Transcript Motion & Forces

Motion & Forces
Describing Motion
Motion
 Speed & Velocity

Newton’s First Law

Newton’s First Law of Motion

An object at rest will remain at rest
and an object in motion will continue
moving at a constant velocity unless
acted upon by a net force.
constant velocity
net force
motion
Motion

Problem:


Is your desk moving?
We need a reference point...

nonmoving point from which motion is
measured
Motion

Motion

Change in position in relation to a reference
point.
Reference point
Motion
Relative Motion
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
If you are sitting in a chair reading this
sentence, you are moving.
You are not moving relative to your desk
or your school building, but you are
moving relative to the other planets in
the solar system and the Sun.
Motion
• Distance
• Distance describes how far an
object has moved.
• The SI unit of length or distance is
the meter (m). Longer distances
are measured in kilometers (km).
• Shorter distances are measured in
centimeters (cm).
Motion
• Displacement is the distance and
direction of an object's change in
position from a reference point.
• Suppose a runner jogs to the 50-m
mark and then turns around and runs
back to the 20-m mark.
• The runner travels 50 m in the original
direction (north) plus 30 m in the
opposite direction (south), so the total
distance she ran is 80 m.
Speed & Velocity

Speed
rate of motion
 distance traveled per unit time

d
s t
distance
speed 
time
Calculating Speed

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The SI unit for distance is the meter
and the SI unit of time is the second
(s), so in SI, units of speed
Sometimes it is more convenient to
express speed in other units, such as
kilometers per hour (km/h).
Speed & Velocity

Instantaneous Speed
speed at a given instant
 Speed odometer reading


Average Speed

the total distance traveled divided by the
total time of travel
total distance
avg. speed 
total time
Changing Instantaneous
Speed

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When something is speeding up or
slowing down, its instantaneous speed is
changing.
If an object is moving with constant
speed, the instantaneous speed doesn't
change.
To determine direction you need to
know the velocity
Speed & Velocity

Problem:

A storm is 10 km away and is moving at a
speed of 60 km/h. Should you be worried?

It depends on the
storm’s direction!
Speed & Velocity

Velocity
speed in a given direction
 can change even when the speed is
constant!

Calculations
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Your neighbor skates at a speed of 4 m/s. You
can skate 100 m in 20 s. Who skates faster?
GIVEN:
d = 100 m
t = 20 s
s=?
d
s t
WORK:
s=d÷t
s = (100 m) ÷ (20 s)
s = 5 m/s
You skate faster!
Calculations

Sound travels 330 m/s. If a lightning bolt
strikes the ground 1 km away from you, how long
will it take for you to hear it?
GIVEN:
WORK:
s = 330 m/s
t=d÷s
d = 1km = 1000m t = (1000 m) ÷ (330 m/s)
t=?
t = 3.03 s
d
s t
Graphing Motion
Distance-Time Graph  slope = speed
A
B

steeper slope =faster speed

Straight line = no motion

flat line = constant speed

Single point = instantaneous
speed
Graphing Motion
Distance-Time Graph
A
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B
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Who started out faster?
 A (steeper slope)
Who had a constant
speed?
 A
Describe B from 10-20
min.
 B stopped moving
Find their average
speeds.
 A = (2400m) ÷ (30min)
A = 80 m/min
 B = (1200m) ÷ (30min)
B = 40 m/min
Graphing Motion
Distance-Time Graph
400
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Distance (m)
300
200
100
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0
0
5
10
Time (s)
15
20
Acceleration is
indicated by a
curve on a
Distance-Time
graph.
Changing slope =
changing velocity
Graphing Motion
Specify the time period
when the object was...
 slowing down
 5 to 10 seconds
 speeding up
 0 to 3 seconds
Speed-Time Graph
3
Speed (m/s)
2
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1
0
0
2
4
6
Time (s)
8
10

moving at a constant
speed
 3 to 5 seconds
not moving
 0 & 10 seconds