Download 3rd Nine Week Benchmark Study Guide

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3
Nine Week Benchmark
Study Guide
1. Define speed, velocity and
distance in your own words.
• Speed is the rate at which an object moves
or how fast it is changing position. The unit
of speed is distance/time like m/s or mph.
• Velocity is both how fast something is
moving and in what direction.
• Distance is a change in an object’s position
measured in meters in the metric system.
2. Calculate speed.
• Speed is calculated by dividing distance by time.
• There is average speed which is the speed you
calculate when speed varies. It is total
distance / total time.
• There is constant speed which is speed that
doesn’t change. Again, this is calculated by
dividing distance traveled by the time it took to
travel that distance.
Example calculation:
1. A school bus driver begins his route at 6:30
a.m. with an odometer reading of 67,930 miles.
He finishes his morning run at 9:30 a.m. and his
odometer then reads 68,005 miles. What is the
average speed of the school bus?
Remember average speed = total distance
total time
Average speed = 68005 – 67930 = 75 miles = 25 mph
3 hrs
3 hrs
Example Calculation:
• A car is traveling at a constant speed. After it
has traveled 30 minutes, it has gone a
distance of 60 km. What is its speed in
km/hour?
Speed = distance / time
Time needs to be in hours 30 minutes = 0.5 hours
Speed = 60 km / 0.5 hrs = 120 km / hr
(about 70 mph)
3. Explain how speed, velocity and
acceleration are all unit rates
A ‘unit rate’ is a how fast something changes
in a unit of time (say 1 second or 1 hour).
The denominator becomes 1 (a unit) when
you divide.
Speed = distance / time
Velocity = distance / time in a given direction
Acceleration = final velocity – initial velocity
time
4. Explain the difference between
velocity and speed
• Speed only tells how fast something is
changing position
• Velocity tells both how fast something is
changing position and in which direction
the object is moving
5. Why do scientists find it necessary to have
a different expression for speed and velocity?
• Scientists often deal with objects that are
changing direction (so velocity is needed)
or several objects moving in relation to
each other (so direction or velocity is
needed). Stating DIRECTION in addition
to SPEED completes the motion picture!
Constant
speed; slope
is the same
distance
Be able to recognize on a graphs
changing and constant speeds
(note that these have the same average speed)
distance
6.
No change
in distance
so speed is
zero
Speed
Speed
increasing;
decreasing;
steeper
slope is ‘flatter’ slope
time
time
7. Define acceleration and name 5 different
ways that an object can accelerate.
•
Acceleration shows how fast an object’s
speed is changing.
An object can accelerate by:
1. Speeding up
2. Slowing down
3. Changing direction
4. Speeding up and changing direction
5. Slowing down and changing direction
8. Calculate acceleration
Acceleration = final velocity – initial velocity
time
A car pulls off of I-85 South to an exit ramp
slowing to a speed of 40 mph from a speed
of 70 mph. It takes the car 1minute to slow
down. What is the rate of acceleration?
Acceleration = 40 – 70 mph = – 30 mph = – 0.50 mph
60 sec
60 sec
sec
The value is negative because the car is slowing down.
Note: units are distance/time/time
9.
Explain why we must know velocity
rather than speed in order to determine
acceleration.
• Because acceleration can be defined as a
change in speed AND/OR direction, an
object moving at a constant speed is
defined as accelerating if it is changing
direction.
• For example, a car making a turn
moving at a constant speed of 25 mph
is considered to be accelerating.
10. Explain the differences of
acceleration and velocity.
• Velocity tells you how fast an object is
moving and the direction in which it is
moving.
• Acceleration tells how fast an object’s
velocity is changing… speeding up,
slowing down and/or changing directions.
11. Explain why an object traveling in a
circle is constantly accelerating even if its
speed/numeric value for velocity is constant.
• The blades of this
windmill are constantly
changing direction as
they turn at a constant
speed. This centripetal
force is an example of
acceleration.
