1. S8P3 – Relationship between the cause and effect between force, mass and the motion of objects

2. Grade 8 Science Standards
S8P3 Students will investigate relationships between force, mass and the motion of objects.
A. Determine the relationship between speed, velocity and acceleration.
B. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

3. Motion/ Reference Point
Motion is defined as an object having a change in position.
But how do you know if something has moved or not? P. 282
Reference Point- A stationary point of comparison used to measure the distance an object has moved.
Position- The point or area occupied by a physical object.

4. Frame of Reference
1 minute!!! Discuss with the person next to you: Is the speed of the ball different relative to the pitcher, the truck driver, and the jet pilot? Why or why not?

5. Distance Distance is the measurement from one point to another.
Distance may refer to a physical length, a period of time, or an estimation.

6. Distance d = 3 cm Distance (d) – how far an object travels.
Does not depend on direction.
Imagine an ant crawling along a ruler.
What distance did the ant travel? cm 1 2 3 4 5 6 7 8 9 10
d = 3 cm

7. Distance Distance does not depend on direction.
Here’s our intrepid ant explorer again.
Now what distance did the ant travel?
d = 3 cm
Does his direction change the answer? cm 1 2 3 4 5 6 7 8 9 10

8. Displacement
Displacement is the direction and the distance in an objects position from its starting and ending points.

9. Displacement
Let’s revisit our ant, and this time we’ll find his displacement.
Distance: 3 cm
Displacement: +3 cm
The positive gives the ant a direction! cm 1 2 3 4 5 6 7 8 9 10 + -

10. Displacement Find the ant’s displacement again. - + Distance: 3 cm
Remember, displacement has direction!
Distance: 3 cm
Displacement: -3 cm cm 1 2 3 4 5 6 7 8 9 10 + -

11. Displacement Find the distance and displacement of the ant. - +
Distance: 7 cm
Displacement: +3 cm cm 1 2 3 4 5 6 7 8 9 10 + -

12. Displacement vs. Distance
Example of distance:
The ant walked 3 cm.
Example of displacement:
The ant walked 3 cm EAST.
An object’s distance traveled and its displacement aren’t always the same!

13. Distance vs. Displacement
You drive the path, and your odometer goes up by 8 miles (your distance).
Your displacement is the shorter directed distance from start to stop (green arrow).
start
stop

14. Practice Problem 1
An athlete runs around a track that is 100 meters long three times, then stops.
What is the athlete’s distance and displacement?
Distance = 300 m
Displacement = 0 m
Why?

15. Practice Problem 2
A whale swims due east (from 0km) a distance of 5km, turns around and goes due west for 2km and finally turns around again and heads 4km due east.
What is the total distance traveled?
What is the displacement?

16. Practice Problem 3 Motion Distance Traveled Displacement X
An object moves from point 1 to point 4 then reverses and ends at point 2
An object moves from point 1 to point 5 then reverses to point 2
An object moves from point 1 to point 3 then reverses to 0
An object moves from point 3 to point 5 and then reverses to point 1
An object moves from point 2 to point 4 and reverses to point 2

17. Rate Rate can be defined as how quickly something changes.
In physics, we calculate the rate of change by dividing the quantity of change by the time it takes for it to occur.

18. Speed speed = distance / time s = d t
Speed (s) – Rate at which an object moves
speed = distance / time s = d t
Speed (m/s)=distance (m) / time (s)
Units: m/s OR km/h
Like distance, speed does not depend on direction.

19. Speed
Speed is a measure of the distance an object moves in a given amount of time.
During a typical trip to school, your car will undergo a series of changes in its speed.

20. Measures of Speed
Constant speed - Speed that does not change (same distance is travelled in the same amount of time)
Instantaneous speed – Speed at a given instant in time (what the speedometer says right now!)
Average speed-- The total distance traveled divided by the total time it took to travel that distance.

21. When completing a formula you MUST show all your work!
Step 1 – write formula
Step 2 – plug in the information you are given
Step 3 – solve
Step 4 – add your units

22. Speed Practice Problems
Melissa shot a model rocket 360 m into the air. It took the rocket 4s to fly that far. What was the average speed of the rocket?
If Jessica ran 5 meters in the first second, 8 meters the next second, and 5 meters the third second to her house, what was her average speed?
What was her instantaneous speed at the end of 2 seconds?

23. Can I determine time given distance and speed? How?
A rocket is traveling at 10 km/s. How long does it take the rocket to travel 30 km?
Can I determine distance given time and speed? How?
A racecar is traveling at 85.0 m/s. How far does the car travel in 30.0 s?

24. S = d / t D S T

25. Calculating Speed, Distance or Time
If you know any two of the variables, you can calculate the missing variable.

26. Velocity The velocity of an object can change if it: Speeds up
Velocity is an object’s speed in a particular direction. If the direction changes, the velocity will also change.
Velocity always includes speed and direction.
The velocity of an object can change if it:
Speeds up
Slows down or
Changes direction

27. Is this speed or velocity?
A man drives 30 km/h to catch a train.
The train was traveling at 10 km/h east.
The train slowed down to 1 km/h as it was pulling into the station.
The train followed the tracks to the right as it sped up and turned to leave the station.
Speed
Velocity
Velocity
Velocity

28. You tell me review! Vs Velocity
An object is moving in a circle at a constant speed of 10 m/ s
We say that it has a constant speed but its velocity is changing. Why?
Direction of Motion
The direction of the object keeps changing.

