Download 1 Study Guide PS2.A: Forces and Motion Learning Target #A

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Study Guide
PS2.A: Forces and Motion
Learning Target #A (background)
Determine when an object is in motion (observer’s frame of reference) and measure its
distance in SI units if cm and km
What is motion?
An object is in motion if its distance from another object is changing.
An object is in motion if it changes position relative to a reference point
How do we decide if we are moving?
To decide if you are moving, you need a reference point.
What is a reference point?
A reference point is a place or object used for comparison to determine if something is in motion.
What objects make good reference points?
Tree, sign, building
What is a frame of reference?
Point of view
What is frame of reference in scientific terms?
Whether an object appears to be at rest or in motion depends on your point of view --- or your “frame of reference.” A
frame of reference uses a coordinate system to establish position.
Why do observers use frame of reference points?
An observer must use frame of reference points in their surroundings to determine distance, speed, and directions of an
observed object.
Which scientists recognized the concept of “frames of reference?”
Galileo, Newton, and Einstein all recognized that the concept of frames of reference was essential when thinking about motion.
What is the International System of Units?
Metric System for measurement
Why do scientists use the International System of Units?
So they can communicate clearly
What is the basic SI unit of length?
Meter (m)
What are other SI units related to the meter in terms of getting bigger or smaller?
(In order) millimeter (mm), centimeter (cm), meter (m), and kilometer (km)
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How do SI units measure distance of an object?
Pencil = centimeters
Bowling ball rolling down a lane = meters
Car traveling from one location to another - kilometers
Learning Target B (background)
Explain and calculate how scientists determine an object’s speed and direction (velocity)
a) Calculate speed, distance, & time
S = d/t
a) Calculate acceleration, force, mass & weight
What is speed?
Speed refers to "how fast an object is moving."
Speed can be thought of as the rate at which an object covers distance.
What are some simple examples that describe speed?
a) A fast-moving object has a high speed and covers a relatively large distance in a short amount of time.
b) Contrast this to a slow-moving object that has a low speed; it covers a relatively small amount of
distance in the same amount of time.
c) An object with no movement at all has a zero speed.
What formula do scientists use to measure speed, distance or time?
a) Speed = distance ÷ time
s=d ÷ t
b) Time = distance ÷ speed
t=d ÷ s
c) Distance = time multiplied by speed
d= t (times) s
What is acceleration?
Acceleration is the rate of change in the speed of an object.
The units for acceleration are meters per second per second or m/s2.
What is a force?
A force is a push or a pull upon an object that results from its interaction with another object.
What is mass?
How heavy something is without the force of gravity
How much matter an object has
What is matter?
Anything that has mass and takes up space
The stuff that makes up everything in the world
All objects are composed of matter
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What are the formulas for calculating force, acceleration, & mass?
F = ma
M=f÷a
A=f÷m
Set up a chart before you begin problem:
Force =
_____
Mass =
_____
Acceleration =
_____
What do the letters in the formula represent?
F = force
M = mass
A = acceleration
Calculate the force based on the following question:
If the car has a mass of 700 kg and a driver pushes the car with an acceleration of .05m/s/s
Chart
Formula
Force =
______
F = M times A
Mass =
700kg
F = 700 times .05
Acceleration = .05m/s/s
F = 35kN
Calculate the mass of a car based on the following question:
If the car has a force of 35kN and a driver pushes the car with an acceleration of .05/m/s/s,
Chart
Formula
Force = 35Kn
M=f÷a
Mass = _______
M = 35/.05
Acceleration = .05m/s/s
M = 700kg
Calculate the acceleration of a car based on the following question:
If the car has a mass of 700 kg and a force of 35kN:
Chart
Formula
Force = 35kN
A=f÷ m
Mass = 700kg
A = 35/700
Acceleration = _______
A = .05m/s/s
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What is weight?
The weight of an object is the force of attraction that the object has to the Earth.
A measure of the gravitational force exerted on an object
Example
Stepping on a bathroom scale actually determines the gravitational force Earth is exerting on you
How do I calculate the weight of an object?
The formula: Weight = mass x acceleration due to gravity
W = mg
How is gravity calculated on the Earth, the Moon, and the Sun?
 On Earth, acceleration due to gravity has a value of about 9.8 N/kg.
