1. Measuring Motion
Chapter 5

2. 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.

3. 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.

4. 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).

5. 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

6. 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?

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

8. 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?

9. 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

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

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

12. 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)

13. Acceleration Calculating Average Acceleration
Avg Accl = 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

14. 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:

15. 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?

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

17. 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.

18. 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.

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

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

21. 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 on force on stationary object
Cause a nonmoving object to start moving.
Ex. Stationary soccer ball kicked

22. 5.3 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

23. 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

24. 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

25. 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

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

27. 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)

28. 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.

29. 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.

30. 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.

31. 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.

32. Mass vs. Weight Mass is the amount of matter in an object.
Unit = kilogram
Measuring device is a balance.
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)