Linear Motion

1
Linear Motion part 2

• Chapter 2

2
Free Fall How Fast

• Gravity causes an apple to accelerate downward
  once it begins falling.
• Free Fall only deals with the affect of gravity
  on an object and not air resistance.
• Elapsed Time the amount of time that has
  elapsed, or passed, since the beginning of the
  fall.

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Free Fall
4
Free Fall

• In table 2.2, what is the acceleration?
• a ? v
• 10 m/s 10 m/s2
• 1s

time interval
5
Free Fall

• Specifically, free fall ( g ) 9.8 m/s2

6
Free Fall

• If a rock was falling _at_ 10 m/s2, how fast would
  it be going at 4.5 s, 8 s, and 15 s?
• What is the equation we will be using?
• a ?v

?t
7
Free Fall

• Do we know a ?
• 10 m/s2
• Do we know time interval?
• 4.5 s
• Do we know ? v ?
• No
• Algebraically solve for ? v
• 10 m/s2 4.5 s 45 m/s

8
Up Down

• So far, we have been looking at objects moving
  straight downward due to gravity.
• If we throw a ball straight up, at the highest
  point, what is its instantaneous speed?
• Zero at that moment in time it is not moving up
  or down.

9
Up Down
An object may not have speed at the top, however,
it has acceleration. Why? How much?
10
Free Fall How Far

• How fast something moves is entirely different
  from how far it moves.
• So, we know in the first second, an object falls
  at 10 m/s, but that does not tell us how far it
  fell.

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Up Down
How far did the ball fall?
12
Free Fall How Far

• To find the distance of our falling object, we
  need to first find its average speed, then
  multiply by time.
• beginning speed final speed time
• 2
• 0 m/s 10 m/s / 2 5 m/s 1 s 5 m

13
Free Fall How Far

• Equations
• d ½ gt2
• v vi vf / 2
• a v/t
• v d/t

14
Free Fall How Far

• An apple drops from a tree and hits the ground in
  1 s. What is its v upon striking the ground? What
  is its v during the 1 s? How high above the
  ground was the apple when it first dropped?

15
Answer

• a v/t or v at
• Given that free fall a 10 m/s2
• v 10 x 1
• v 10 m/s

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Next

• Average velocity vi vf /2
• Initial speed of the apple was 0
• Final speed was 10 m/s
• 0 10/2 5 m/s

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Finally, how far?

• d 1/2gt2
• d ½ (10 m/s2)(1s)2
• d 5 meters

18
Air Resistance and Falling Objects

• Drop a feather and a coin in a vacuum, the two
  will accelerate equally.
• Air resistance is responsible for different
  accelerations when in regular free fall.