12. Define and describe the difference
between balanced and unbalanced forces.
• You must have more than one force to have
balanced or unbalanced forces.
• Balanced forces combine for a net force of zero
and result in no change in motion when applied to
an object at rest or no change in motion when
applied to an object moving at a constant speed
and in a straight line. In other words, balanced
forces don’t cause acceleration.
• Unbalanced forces combine for some positive or
negative net force. Unbalanced forces cause a
change in motion.
12a, b. Explain and draw force diagrams including
both balanced and unbalanced forces and how
these forces affect motion.
Balanced forces result in no change in an
object’s motion. The net force (after adding
forces) is zero.
FRICTION OPPOSING MOTION
WHEELS PUSHING ON GROUND
Net Force = 0 resulting in constant speed (no acceleration)
12a, b. Explain and draw force diagrams including
both balanced and unbalanced forces and how these
forces affect motion.
• Unbalanced forces result in a change in an object’s
motion causing the object to accelerate (speed up, slow
down or change direction). The net force (after adding
forces) is either positive or negative.
Net force = 75 + (- 50) = 25 N
- 50 N
FRICTION OPPOSING MOTION
+ 75 N
WHEELS PUSHING ON GROUND
Net force is positive resulting in positive acceleration Δ car speeds up.
12c. Explain why we need to know a net
force to determine an object’s motion.
• Balanced forces have a NET FORCE of
ZERO resulting in NO CHANGE in an
object’s motion.
• Unbalanced forces have a POSITIVE or
NEGATIVE NET FORCE resulting in a
CHANGE in an object’s motion.
12d. Be able to identify the direction of
motion based on the net force.
An object will move in the direction of the net
force if the net force is not zero.
Net force is – 55 N
Friction 20 N
Pushing with a force of - 75 N
Pushing with a force of - 75 N
Net force is – 55 N
Friction 20 N
12d. Be able to identify the direction of
motion based on the net force.
An object will move in the direction of the net
force if the net force is not zero.
He pulls up against the
pull down of gravity. The
magnitude of the forces
is equal to the weight of
the boxes with an equal
force. The net force is
zero, so the boxes do not
fall or move up.
12d. Be able to identify the direction of
motion based on the net force.
An object will move in the direction of the net
force if the net force is not zero.
+ 40 N
+ 40 N
- 10 N Net force + 30 N
The net force is + 30 N
to the right (+ direction).
Because the net force
is not zero, this pair of
forces is unbalanced
resulting in motion of
the grocery cart.
- 10 N
13. What is the abbreviation for and unit name for
force in the international system of measurement?
The unit of measure for force is the Newton,
abbreviated N.
5
5
He’s lifting
barbells with a
weight of 5 N!
14. Newton’s Laws: Explain each
in your own words
Newton’s First Law has to do with inertia which is related to
an object’s mass.
The more mass or inertia an object has, the harder it is to
get it to move OR the harder it is to change its movement.
Also, objects that aren’t moving or that are moving at a
constant speed and in a straight line will keep doing what
they’re doing unless an unbalanced force causes the motion
to change.
The baseball has low inertia
(mass) and we can make it
move with minimal force. It
will just sit here unless an
unbalanced force is applied.
14. Newton’s Laws: Explain each
in your own words
Newton’s Second Law tells us how force, mass
and acceleration are related.
Basically, if you want something to move, that
something has a mass (m). To get it to move, you
have to apply a force (F). The equation F = ma
will tell you the rate of acceleration (a).
If the dude wants to make
the ball speed up a lot (high
rate of acceleration), he is
going to have to apply a big
kick (force) to it.
14. Newton’s Laws: Explain each
in your own words
Newton’s Third Law tells us that forces
come in pairs. When a force acts on an
object, that object exerts an equal and
opposite force back on the first object.
Example: When you walk down
the street, you push off of the
street and the street
pushes back on you.
15. Calculate acceleration given a
force and a mass.
