1. Measuring Motion
Chapter 5

2. Section 1: Measuring Motion
Objectives:
Describe the motion of an object by the position of the object in relation to a reference point
Identify the two factors that determine speed
Explain the difference between speed and velocity
Analyze the relationship between velocity and acceleration
Explain how changes in motion can be measured and represented in a graph

3. Observing Motion by Using a Reference Point
How do you know an object is moving?
When you watch the motion of an object, you are actually watching the object in relation to another object that appears to stay in place.
The object that appears to stay in place is a reference point.

4. Observing Motion by Using a Reference Point
When an object changes position over time relative to a reference point, the object is in motion.
Common Reference Points:
tree
building
door
objects in motion – a bird flying while in a hot air balloon.

5. Speed Depends on Distance and Time
Speed-Distance traveled by an object divided by the time taken to travel that distance.
Ex. A balloon traveled 50 m in 10 s. What is the balloons speed?
The SI unit for speed is meters per second (m/s).

6. Determining Average Speed
Most of the time, objects do not travel at a constant speed – you probably do not walk at a constant speed from one class to the next.
Average speed= total distance
Total time

7. Section 1: Measuring Motion
SPEED = distance
time d s t

8. Section 1: Measuring Motion
Write the formula
Identify variables
Plug in numbers
Solve and circle
A snail moves 1mm in 5 seconds. What is the speed in METERS/SECOND?

9. Section 1: Measuring Motion
Write the formula
Identify variables
Plug in numbers
Solve and circle
A car is driving along at 25km/hr. How long will it take me to drive 400km?

10. Section 1: Measuring Motion
A bee is flying by at a speed of 1.5 m/s. How far will the bee travel in one minute?
Write the formula
Identify variables
Plug in numbers
Solve and circle

11. Average Speed Examples:
An athlete swims a distance from one end of a 50m pool to the other end in a time of 25s. What is the athletes average speed?
Jake jogs to a store 72 m away in a time of 36s. What is Jake’s average speed?

12. Recognizing Speed on a Graph
Distance vs. Time
What is the average speed?
Distance (m)
Time (s)

13. Section 1: Measuring Motion
Speed ≠ Velocity
_____________ is the speed of an object in a given direction
Velocity MUST include a ___________!
Speed
direction

14. Section 1: Measuring Motion
Which is it? Speed or Velocity?
7 miles per hour
2.345 inches/minute east
18.7 ft/s down
7.77 km/day

15. Velocity: Direction Matters
Imagine that 2 students leave the same classroom at the same time. They both walk at 10km/hr for 5 min, 12 km/hr for 8 min, and 5 km/hr for 10 min. Why don’t they end up at the same place?

16. Velocity Velocity- Speed of an object in a particular direction
You must always include a direction in your answer
Constant velocity only occurs if neither speed nor direction changes.
Velocities can be added or subtracted together to give a resultant velocity.
Add velocities that are in the same direction.
Ex: Walking on a bus as the bus is moving forward.
Subtract velocities that are in opposite directions.
Ex. Walking to the back of the bus while it is moving forward

17. Finding Resultant Velocities
Same direction
Resultant Velocity =
2 km/min east
15 km/min east

18. Finding Resultant Velocities
Opposite directions
Resultant Velocity=
2 km/min west
15 km/min east

19. Acceleration Acceleration is the rate at which velocity changes.
Velocity changes if:
speed changes
direction changes
both change
Increase in velocity = positive acceleration
Decrease in velocity = negative acceleration (deceleration)

20. Acceleration Calculating Average Acceleration
Avg. Acceleration = final velocity-starting velocity
time it takes for velocity to change
A = vf – vi = A = Δv
t t
Velocity is expressed in m/s and time is expressed in s.
Therefore, acceleration is expressed in meters per second per second (m/s/s) or m/s2

21. Calculating Acceleration
0:02
0:03
0:04
0:05
0:01
1 m/s m/s m/s m/s m/s
Equation:
Calculation:

22. Acceleration Problems
A skater goes from a standstill to a speed of 6.7 m/s in 12 seconds.  What is the acceleration of the skater?
A plane passes point A at a velocity of 240 m/s north. Forty seconds later, it passes point B at a velocity of 260 m/s north. What is the planes average acceleration?