On the Moon, acceleration due to gravity has a value of about 1.63 N/kg.
On the Sun, acceleration due to gravity has a value of about 274.1 N/kg.
Compare the moon’s gravity to Earth:
The moon’s gravity is 1/6 of the earths
9.8m/s/s divided by 6
= 1.6N
What is the difference between mass and weight?
If you go from the Earth to live on the Moon you would have the same mass, but your weight would be
different.
 This is because the force of gravity changes depending on which planet you are on.
Calculate your weight on Earth in Newtons if your mass is 70kg
70Kg times 9.8 = 686N on Earth
Calculate your weight on the Moon in Newtons if your mass is 70kg
70kg times 1.63 = 114.1N
How does Newtons relate to our mass on Earth?
On the Earth the force of gravity is 9.81 Newtons per kg.
This means that for every kg of mass you would have a force downwards (your weight) of 10 N. on the Earth
Why does a person’s weight change compared to Earth as on the Moon?
 The gravitational pull on the Moon is much less than the pull on Earth
 The moon is much smaller than the earth.
How can weight be calculated on the moon versus weight on the Earth?
The force of gravity on the moon is only about one sixth as strong as gravity on earth.
What is the weight of a person on the moon that weighs 54lbs and 210lbs?
1/6 times 54/1
= 54/6
= 9lbs
1/6 times 180/1
=210/6
= 35lbs
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Is the speed of a moving object constant?
No, an object will increase and decrease speed at all times
What is velocity?
 The rate at which an object changes its position.
 The direction at which an object is moving
 Speed and direction working together
 Velocity is very important for air traffic controllers to keep planes from colliding.
What is a real life explanation for velocity?
Imagine a person moving rapidly - one step forward and one step back - always returning to the original
starting position. While this might result in a frenzy of activity, it would result in a zero velocity. Because the
person always returns to the original position, the motion would never result in a change in position. Since
velocity is defined as the rate at which the position changes, this motion results in zero velocity. If a person in
motion wishes to maximize their velocity, then that person must make every effort to maximize the amount
that they are displaced from their original position. Every step must go into moving that person further from
where he or she started. For certain, the person should never change directions and begin to return to the
starting position.
Why is velocity direction aware?
When evaluating the velocity of an object, one must keep track of direction.
It would not be enough to say that an object has a velocity of 55 mi/hr. One must include direction
information in order to fully describe the velocity of the object.
For instance, you must describe an object's velocity as being 55 mi/hr, east.
What is the difference between speed and velocity?
Speed does not keep track of direction; velocity is direction aware.
What is an example of a task that describes the direction of velocity?
 The direction of the velocity is simply the same as the direction that an object is moving.
 It would not matter whether the object is speeding up or slowing down.
 If an object is moving rightwards, then its velocity is described as being rightwards.
 If an object is moving downwards, then its velocity is described as being downwards.
So an airplane moving towards the west with a speed of 300 mi/hr has a velocity of 300 mi/hr, west. Note that
speed has no direction and the velocity at any instant is simply the speed value with a direction.
How do I calculate speed of an object?
 Set up a chart before you begin problem:
Speed =
_____
Distance =
_____
Time =
_____
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John is traveling for 2 hours at a rate of 65 mph. How far did he travel?
Speed =
65 mph
Distance =
______
Time =
2 hours
d = ts (t times s)
d = (2 X 65)
d = 130 miles
Randy drove 630 miles at a rate of 70 mph. How much time did he actually travel?
Speed =
70 mph
Distance =
630 miles
Time =
_____
t = d/s
t = (630 70)
d = 9 hours
How do I calculate the velocity of an object?
Jackie is traveling for 3 hours at a rate of 60mph east. How far did he travel?
Velocity (speed) =
60 mph/east
Distance =
______
Time =
3 hours
Formula
d = s times t
d = 60mph east X 3 d = 180 miles east
Learning Target C (background)
Explain how a force is described
a) determine how unbalanced and balanced forces relate to an object’s motion
b) explain how the greater the mass, the greater the force needed to change motion
c) identify factors that affect the gravitational force between two objects
What is a Force?
a push or a pull
How is a force described ?
by its strength and by the direction in which it acts
What SI unit is used to measure the strength of a force?
