1. Describing Motion Motion Speed & Velocity
Motion & Forces
Describing Motion
Motion
Speed & Velocity

2. Newton’s First Law Newton’s First Law of Motion constant velocity
Law of inertia
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

3. Huh?
Theory of Relativity
Must have a reference point

4. Motion Problem: We need a reference point... Is your desk moving?
nonmoving point from which motion is measured

5. Motion Motion Change in position in relation to a reference point.

6. Relative Motion
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.

7. 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).

8. 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.

9. 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.

10. d s t Speed & Velocity Speed rate of motion
distance traveled per unit time

11. 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).

12. Speed & Velocity Instantaneous Speed Average Speed
speed at a given instant
Speed odometer reading
Average Speed
the total distance traveled divided by the total time of travel

13. Changing Instantaneous Speed
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

14. 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!

15. Speed & Velocity Velocity speed in a given direction
can change even when the speed is constant!

16. t d s Calculations d = 100 m s = d ÷ t t = 20 s s = (100 m) ÷ (20 s)
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 = ?
WORK:
s = d ÷ t
s = (100 m) ÷ (20 s)
s = 5 m/s
You skate faster! s d t

17. t d s Calculations s = 330 m/s t = d ÷ s d = 1km = 1000m
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:
s = 330 m/s
d = 1km = 1000m
t = ?
WORK:
t = d ÷ s
t = (1000 m) ÷ (330 m/s)
t = 3.03 s s d t

19. Graphing Motion speed slope = steeper slope = Straight line =*
07/16/96
Graphing Motion
Distance-Time Graph A B
speed
slope =
steeper slope =
Straight line =
flat line =
Single point = instantaneous speed
faster speed
no motion
constant speed *

20. Graphing Motion Who started out faster? A (steeper slope)
Who had a constant speed? A
Describe B from min.
B stopped moving
Find their average speeds.
A = (2400m) ÷ (30min) A = 80 m/min
B = (1200m) ÷ (30min) B = 40 m/min
Distance-Time Graph A B

21. Graphing Motion
Distance-Time Graph
Acceleration is indicated by a curve on a Distance-Time graph.
Changing slope = changing velocity

22. Graphing Motion Specify the time period when the object was...
slowing down
5 to 10 seconds
speeding up
0 to 3 seconds
moving at a constant speed
3 to 5 seconds
not moving
0 & 10 seconds
Speed-Time Graph