1. Motion, Acceleration, and Forces
Chapter 3
Motion, Acceleration, and Forces

2. Motion and Velocity 3-1

3. Motion Motion occurs when an object changes position.
The distance between an object must change in relation to a reference point.
A reference frame or reference point can be something stationary or moving.

4. Are You In Motion?
You are seated on an airplane flying. Are you in motion compared to the ground?
Are you in motion compared to the seat in front of you?

5. Distance
A runner jogs 50 m North, then runs 30 m South. How far was the total distance run?
For a moving object, distance is the length of the path that the object traveled, so:
50 m + 30 m = 80 m
However, even though she ran 80 m, she is only 20 m North of the starting point. We need a new term to describe this.

6. Displacement
Displacement: The distance AND direction of an object’s change in position from the starting point

7. What is the Displacement 1?
Jon runs 200 meters north, turns and heads 150 meters south. Displacement?
Since Jon runs in opposite directions you subtract: 200 m – 150 m = 50 m North
You walk 30 meters east; then you head 10 meters west. Displacement?
Since you walk in opposite directions you subtract: 30 m – 10 m = 20 m East

8. What is the Displacement 2?
A squirrel runs 4.8 m South across a lawn, then runs 2.3 m in the opposite direction. What is the squirrel’s displacement from its starting point?
4.8 m – 2.3 m = 2.5 m South

9. Vectors Vector: a quantity that is both a size and direction.
Displacement is a vector.
Distance is NOT a vector!

10. Speed
Speed: the distance an object travels per unit of time, or distance/time.
SI unit for speed is usually measured in meters per second (m/s)
EX: 5m/s, 20 km/hr, 9mm/min

11. Calculate the Speed! A race car travels 7000 km in 10 hours
An ant travels 75 cm in 15 seconds
A snail moves 4 cm in 2 minutes
An airplane travels 20,000 km in 3 hours
You took 5 hours to drive 550 km.

12. Instantaneous Speed
Instantaneous speed: the speed of an object at a given point in time
Can be measured with a speedometer or radar gun

13. Average Speed
Average Speed: the total distance an object travels divided by the total amount of time it takes to travel
Average speed (in meters/second) = total distance (meters) / total time (second) ṽ = d / t. Now solve this 1-step equation: What is the average speed of a car that travels a distance of 750 m in 25 s?

14. Average Speed Problem 1 Identify the known values:
Travels a distance of 750 m, so d = 750 m
In 25 seconds, so t = 25 s
Identify the unknown value:
What is the average speed, ṽ = ? m / s
Insert known values and solve problem:
ṽ = (d / t) = (750 m / 25 s) = 30 m / s
Check answer and multiply: 30 x 25 = 750

15. Average Speed Problem 2
A bus leaves at 9 am and travels 350 km. After lunch it travels another 250 km until it stops at 3 pm. What is its average speed?

16. Velocity
Velocity: an object’s speed and direction Ex: 5 m/s north, 20 km/hr southeast
Objects have different velocities if they are moving at different speeds or directions.
Is velocity changing if you are on a merry-go-round?

17. Vectors or Not?
Speed?
Velocity?
Distance?
Displacement?

18. Graphing Motion
A distance-time graph gives the speed of an object in motion.
Time: x-axis
Distance: y-axis

19. Distance-Time Graphs
The steeper the slope on these graphs, the faster the object or person traveled.
A horizontal line means an object was at rest.

20. Distance-Time Graphs
A straight line indicates the object traveled at a constant speed.

24. Make Distance-Time Graphs
Title
Label both the x and y axis
Examine data and create the scale
Place a dot where the distance matches the time
Connect the dots when you are finished
Key/color code lines

25. GRAPH 1: THE TRIP
John drives to the store from home. He travels 1 kilometer in the first 6 minutes. During the next 10 minutes John travels another 2 kilometers. John stays at the store for 20 minutes. The trip back home takes John 20 minutes. Graph the trip.

