Describing motion

1
Describing motion

• We have defined motion.
• We have described motion in terms of displacement
  vs. distance from some F of R

2
Three kinds of motion

• Relative to some F of R it turns out there are 3
  basic ways objects can move
• 1. Translational motion which is basically
  straight line motiona car going down a street,
  you walking down the hall..
• 2. Circular motion which is movement around some
  central point, a Ferris wheel, a Merry-go-Round
  or a Carousel.
• 3. Vibrational motion is simply a repeating, back
  and forth motion relative to some rest
  position. This includes heartbeats, all sound is
  vibrational.

3
Quantification of motion

• The three ways we have listed are qualitative
  descriptions. Science wants quantification
  wherever possible.
• Therefore we can begin to ask about HOW fast the
  car is going down the road or how fast the
  merry-go-round is spinning or how fast your heart
  is beating.

4
SPEED

• Speed is simply the distance an object travels
  from some F or R in what amount of time
• 6 m/s or 50 ft/min or 55 miles/hour
• This is a quantity and where there are numbers
  there are equations in science

5
Equations

• In science equations are blueprints, they tell us
  what to do with the numbers we have, the data we
  have, so we can make calculations so we can
  quantify our descriptions of events in the real
  world.
• S d/t
• Fma
• Wt. mg
• S vt .5 A TT
• These are a few equations. When we understand
  what the letters mean, the equation will tell us
  what to do with the numbers, add, subtract,
  multiply or divide them.

6
Describing motion with words and numbers

• When an objects position from another object is
  changing we can be sure that the object is
  moving.
• How quickly it changes its position is called
  speed or velocity.If we are ONLY interested in
  the distance covered in a certain time that is
  speed BUT if we want to know the displacement
  change in a certain time then that is VELOCITY.
• So if a ball rolls 20 m across the floor in 10
  seconds, its speed is?
• Well what shall we do with the data, 20m and 10
  seconds?
• There is an equationit is the rule for finding
  speed. The equation(the rule) is
• S D/T distance divided by time
• S 20m/10sec 2 m/s.
• But is the ball rolls 20 m South in 10 seconds
  then that is displacement/time velocity. Our
  answer is 2 m/s South. Velocity is a vector. It
  is speed direction.

7
Average vs Instantaneous speeds

• The ball rolled 20m in 10 sec. so we say its
  speed is 2 m/s. What does that mean?
• It means the average speed of the object was 2
  m/s. To find that one takes the entire distance
  and divide by the entire time of the trip. Let us
  say you go 400 miles in 8 hours. Speed? S
  D/T400miles/8 hours 50 miles/hour.
• Now what does that mean? It means your AVERAGE
  speed for the trip was 50 mi/h. Sometimes you
  might have gone faster, sometimes slower,
  sometimes stopped altogetherbut on average, you
  traveled 50 miles/hour.
• So what is your instantaneous speed? That would
  be on your speedometer. Look at it. That is the
  speed you ARE going at that moment. The
  instantaneous speed is your speed in some SMALL
  interval of timeover say a fraction of a second.

8
acceleration

• What if your speed changes, you go faster or
  slower or
• It turns out that is acceleration.
• Acceleration is ANY change in speed or direction
  or any change in velocity.
• We can quantify how FAST we change our speeddo
  you gradually come to a stop, change your speed
  slowly or do you have to slam on the brakes and
  change your speed in a hurry!
• At a stoplight do you press slowly on the gas
  pedal and speed up gently or do you peel away
  from the light with tires screechingthat is you
  change your speed quickly.
• THAT is what acceleration tells ushow fast you
  change your speed or direction.

9
AV v / T-t

• The equation has a couple of forms which I will
  discuss with you on the board.
• But they all say the same thing
• Acceleration the final velocity the initial
  velocity divided by the time it took to make that
  schange in speed is acceleration.

10
Joe Schmoe

• Joe Schmoe speeds up from 20 m/s to 40 m/s in 2
  seconds.
• His acceleration would be
• A 40 m/s 20 m/s / 2 s
• A ?
• 10 m/s/s.
• Notice the units are distance/time/time.
• m/s/s

11
10 m/s/s means???

• How fast is Joe changing his speed?
• If his acceleration is 10 m/s/s then that means
  what?
• Joe is going 10 m/s FASTER every second.
• On Earth gravity is 9.8 m/s/sthe pull of gravity
  accelerates ALL masses at 9.8 m/s/s
• So a falling object speeds up
• It will go 9.8 m/s FASTER every second that it is
  falling

12
So.?

• A rock falls off a cliff
• Its initial velocity is zero
• BUT once it starts to fall its acceleration is
  9.8 m/s/s.
• At the end of 1 second the rock is going..
• 2 sec. ?
• 3. sec.?

13
Quiz

• 1. We have listed three kinds of motion. Identify
  each type and give two examples of each type.
• 2. A person accelerates at 3.5 m/s/s. In a
  complete sentence explain exactly what that
  means.
• 3. We have discussed motion both in qualitative
  terms and in quantitative terms. Take one
  situation a skateboarder headed downward on a
  halfpipe track and describe the motion of the
  skateboarder BOTH qualitatively and
  quantitatively.