UNIT 2: Physics

Chapter 3: Describing Motion (pages 68-95)

I. Describing Motion

A. Motion

1. Motion occurs when an object changes position

  

2. To measure the position of an object, a reference frame must be chosen

a. Reference frame- a group of objects that are not moving relative to each other

b. One point in reference frame is chosen as the reference point

  

3. Motion is relative- motion of object depends on the reference frame that is chosen

 

4. Distance and Displacementa. Displacement- distance and direction of an object’s final position from its initial position

1). Includes both a size and direction2). Vectors describe something with

both size and directionb. Distance alone is not a vector

What is your displacement if you walked 100 meters North, then 10 meters West, then 100

meters South, and finally 10 meters East?   

100 m North

10 m West

100 m South

10 m East

Starting point

Your total distance traveled is 220 meters, but your displacement is zero.

B. Speed- distance an object travels in a given amount of time

a. SI unit of speed is meters per second (m/s)

b. Instantaneous speed- speed at any given instant. Car speedometer measures

instantaneous speed

  

m/s

c. Average speed- total distance traveled divided by the total time traveled  

t

dv

C. Velocity- speed of the object and its direction

1. is a vector quantity

2. Size of objects velocity is speed

  

3. Object will have different velocities if have different speeds or move in different directions

4. The velocity of an object can change even if the speed of the object remains constant (e.g. driving in a circle)

  

D. Graphing Motion

1. Motion of object over a period of time can be shown on a distance-time graph

 

 a. Time is plotted on horizontal axis (x-axis)

b. Distance plotted along vertical axis (y-axis)

2. The slope (steepness) of a line graph equals the speed

 

 

a. When slope is zero (line is horizontal) the speed is zero

b. Steeper the slope, the faster the speed

Remember- slope equals rise over run

What is the relationship between the steepness of the line and the objects speed?

What does a flat line represent?

Which runner started out fastest?

Which runner finished the 100 meters first?

What was Charlie doing from 8-13 seconds?

II. Acceleration

A. Acceleration, Speed, and Velociy

1. Acceleration- change in velocity divided by the time for the change

to occur

 

2.Velocity is a vector, so change in velocity can be either change in speed or direction of the motion

3. Acceleration is a vector (has size and direction)

 

4. Any time an object changes direction, its velocity changes and it is accelerating

 

B. Calculating Acceleration1. Can calculate an objects average

acceleration 

if

if

tt

vva

vf = final velocity, vi = initial velocity

tf = final velocity, ti = initial velocity

2. Speed-Time Graphs- for object moving in one direction, the acceleration can be found from a speed-time graph

a. vertical axis is the objects speed

b. horizontal axis is the time

  

Describe what is occurring by looking at the speed vs. time graph between the start and point A. A to B, B to C, C to D, D to E, and from E to F.

3. Positive acceleration- positive slope (speed is increasing)  

4. Negative acceleration- negative slope (speed is decreasing)

 

C. Amusement Park Acceleration- roller coasters can give riders large accelerations

 

II. Motion and Forces

A. What is force?

1. force- is a push or pull that oneobject exerts on another

2. Force is a vector (size and direction)

3. SI unit of force is newtons (N)

 

4. A force can cause the motion of an object to change

5. Unbalanced forces change velocity

 

 a. If net force is zero no change in velocity

b. If net force is not zero (unbalanced forces) velocity will change

B. Friction- force that opposes the sliding motion of two surfaces that are in contact

 

 1. The size of frictional force depends on the materials the surfaces are made from and the roughness of the surfaces

 2. Frictional force between two surfaces increases when the force pushing the surfaces together increases  

Where would you find the greatest friction occurring on a typical roller coaster?

3. Static friction- friction that prevents two surfaces in contact from sliding past each other   

Frictionforce

Pulling force

Frictionforce

Pulling force

If pulling force exceeds friction you produce movement   

4. Sliding friction- the force that acts in the opposite direction to the motion of a surface sliding on another surface.  

C. Air Resistance- frictional force opposes motion of objects that move through air

 

  

1. Air resistance acts in direction opposite to velocity of an object moving in air

2. Size of air resistance depends on the size and shape of an object

3. Terminal velocity- the maximum velocity a falling object will reach

a. Depends on objects size, shape, and mass of falling object

b. Object reaches terminal velocity when net force equals zero