26. GRAPH 2: THE JOG
Jon and Jan leave their house North. In the first 0.1 hour he jogs 700m & she jogs 800m. Uphill they slow as he jogs 500m & she jogs 550m North in the next 0.15 hour, then they walk home South for the next ¼ hour. Graph the jog in m/min. What is their average speed in meters per minute? Hint: average speed is ṽ = total d / total t

27. Graph 3: The Race Anita Distance (m) Ashley Distance (m) Time (s) 2.5
2.5 3
9.5 11 9
21.5 26 19
38.5 46 34

28. Acceleration 3-2

29. Acceleration
Acceleration: rate of change in velocity. This can be calculated by dividing the change in velocity by the time it takes for the change to occur!
Acceleration = (Vf - Vi) / (Tf - Ti)
Acceleration = Final velocity - Initial velocity
Final time - Initial time
The word initial means starting

30. Acceleration Units
The typical unit for acceleration is meters per seconds squared or m/s2
Other examples: km/hr2, m/min2
You would read this as kilometers per hour per hour and meters per minute per minute
There will always be 2 units of time. Ex: km/hr/min

31. Acceleration Cont. Acceleration can be positive, negative, or zero.
Positive acceleration: an object is speeding up
Negative acceleration: an object is slowing down
Zero acceleration: an object is traveling at a constant speed

32. Is it accelerating? Remember, an object IS accelerating if it:
speeds up
slows down
changes direction

33. Calculate Acceleration
A car that was at rest suddenly increases its speed to 65km/hr in 6 seconds traveling west. Acceleration?

34. Calculate Acceleration
A car is traveling south on the freeway at 95 km/hr. The car takes 5 seconds to slow to a speed of 45 km/hr. Acceleration?

35. A bicyclist is riding at 15 meters per minute northwest
A bicyclist is riding at 15 meters per minute northwest. While traveling up a steep hill, the bicyclist slows to a speed of 6 meters per minute in 3 seconds.

36. A person waiting to run a race suddenly sprints to a speed of 4 m/s in 2 seconds while traveling east.

37. An airplane traveling north at 20,000 km/hr takes 15 minutes to slow to a speed of 10,000 km/hr.

38. Graphing Acceleration
These are called speed-time graphs. Time is plotted again on the x-axis and speed/velocity is on the y-axis.

39. Speed-time Graphs
A line with a positive slope means that there is positive acceleration (speeding up).
If the slope of the line is negative, the object has negative acceleration (slowing down).
If there is a horizontal line on this graph, the object is traveling at a constant speed and has zero acceleration.

40. Speed-Time Graph

44. Forces 3-3

45. Forces A force is a push or a pull.
A force can be from a living thing, wind, moving water, magnetism, friction or gravity.

46. Net Force Net Force: total sum of all forces acting on an object.
If two or more forces are acting on an object (in same direction), you can add the total amount of forces to get the net force.
Force is measured in Newtons, which is represented by a capital N.

47. Balanced Forces
Balanced forces are forces on an object that combine to give a zero net force
Balanced forces do not change the motion of the object
40 N
40 N

48. Unbalanced Forces
Unbalanced forces are forces that combine to produce a net force that is not equal to zero (opposite directions).
This will cause the velocity of an object to change.
20N
15N 3N

49. Friction
Friction: a force that opposes motion when two surfaces are in contact.
Friction is what slows objects down.
Friction will turn kinetic energy into thermal energy (heat).

50. More about Friction
The rougher an object, the greater the amount of friction that object has.
Gravity can increase the amount of friction between two objects sliding past each other.

51. Static Friction
Static friction: Frictional force that prevents two surfaces from sliding past each other.
When friction is greater than the forces acting on it, the object does not move.

52. Sliding Friction
Sliding Friction: the frictional force that opposes motion of two surfaces sliding past each other.

53. Rolling Friction
Rolling Friction: frictional force from round surfaces

54. Fluid Friction
Fluid Friction: frictional force occurring when a solid material passes through a fluid.
A fluid is any substance that can change its shape easily.
Liquids and gases are fluids.

55. Air resistance Air resistance is a type of fluid friction.
Air resistance opposes motion in the air. Over time, enough air resistance will generate thermal energy.

56. Video Clip

57. Air resistance
Air resistance is part the reason why objects appear to fall at different accelerations.
In the absence of air, all objects fall at the same rate of acceleration
Acceleration due to gravity: 9.8m/s2

58. Terminal Velocity
When air resistance and gravity’s forces become equal, a falling object’s velocity will stay constant.
This is terminal velocity.