Physics Grade 12

Unit 2: Kinematics (How Things Move)

Note 3: Acceleration & Acceleration Due to Gravity


Reference Text Sections 1.2, 1.3

Recall Definitions:

Acceleration – rate of change of velocity        

Instantaneous Acceleration – acceleration at one point in time


DEMO: heavy and light objects accelerating at same speed.  Discuss Aristotles’ mistakes and Galileo’s discovery….


Gravity – force of attraction between masses

Acceleration Due to Gravity – Approx. 9.8 m/s2

Free Fall – motion of an object falling to earth with only gravity acting on it.

Terminal Speed – Air resistance becomes so great that there is no more acceleration.


Solving Constant Acceleration Problems (the formulas to use J):


             These can be derived from Velocity vs Time graph


Example 1:


Find a baseball’s acceleration if the pitcher’s wind up is 3m and the speed is 50 m/s.






Example 2:

A steel ball is thrown from the ledge of a building so that it has an initial velocity of magnitude 15.0 m/s.  The ledge is 15.0 m above the ground (assume no air resistance).

a)   What are the total flight time and the speed of impact at the ground if the initial velocity is upward? (Answer: flight time = 3.86 s, impact speed = 22.8 m/s)

b)   What are these two quantities if the initial velocity is downward?

      (Answer: flight time = 0.8 s, impact speed = 22.8 m/s)

c)   Based on your answers to a) and b), write a concluding statement.(Answer to this will be the final velocity is independent of whether the ball is thrown up or down)


Example 3:

Car A is traveling at 11 m/s [N] when it passes a stationary Car B.  When A is 20m ahead of B, B begins to accelerate at a uniform rate of 4.0 ms-2 [N] and at the instant, A begins to accelerate at a uniform rate of 2 ms-2 [S].  Determine when and where B catches up with A.