29. Graphs show relationships
A good way to show a relationship between two variables is to use a graph.
A graph makes it easy to see if changes in one variable cause changes in the other variable (the effect). D
Distance

30. The distance vs. time graph used to graph the speed of an object or compare speeds of multiple objects
To graph data, you put time on the horizontal (x) axis – this is your independent variable.
Distance goes on the vertical (y) axis – this is your dependent variable.
Distance
Distance

31. The distance vs. time graph
Distance vs. time data tells you the runner’s position at different points in time.
The runner is at 50 meters after 10 sec., 100 meters after 20 sec. and 150 meters at 30 sec.
Distance
Distance

32. A straight, diagonal line indicates…
Constant Speed
Why is the line in a constant speed graph straight and diagonal?
The object is traveling the same distance in the same amount of time.

33. An Object At Rest Object’s AT REST are not moving
A horizontal line on a Distance vs. Time graph has NO SLOPE = 0 speed

34. A curved line indicates…
Changing Speed

35. Distance-Time Graph and Changing Speed
What do the different lines indicate when an object is changing speed?
Downward Curve
Horizontal Line
Upward Curve
Slowing down
Stopping
Speeding Up

36. Time/Distance Graph

37. Terry, Jade and Jerome raced each other
Terry, Jade and Jerome raced each other. Plot their data on a distance vs. time graph!

38. Comparing speeds on a Distance – Time Graph
1. Find the average speed of each line.
2. Compare the steepness
of each line.
The steeper slope indicates:
Faster Speeds

39. Distance vs. Time Graphs
Which graph shows the faster moving object?

40. Math Connection What does the slope tell you on a distance vs
Math Connection What does the slope tell you on a distance vs. time graph?
Rise =
Run =
Rise/Run =
Distance (y axis)
Time (x axis)
Speed or velocity

41. Force
A push or pull that causes an object to move, stop, or change direction
A force will cause an object with mass to accelerate.

42. Forces can affect motion in several ways:
→ They can make objects start moving.
→ They can make objects move faster.
→ They can make objects move slower.
→ They can make objects stop moving.
→ They can make objects change direction.
***These are ALL forms of acceleration!!

43. Acceleration & Deceleration
Acceleration is an increase in speed. It is a positive quantity.
Deceleration is a decrease in speed. It is a negative quantity.
Acceleration can also occur because of a change in direction.

44. Types of acceleration Increasing speed Decreasing speed
Example: Car speeds up at green light
Decreasing speed
Example: Car slows down at stop light
Changing Direction
Example: Car takes turn (can be at constant speed)
screeeeech

45. Calculating Acceleration
Acceleration (m/s2) = final speed – initial speed
time
a= sf – si t
Calculate the acceleration of a car whose speed changes from 6 m/s to 12 m/s in 3 s.
Calculate the acceleration of a bus whose speed changes from 30 m/s to 10 m/s in 5 s.

46. Graphing Acceleration Speed – Time Graphs
Shows how changes over TIME
X axis =
Y axis =
SPEED
TIME
SPEED

47. Object at Rest The speed is zero and does not change
Horizontal line (like a
distance – time graph y = 0 )

48. Constant Speed The line will be HORIZONTAL
The further the line is from the
the
It is moving.
X - AXIS
FASTER SPEED

49. Speeding Up CLOSER BEGINNING LOWER UPWARD INCREASES
The line on a speed – time graph is
to the x-axis in the of the
time period when it has a speed.
CLOSER
BEGINNING
LOWER
The line slants toward the
right side of the graph as the
speed
UPWARD
INCREASES

50. Slowing Down FAR DECREASES CLOSER DOWNWARD ZERO
When initially starting to slow down the
point representing speed is
from the x-axis.
FAR
As speed the points
representing speed get
to the x-axis.
DECREASES
CLOSER
The line on a speed-time graph slopes
to the right.
DOWNWARD
When the line touches the x-axis, the speed is and the object
stopped.
ZERO

51. Mass & Inertia Mass is the amount of matter in an object.
Inertia- the tendency of an object to resist a change in its motion.

52. More on Inertia
As an object’s mass increases…so does it’s inertia. WEAR YOUR SEATBELT!!!!

53. Momentum
Momentum- A measure of how hard it is to STOP a moving object and it depends on the mass and velocity of the object.
Which would be harder to stop….
A 68 kg. guy going 10 m/s on a bicycle or a
2 g. bullet fired from a rifle at m/s? Why ?
Momentum = mass X velocity
(g or kg x m/s ) = g or kg x m/s

54. Law of Conservation of Momentum
In any collision between objects, momentum is transferred from one object to another. (example…playing pool or bowling.)
The total momentum will stay the same unless acted on by an outside force like gravity or friction.
Let’s make predictions based on size and directions: small- big, Big-small, same size

55. Momentum Simulation