How fast will a baseball accelerate if it has a
mass of 0.145 kg and is hit with a force of
3400 N?
F = ma solving for a gives us
a = F/m
a = 3400 N / 0.145 kg
a = 23,400 m/s/s
16. Understand the relationship between
force, mass and acceleration
Newton’s Second Law tells us how force,
mass and acceleration are related.
For constant mass:
F = ma as F↑ a↑ and as F↓ a↓
For constant Force:
F = ma as m↑ a↓ and as m↓ a↑
For constant acceleration:
F = ma as F↑ m↑ and as F↓ m↓
17. Define and explain the
properties of inertia
Inertia is basically an object’s tendency to resist a
change in its current state of motion. Inertia is
proportional to mass.
An object that has low inertia (mass) will be easy
to move if it is not moving.
An object that has high inertia (mass) will be hard
to move if it is not moving
The same is true of changing the object’s motion if
it is moving…
18a and 18b: Give examples of objects with
greater inertia and explain why it’s harder to make
this object accelerate.
The more mass something has, the more
inertia it has. It’s hard to make this object
accelerate (speed up, slow down and/or
change direction) because it has a lot of
MASS to move differently.
19. How do we measure weight
and inertia?
• Weight is measured with a scale that
shows how much gravity is pulling on an
object.
• Inertia is measured indirectly by
determining how much force has to be
applied to make something accelerate.
20. Friction
• Friction is a force that opposes motion.
Friction is due to contact of surface and
the force between them.
• There are several types: sliding, rolling,
fluid (including air resistance) and static
(between two surface that are not moving).
21. Explain what happens in terms of
Newton’s Laws to an object with balanced
forces when friction is added.
Because friction is a FORCE that acts in the
opposite direction of an applied force,
friction will SLOW an object that is moving
at a constant speed.
Static friction is also responsible for a small
part of an object’s inertia (its tendency to
NOT move)
22.
Be able to identify the direction of frictional
force in an illustration when the direction of motion
is identified.
Friction OPPOSES motion so the friction
force arrow will be in the opposite direction
of motion.
MOTION
friction
friction
23. What are some ways to reduce
friction?
You can reduce friction by:
- Minimizing force between objects
- Making surfaces more regular or smooth
- Use wheels or change sliding to rolling
friction
- Use a fluid as a lubricant – water, oil,
grease
24 and 25. Define gravity and state
the Law of Universal Gravity
Gravity is a pulling force exerted by anything
that has mass.
The Law of Universal Gravity says that:
ALL objects attract each other with a force
of gravitational attraction. Gravity is
universal.
26 and 27. Where does gravity
exist and what affects it?
Gravity exists everywhere.
Gravity is affected by an object’s mass and
it’s proximity (closeness) to other objects.
Gravity increases as an object’s mass goes
up (and goes down as mass goes down).
Gravity increases as objects get closer
together (and decreases as they get
farther apart).
28. Be able to explain the relationship
between mass and weight.
MASS is how much stuff is in something.
WEIGHT is a measure of how gravity pulls
on something’s mass.
MASS will not change. WEIGHT changes
depending on the source of gravity.
29. Be able to find both the acceleration,
speed and distance an object will reach
when dropped.
Remember that the pull of gravity (acceleration)
here on Earth is g = 9.8 m/s/s. This is a
CONSTANT and does not change (for the most
part).
To find an object’s speed after a certain number of
seconds, multiply 9.8 m/s/s by the number of
seconds.
v = gt where g = 9.8 m/s/s and t is time
To find the distance traveled, use the formula
h = 1/2 g t² where g = 9.8 m/s/s
and t is the time in seconds
30. Explain how satellites and
orbits work.
• Satellites are able to remain in an orbital
motion because of a balance between
gravity pulling on an object (free fall) and
its forward motion at a constant speed.
When these are in balance, the satellite
maintains its orbital motion.
• This force is called centripetal force.
