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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
Motion and Position




You don't always need to see something
move to know that motion has taken
place.
A reference point is needed to
determine the position of an object.
Motion occurs when an object changes
its position relative to a reference
point.
The motion of an object depends on the
reference point that is chosen.
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.
Motion
Problem:
 You are a passenger in a car stopped at a
stop sign. Out of the corner of your eye,
you notice a tree on the side of the road
begin to move forward.
 You have mistakenly set yourself as the
reference point.
Speed & Velocity

d
Speed
rate of motion
 distance traveled per unit time

s t
distance
speed 
time
Calculating Speed


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
Average Speed

the total distance traveled divided by the
total time of travel
total distance
avg. speed 
total time
Instantaneous Speed


A speedometer shows how fast a car is
going at one point in time or at one
instant.
The speed shown on a speedometer is
the instantaneous speed. Instantaneous
speed is the speed at a given point in
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.
Speed describes only how fast
something is moving
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

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?
WORK:
GIVEN:
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
A

slope =

steeper slope =
faster speed

B
speed


straight line =
no motion
flat line =
constant speed
Single point =
instantaneous speed
Graphing Motion

Distance-Time Graph
A


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

Distance (m)
300
Acceleration is
indicated by a curve
on a Distance-Time
graph.
200

100
0
0
5
10
Time (s)
15
20
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