23. Centripetal Acceleration
Centripetal Acceleration- the acceleration that occurs in a circular motion.
Ex: Ferris wheel, the moon’s orbit

24. Section 2: What is a Force?
Objectives
Describe forces, and explain how forces act on objects.
How do you determine the net force when more than one force is acting on an object?
Compare balanced and unbalanced forces.
Describe how unbalanced forces cause changes in motion.

25. 5.2 Forces
A force is a push or pull that causes a resting object to move, or it can accelerate a moving object by changing its speed or direction.
Unit = Newton (N)
1 kg m/s2
Combination of all forces acting on an object is called the net force.

26. Types of Forces Balanced Force
When the forces on an object are balanced, the net force on an object is zero and there is no change in the object’s motion.
Unbalanced force
When the forces on an object are unbalanced, there is a net force and the object accelerates.

27. Calculating Net Force Forces in the same direction
Add forces to determine net force

28. Calculating Net Force Forces in opposite directions
Subtract forces to determine net force

29. Forces Affect of force on moving object
Change in speed or direction
Ex. When soccer ball is passed to another player and is kicked
Affect of force on stationary object
Cause a nonmoving object to start moving.
Ex. Stationary soccer ball kicked

30. Section 3: A Force that Opposes Motion
Objectives:
Describe friction
Explain why friction occurs
List the two types of friction, and give examples of each

31. Friction
Friction - A force that opposes motion between two surfaces that are in contact.
Friction causes moving objects to slow down and eventually stop.
What causes friction?
When the hills and valleys of one surface come in contact with the hills and valleys of the other surface

32. Friction
Rough surfaces have more hills and valleys than smooth surfaces do.
The rougher the surface, the greater the friction.
Ex. A soccer ball rolling on grass, verses a hockey puck on ice.
The greater the weight
of the object, the greater
the friction will be

33. Types of Friction
Static Friction is the friction force that acts on a stationary object. It opposes the applied force.
Kinetic Friction is the force that opposes the direction of motion of an object as it slides over a surface.
Solid over solid

34. Types of Kinetic Friction
Rolling Friction is the force of friction felt on rolling objects
Example: wheels and balls.
Fluid friction opposes the motion of a moving object in a fluid (gas or liquid)
Example: Air resistance

35. Section 3: A Force that Opposes Motion

36. Is friction helpful or harmful?
Movement of tires
Walking
Eraser
Harmful
Machine wear
Erosion
Burns/blisters

37. Decreasing Friction
Lubricants- Substances that are applied to surfaces to reduce the friction between the surfaces.
Ex: motor oil, wax, grease
Lubricants are usually liquids, but can be solids or gases. (Air in air-hockey)

38. Decreasing Friction
Switching from sliding friction to rolling friction.
EX. Ball bearings are placed between the wheels and axles of skates
Make surfaces that rub against each other smoother.

39. Gravity
Gravity is a force of attraction that acts between objects with mass.
Objects close to Earth accelerate at 9.8m/s2.
All objects fall at the same rate.

40. Gravity
If gravity affects all objects with mass – why aren’t all objects stuck together?
Masses of most objects are too small, you can’t detect this force
Earth has a huge mass, so the gravitational force of Earth is large. It pulls everything toward the center of the Earth. – Dropped objects fall to the floor.

41. Law of Universal Gravitation
All objects in the universe attract each other through gravitational force.
The size of the force depends on the masses of the objects and the distance between the objects.
Greater the mass the greater the force.
Smaller the distance, the greater the force.

42. Mass vs. Weight
Weight is a measure of gravitational force on an object.
Unit – Newton
Measuring device is a spring scale.
Weight = mass x gravity
N = (kg) x (m/s2)
Mass is the amount of matter in an object.
Unit = kilogram
Measuring device is a balance.