31. Identify the force of gravity that would
produce the greater or lesser acceleration.
Gravity from a less massive object will
produce less acceleration.
Gravity from a more massive object will
produce more acceleration.
Gravity from objects farther from each other
will produce less acceleration.
Gravity from objects that are closer together
will produce more acceleration.
32 and 33. Why can zero gravity never exist
and why can an object never truly be
weightless?
Gravity exists everywhere.
An object can never truly be weightless
because gravity, that is everywhere, gives
an object weight due to the force pulling
on its mass.
34 and 35.
• What are some tools/instruments used to
measure mass and/or weight?
– Mass- balance
– Weight- scales
• What is the relationship of mass and
weight with regards to the force of gravity?
Mass- stays the same regardless of gravity
Weight- depends on gravitational force
36a. Define work and explain how you can
determine if work is done on an object
Work (scientifically) is done on an object
when a force is applied and results in the
object moving in the SAME direction as the
applied force.
Example: When you write with a pencil, you
are doing work on the pencil because the
pencil moves in the direction in which you
push.
37. Practice calculating work and
know the unit of measure.
Work = Force X Distance (in same direction)
Newtons X meters = N ● m = joule
If you push a box with a force of 30 N and
the box moves 4 meters, how much work
have you done on the box?
Work = F ● d = (30 N)(4 m) = 120 joules (j)
38 and 39. Define power and practice
calculating it. What is its unit of measure?
Power is the RATE at which work is being
done.
Power = work
time
Example: How much power
does a machine have that
can do 500 j of work in 50
seconds?
Power = 500 j / 50 sec = 10 watts
40. Explain how energy is conserved
using a simple machine.
Because energy cannot be created or destroyed (law
of conservation of energy) using a machine does not
enable us to get more energy out than we put in.
A machine only makes the work easier to do by
changing the size of the force required or the
direction of the force. If the size of the force changes,
the distance the object moves must change too.
Remember: Work in = Work out
Most of the time it is Work in = Work out + heat
41. Explain how changing the output force
affects the distance and the amount of work
done. (see page 195)
The output force is the force that results from using a
machine. The input force is the force put into the machine.
Because the equation for work is W = F x d decreasing the
output force will increase the distance and increasing the
output force will decrease the distance.
This is the force/distance trade-off.
41a and b. Explain how work input and work
output are different but related. How do machines
make work easier?
Because work is constant, the amount of work we put
in is equal to the amount we get out. HOW that work
gets done (force x distance) is how they differ.
Because machines only make doing the work
EASIER, machines do not change the AMOUNT of
work done.
The goal of machines is to make the forces and
distances change to make the work easier.
42. Explain how work changes if the force
or distance is changed.
Because the equation for work is:
Work = Force x distance
as F ↑
d ↓ (work stays same)
as F ↓
d ↑ (work stays same)
If the force stays the same (magnitude) and
only the direction changes, the distance will
be the same.
43. Explain how the six types of simple
machines operate and give an example of
each.
See Section 8.3 for this.
Make sure you look at the diagrams to
understand how each works and the
differences between the three types of
levers.
44. Define a compound machine and
identify the simple machines that make up
the machine.
A compound machine is a machine made of
two or more simple machines.
A bicycle is a compound machine made of
wheels and axles, pulleys (gears and chain),
levers (the pedals) and screws that hold
things in place.
45. What is mechanical advantage?
The mechanical advantage of a machine
tells you how many times the machine
multiplies the force.
MA = output force
input force
Example: If a machine’s
input force is 10 Newtons and
the output force is 50 N, the
mechanical advantage is:
MA = 50 N = 5
10 N
46. What is mechanical efficiency?
P. 197
Mechanical efficiency is a comparison of the
machine’s work output with the machine’s input.
Mechanical efficiency = work output x 100
work input
Most machines generate less work (output) due to
the formation of heat energy. A machine that
has 100% efficiency is called an ideal machine.