Chapter 11 section 2
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«num»
Compare event A to event B.
When motion is in a straight line, vectors add and subtract
easily; In this case displacement. Direction of the vectors
indicates the sign (add or subtract).
A
B
Add the magnitudes of two displacement vectors that
have the same direction.
Two displacement vectors with opposite directions are
subtracted from each other.
Displacement only explains part of the story:
Ch 11 Section 2: Speed and Velocity
Speed is the ratio of the distance an object moves to the
amount of time the object moves.
The SI unit of speed is meters per second (m/s).
The two ways to describe the speed of an object are average
speed and instantaneous speed.
1. Average speed is computed for the entire time of a trip.
2. Instantaneous speed, by contrast, is measured at a
particular instant.
In different situations, either one or both of these measurements may be a
useful way to describe speed.
Average speed: Sometimes it is useful to know how fast
something moves for an entire trip. Average speed, vavg, is the
total distance traveled, d, divided by the time, t, it takes to
travel that distance.
During the time an object is moving,
its speed may change, but this
equation tells you the average speed
dtotal
vavg =
ttotal
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over the entire trip. Sometimes, you need to know how fast
you are going at a particular moment.
Instantaneous speed, vins, is the rate at which an object is
moving at a given moment in time; go to page 335.
A distance-time graph is a good way to describe motion. On a
line graph, slope is the change in the vertical axis value
divided by the change in the horizontal axis value.
The slope of a line on a distance-time graph is speed, or the
change in the distance divided by the change in time.
Velocity: is a description of both speed and direction of
motion. Velocity is a vector.
A change in velocity can be the result of a change in speed,
a change in direction, or both.
Sometimes the motion of an object involves more than one
velocity. Two or more velocities add by vector addition.
(Just like Displacement) When motion is in a straight line,
vectors add and subtract easily; In this case velocity.
Direction of the vectors indicates the sign (add or
subtract).
Examples of distance time graphs:
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Chapter 11 section 2
«F_Name» «L_Name»
«num»
In A the river and the boat are in a straight
line and same direction; so you add their
values to get the resultant.
In C the river and the boat are in a straight
line but opposite direction; so you
subtract their values to get the resultant.
B is the tricky one, the river and the boat
are not in a straight line; in this case you
will have to add “graphically”. Since the
two vectors are at a right angle to each
other we will use Pythagoreans theorem.
Z2 = X2 + Y2
Homework Page 337 #’s 1-7 and the 11.2
Handout
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Page 337 #’s 1-7
1. Velocity describes both speed and direction of motion.
2. The slope of a line on a distance-time graph is equal to
speed.
3. Average speed is calculated for the entire duration of a
trip, whereas instantaneous speed is determined at a
single moment.
4. Two or more velocities can be combined by vector
addition.
5. A speedometer measure speed at the current moment, so
it shows instantaneous speed, not average speed. Because
a speedometer does not show direction it does not show
velocity.
6. Use a stopwatch to measure the time for the car to travel
down the incline. The average speed would be calculated
by dividing the distance traveled by the total time.
7. Slope is equal to the change in vertical value divided by
the change in horizontal value. On a distance-time
graph, the change in vertical value is a distance and the
change in horizontal value is a time. Therefore, the slope
is distance divided by time, which equals average speed.
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