The strength of a force is measured in the SI unit called the newton (N)
What is the weight of an apple in terms of Newtons?
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Why do scientists used an arrow to show direction and strength of a force?
The direction and strength of a force can be represented by an arrow (which points in the direction of the
force)
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What is a net force ?
the combination of all forces acting on an object
What does the net force determine?
whether an object moves and also in which direction it moves
What happens when forces act in the same direction?
the net force is found by adding the strength of both together
What happens when forces act in opposite directions?
the net force is found by subtracting the strength
What is an Unbalanced Force?
a) Whenever there is a net force acting on an object, the forces are unbalanced.
b) Unbalanced forces can cause an object to start moving, stop moving, or change direction
c) An unbalanced force has kinetic energy
What is a Balanced Force?
Equal forces acting on one object in opposite directions
a) Balanced forces acting on an object do not change the object’s motion
b) A balanced force has potential energy
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What are 2 types of mechanical energy?
Potential
Kinetic
What is potential energy?
Energy that is stored
The object is not moving
What are examples of potential energy?
Table, wall, pencil not being touched, basketball sitting on a rack
What is kinetic energy?
Energy of motion
An object starts moving
How is an object with potential energy affected when its energy turns into kinetic energy?
The object will begin to move and the speed and velocity will depend on what caused it to move
Why does a bus have more kinetic energy than a car, if they are traveling at the same speed?
The bus has more mass
The more mass an object, the more kinetic energy it will have
What is gravity?
A force that pulls objects toward each other
Gravity is what holds us down on the earth's (or moon's) surface.
What is the Law of Universal Gravitation?
Gravity acts everywhere in the universe, not just Earth
What factors that affect the gravitational force between two objects?
1) Mass
2) Distance
What is mass?
Measure of the amount of matter in an object
What is the SI unit for mass?
The SI unit for mass is the kilogram
The more mass an object has, the greater its gravitational force
What is gravitational force?
It depends on the distance between two objects.
The farther apart two objects are, the lesser the gravitational force between them.
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Learning Target #1
I can define Newton’s Third Law
Learning Target #2
I can use Newton’s Third Law to design and describe the motion when two
objects collide
What is Newton’s third law of motion?
If one object exerts a force on another object, then the second object exerts a force of equal
strength in the opposite direction on the first object
 According to Newton, whenever objects A and B interact with each other, they exert
forces upon each other
 “For every action, there is an equal but opposite reaction”
What does “For every action, there is an equal but opposite reaction” mean?
 The statement means that in every interaction, there is a pair of forces acting on the
two interacting objects.
 The size of the forces on the first object equals the size of the force on the second
object.
 The direction of the force on the first object is opposite to the direction of the force on
the second object.
 Forces always come in pairs - equal and opposite
action-reaction force pairs.
Examples:
 Gymnast doing flip with a vaulting horse
 Kayaker’s paddle pulls on the water
 Dog leaps by pushing on the ground
What are examples of Newton’s third law?
I. When you sit in your chair, your body exerts a downward force on the chair and the chair
exerts an upward force on your body.
 There are two forces resulting from this interaction - a force on the chair and a force
on your body.
 These two forces are called action and reaction forces and are the subject of
Newton's third law of motion.
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II.
A variety of action-reaction force pairs are evident in nature.
Consider the propulsion of a fish through the water.
 A fish uses its fins to push water backwards.
 But a push on the water will only serve to accelerate the water.
 Since forces result from mutual interactions, the water must also be pushing the fish
forwards, propelling the fish through the water.
 The size of the force on the water equals the size of the force on the fish;
 the direction of the force on the water (backwards) is opposite the direction of the
force on the fish (forwards).
 For every action, there is an equal (in size) and opposite (in direction) reaction force.
Action-reaction force pairs make it possible for fish to swim.
III.
Consider the flying motion of birds.
A bird flies by use of its wings.
 The wings of a bird push air downwards.
 Since forces result from mutual interactions, the air must
also be pushing the bird upwards.
 The size of the force on the air equals the size of the force
on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on the
bird (upwards).
 For every action, there is an equal (in size) and opposite (in direction) reaction.
 Action-reaction force pairs make it possible for birds to fly.
IV.
Consider the motion of a car on the way to school.
 A car is equipped with wheels that spin. As the wheels spin, they grip the road and
push the road backwards.
