FORCE AND MOTION

CHAPTER ONE SECTION ONE MEASURING MOTION

WHAT IS MOTION? Motion is a change in an objects position over time when compared with a reference point.

Reference point or frame of reference is the background or object that is used as comparison to the object in motion. Most common reference point is the Earth’s surface. Other reference points are buildings, trees, mountains etc. What reference point do the balloonists use?

Motion is not absolute, it depends on the observer. For example, if Jack and Jill are sitting in a room, Jack sees Jill at rest and Jill sees Jack at rest. BUT, if we zoom out, we see that Jack and Jill are sitting on a bus moving at a constant speed of 25mph. From the reference INSIDE the bus, Jack sees Jill at rest and Jill sees Jack at rest.

Jill’s mother, waiting for the bus, sees Jill’s bus go by at 25mph. She sees Jill traveling 25mph. Jack on the other hand, sees Jill as going 0mph. What they observe seems to depend on their reference point.

While riding the bus, Jill begins throwing cookies to Jack. Jill can throw cookies at a speed of 30 mph. From their reference point, Jack and Jill both see the cookies going 30mph.

As Jill’s mom observes the cookie tossing from where she is standing on the side of the road, she sees the cookies going 30mph on a bus moving 25mph in the same direction. SO, she observes the cookies going 55mph!!!!!!

As Jill continues to throw 30mph cookies at Jack, the bus drives by Jack’s mom on a bicycle. She is riding 10mph in the same direction as the bus is traveling 25mph. From her point of reference, Jack seems to only be going 15mph (25mph – 10mph) and she see the tossed cookies going 45mph (15mph + 30mph).

As the cookie tossing between Jack and Jill continues, a car drives by going 40mph in the other direction. The observer in the car sees Jack going 65mph (40mph + 25mph). The observer then notices the flying cookies to be traveling a whopping 95mph!!!!!! (40mph + 25mph + 30mph).

SO WHAT IS SPEED? Speed is the rate at which an object moves. This is determined by the distance traveled and the time it takes to travel. SO… SPEED = DISTANCE / TIME UNITS FOR SPEED: meters per sec (m/s) miles per hour (mi/hr), feet per sec (ft/s)

AVERAGE SPEED VS CONSTANT SPEED Average Speed = Total distance traveled divided by Total time of the trip

AVERAGE SPEED VS CONSTANT SPEED Constant speed – speed that does not change. Every plot point on the graph has the same speed When you see a graph like this, you will know that the object is traveling at a constant speed. Double check yourself by calculating the speed at each point.

SOLVE THIS PROBLEM While on vacation, Lisa traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed?

SOLVE THIS PROBLEM While on vacation, Lisa traveled a total distance of 440 miles. Her trip took 8 hours. What was her average speed? SPEED – 55 MILES PER HOUR SPEED = DISTANCE/TIME OR SPEED = D/T DISTANCE – 440 MILES TIME – 8 HRS

Which graph shows constant speed?

VELOCITY Velocity is the speed of an object in a specific direction. Speed and Velocity do not mean the same thing Velocity changes as speed or direction changes.

Which of the following are examples of velocity? 1.25 m/s forward 2.1/500 km/h 3.55 m/h south 4.10, 000 m/s X X

COMBINING VELOCITIES 2 velocities in the SAME DIRECTION – add them together to find the resultant velocity. 2 velocities in DIFFERENT DIRECTIONS – subtract the smaller velocity from the larger velocity to find the resultant velocity.

Acceleration is the rate at which velocity changes. SO…..if an object speeds up, slows down or changes direction, it has ACCELERATED.

ACCELERATION – CHANGES IF SPEED, DIRECTION OR BOTH CHANGES

TYPES OF ACCELERATION Positive Acceleration – occurs when velocity increases. Negative Acceleration – occurs when velocity decreases. This is also called deceleration.

SO… what about this? What type of motion does it have? Is the object accelerating?

Is the object changing speed? Is the object changing direction? YES!!!! An object traveling in a circle is constantly changing direction. SO… is the object accelerating? NO!

Objects that are continuously accelerating is called CIRCULAR MOTION. This means that the direction is constantly changing.

LET’S PRACTICE

1. Consider a car moving with a constant velocity of 10m/s for 5 seconds. Look at the data table and draw a graph of the results into your notes. Does your graph look like this? What kind of graph is this? What is the speed? The object is traveling at a constant speed of 10 m/s for 5 seconds.

2. Consider a car moving with a constant velocity of 5 m/s for 5 seconds, stopping abruptly, and then remaining at rest for 5 seconds. Graph the information into your notes. Does your graph look like this? Whenever there is a flatline, this means motion has stopped.

3. Look at the picture and graph the information. Does your graph look like this? What is the speed? Graphs that look like this always show ……. ACCELERATION Speed is NOT the same at every point along the line

Look at the two cars below. The blue car starts “ahead” of the red car (which actually starts off the screen). Since the red car is moving faster, it eventually catches up with and passes the blue car. Since the red car passes the blue car, there is an instant where they occupy the same position. At how many seconds and at what distance does this occur? What speed are the cars traveling at this position?

Which car or cars are accelerating? The red car is traveling at a constant speed. Since it is constant nothing is changing!!!!! Only the blue and the green cars are accelerating. But why not the red?

Which car is experiencing the greatest acceleration? IT MUST BE THE BLUE CAR BECAUSE IT OVERTAKES ALL THE OTHERS

Look at the distance/time graph. Each one of the three lines corresponds to the motion of one of the three cars. Match the appropriate line to the particular color of car.

LET’S PRACTICE

Drag Car 1 traveled 240m in 12 sec. Its average speed was 20m/s. I know it traveled at a constant speed because of the diagonal line on the graph and the fact that the speed was the same at every point along the graph.

Drag Car 2 traveled 150m in 12 sec. Its average speed was 12.5m/s. I know the car accelerated for the first 9 sections of the race and then came to an abrupt stop at 10 seconds and remained stopped for the remainder of the race.

Drag Car 3 traveled 112m in 12 sec. Its average speed was 9.3m/s. This car accelerated throughout the entire race going faster at the beginning but very slowly at the end.

Drag Car 4 traveled 315m in 12 secs. Its average speed was 26.3m/s. Car 4 accelerated for the first two seconds and then came to an abrupt stop. It stayed still for 3 seconds. It started moving again and accelerated very rapidly until the end of the race.

THINK ABOUT RIDES IN AN AMUSEMENT PARK 1. WHICH RIDES HAVE CONSTANT SPEED? 2. WHICH RIDES HAVE CIRCULAR MOTION? 3. WHICH RIDES ACCELERATE?

END CHAPTER ONE SECTION ONE

Grandma called my house and asked to borrow a cup of sugar. Since she only lives 1 block away, I told her I would be right there. For the first 2 mins, I ran 100m in 2 minutes at a constant speed of 50m/min. I saw Susie in her yard and I stopped to talk to her for 1 minute. I continued running for 1 more minute. I went 50 more meters at a speed of 37.5m/min. I reached Grandma’s house in 4 minutes. I gave Grandma the sugar and talked with her for 2 minutes, then I started home. This time I walked 50 meters for 2 minutes at a speed of 25m/min. Just then, I saw a cute puppy on the sidewalk and I stopped to pet it for 1 minute. I then continued to walk home. It took me 3 more minutes to get home. Since I was tired, I decelerated from 22m/min, to 21.8m/min, to 20.8 m/min. Total time: 12 minutes Total Dist: 250meters Avg Speed: 20.8m/min