Weight mg because, when gravity is the only
  force acting on the object it produces an
  acceleration of g.

54
A falling skydiver of mass 100 kg experiences 500
N of air resistance. The acceleration of the
skydiver is

• A) 0.2 g
• B) 0.3 g
• C) 0.4 g
• D) 0.5 g
• E) more than 0.5 g

55
As a ball falls, if we call the action force the
pull of the Earths mass on the ball. The
reaction force is the

• A) air resistance against the ball.
• B) acceleration of the ball
• C) pull of the balls mass on the Earth
• D) none of these

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Be sure you understand

• Momentum mv kg m/s
• Impulse Ft N s
• NET Ft change in mv
• Work Fd (in direction of F)
  N m
• KE (1/2)mv2 joules (J)
• NET work change in KE
• PE energy stored in
  position
• Grav PE mgh
• Power work/time watts (W)

57
Padded dashboards in cars are safer in an
accident than non padded ones because an occupant
hitting the dash has

• A) increased time of impact
• B) decreased time of impact
• C) decreased impulse
• D) increased momentum

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A ball is moving at 3 m/s and has a momentum of
48 kg m/s. What is the balls mass?

• A) 4 kg
• B) 12 kg
• C) 16 kg
• D) 144 kg
• E) none of these

59
If an object has kinetic energy, then it must
also have

• A) impulse
• B) momentum
• C) acceleration
• D) force
• E) none of these

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A bullet has more kinetic energy than the
recoiling rifle from which it is fired because
the force on the bullet acts over a

• A) longer time
• B) longer distance
• C) longer time and greater distance

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A 1-kg ball dropped from a height of 2 m rebounds
only 1.5 m after hitting the ground. The amount
of energy converted to heat is

• A) 0.5 J
• B) 5 J
• C) 50 J
• D) insufficient information
• PE difference is 1kg10 m/s20.5 m

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Good Luck!