 Since forces result from mutual interactions, the road must also be pushing the
wheels forward.
 The size of the force on the road equals the size of the force on the wheels (or car);
the direction of the force on the road (backwards) is opposite the direction of the
force on the wheels (forwards).
 For every action, there is an equal (in size) and opposite (in direction) reaction.
 Action-reaction force pairs make it possible for cars to move along a roadway
surface.
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Learning Target #3
I can define and describe Newton’s 1st and 2nd law
Learning Target #4
I can plan an investigation (experiment) to show that an object’s motion will
depend on the mass of the object and the forces (balanced or unbalanced)
acting on the objects
What is a force?
A force is a push or a pull upon an object that results from its interaction with another object.
What is an unbalanced force?
a) Whenever there is a net force acting on an object, the forces are unbalanced.
b) Unbalanced forces can cause an object to start moving, stop moving, or change
direction
c) On Earth, gravity and friction are unbalanced forces that often change an object’s
motion
Examples:
Clothes on the floor will stay until you pick them up
Tennis ball flies through the air, ONCE you hit it with a racket
What is inertia?
The tendency of an object to resist a change in motion.
 It does not matter whether an object is moving or not, it RESISTS any change to its motion
 Inertia depends on mass – some objects have more mass than others
 The greater the mass of an object, the greater the inertia, and the greater the force required to
change its motion
Example:
Moving forward in your seat when a car stops suddenly
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Newton's First Law
Isaac Newton (a 17th century scientist) put forth a variety of laws that explain why objects
move (or don't move) as they do. These three laws have become known as Newton's three laws
of motion. The focus of Lesson 1 is Newton's first law of motion - sometimes referred to as the
law of inertia.
What is Newton’s first law of motion called?
Law of Inertia
Newton's first law of motion is often stated as
An object at rest stays at rest and an object in motion stays in motion with the same speed
and in the same direction unless acted upon by an unbalanced force.
 states that an object at rest will remain at rest, and an object moving at a constant
velocity will continue moving at a constant velocity, unless it is acted upon by an
unbalanced force
There are two parts to this statement
 one that predicts the behavior of stationary objects
 the other that predicts the behavior of moving objects
The two parts are summarized in the following diagram.
The behavior of all objects can be described by saying that
objects tend to "keep on doing what they're doing" (unless
acted upon by an unbalanced force).
 If at rest, they will continue in this same state of
rest. If in motion with an eastward velocity of 5
m/s, they will continue in this same state of motion
(5 m/s, East).
 If in motion with a leftward velocity of 2 m/s, they will continue in this same state of
motion (2 m/s, left).
 The state of motion of an object is maintained as long as the object is not acted upon
by an unbalanced force.
 All objects resist changes in their state of motion - they tend to "keep on doing what
they're doing."
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Suppose that you filled a baking dish to the rim with water and walked around an oval track
making an attempt to complete a lap in the least amount of time. The water would have a
tendency to spill from the container during specific locations on the track. In general the water
spilled when:
 the container was at rest and you attempted to move it
 the container was in motion and you attempted to stop it
 the container was moving in one direction and you attempted to change its direction.
The water spills whenever the state of motion of the
container is changed. The water resisted this change in its
own state of motion. The water tended to "keep on doing
what it was doing."
 The container was moved from rest to a high speed
at the starting line; the water remained at rest and
spilled onto the table.
 The container was stopped near the finish line; the water kept moving and spilled over
container's leading edge.
 The container was forced to move in a different direction to make it around a curve; the
water kept moving in the same direction and spilled over its edge.
The behavior of the water during the lap around the track can be explained by Newton's first
law of motion.
Everyday Applications of Newton's First Law
There are many applications of Newton's first law of motion.
Consider some of your experiences in an automobile.
Have you ever observed the behavior of coffee in a coffee cup filled to the rim while starting a
car from rest or while bringing a car to rest from a state of motion?
Coffee "keeps on doing what it is doing."
 When you accelerate a car from rest, the road provides an unbalanced force on the
spinning wheels to push the car forward; yet the coffee (that was at rest) wants to stay
at rest.
 While the car accelerates forward, the coffee remains in the same position;
subsequently, the car accelerates out from under the coffee and the coffee spills in your
lap.
 On the other hand, when braking from a state of motion the coffee continues forward
with the same speed and in the same direction, ultimately hitting the windshield or the
dash. Coffee in motion stays in motion.
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Have you ever experienced inertia (resisting changes in
your state of motion) in an automobile while it is
braking to a stop?
 The force of the road on the locked wheels
provides the unbalanced force to change the
car's state of motion, yet there is no unbalanced
force to change your own state of motion.
 Thus, you continue in motion, sliding along the
seat in forward motion. A person in motion stays in motion with the same speed and in
the same direction ... unless acted upon by the unbalanced force of a seat belt.
 Yes! Seat belts are used to provide safety for passengers whose motion is governed by
Newton's laws.
 The seat belt provides the unbalanced force that brings you from a state of motion to a
state of rest.
 Perhaps you could speculate what would occur when no seat belt is used.
There are many more applications of Newton's first law of motion. Several
applications are listed below.






Blood rushes from your head to your feet while quickly stopping
when riding on a descending elevator.
The head of a hammer can be tightened onto the wooden handle by
banging the bottom of the handle against a hard surface.
A brick is painlessly broken over the hand of a physics teacher by
slamming it with a hammer. (CAUTION: do not attempt this at
home!)
To dislodge ketchup from the bottom of a ketchup bottle, it is often
turned upside down and thrusted downward at high speeds and
then abruptly halted.
Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
While riding a skateboard (or wagon or bicycle), you fly forward off the board when
hitting a curb or rock or other object that abruptly halts the motion of the skateboard.
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Newton's Second Law
What is Newton’s second law of motion?
Acceleration depends on the object’s mass AND on the net force acting on the object
What is acceleration?
is measured in meters per second per second (m/s²)
Mass is measured in kilograms (kg)
Force = kilograms times meters per second per second
Newton's second law of motion pertains to the behavior of objects for which all existing forces
are not balanced.
The second law states that the acceleration of an object is dependent upon two variables
 the net force acting upon the object
 the mass of the object.



The acceleration of an object depends directly upon the net force acting upon the
object, and inversely upon the mass of the object.
As the force acting upon an object is increased, the acceleration of the object is
increased.
As the mass of an object is increased, the acceleration of the object is decreased.
Newton's second law of motion can be formally stated as follows:
 The acceleration of an object as produced by a net force is directly proportional to the
magnitude of the net force, in the same direction as the net force, and inversely
proportional to the mass of the object.
This verbal statement can be expressed in equation form as follows:
a = Fnet / m
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The above equation is often rearranged to a more familiar form as shown below. The net force
is equated to the product of the mass times the acceleration.
Fnet = m * a
The Fnet = m • a equation is often used in algebraic problem solving. The table below can be
filled by substituting into the equation and solving for the unknown quantity.
Net Force
Mass
Acceleration
(N)
(kg)
(m/s/s)
1.
10
2
5
2.
20
2
10
3.
20
4
5
4.
10
2
5
5.
10
1
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The numerical information in the table above demonstrates some important qualitative
relationships between force, mass, and acceleration.
 Comparing the values in rows 1 and 2, it can be seen that a doubling of the net force
results in a doubling of the acceleration (if mass is held constant).
 Similarly, comparing the values in rows 2 and 4 demonstrates that a halving of the net
force results in a halving of the acceleration (if mass is held constant).
 Acceleration is directly proportional to net force.
Furthermore, the qualitative relationship between mass and acceleration can be seen by a
comparison of the numerical values in the above table.
 Observe from rows 2 and 3 that a doubling of the mass results in a halving of the
acceleration (if force is held constant).
 And similarly, rows 4 and 5 show that a halving of the mass results in a doubling of the
acceleration (if force is held constant).
 Acceleration is inversely proportional to mass.
 The analysis of the table data illustrates that an equation such as F net = m*a can be a
guide to thinking about how a variation in one quantity might effect another quantity.
 Whatever alteration is made of the net force, the same change will occur with the
acceleration. Double, triple or quadruple the net force, and the acceleration will do the
same.
 On the other hand, whatever alteration is made of the mass, the opposite or inverse
change will occur with the acceleration.
 Double, triple or quadruple the mass, and the acceleration will be one-half, one-third or
one-fourth its original value.