Everything Science Grade 12

CHAPTER 6. MOTIONIN TWO DIMENSIONS 6.2

Example 5: Describing Projectile Motion

QUESTION

A cricketer hits a cricketball from the ground and the following graph ofveloc-

ity vs. time was drawn.Upwards was taken as positive. Study the graphand

follow the instructions below:

1. Describe the motionof the ball according tothe graph.


2. Draw a sketch graphof the corresponding displacement-time graph.La-
   bel the axes.


3. Draw a sketch graphof the corresponding acceleration-time graph. Label
   the axes.

�

�

�

19,6

-19,6

2 4

time (s)

velocity (m

·s

−

1

)

�

�

SOLUTION

Step 1 : Describe the motion of the ball.

We need to study the velocity-time graph to answer this question.

We will break the motion of the ball up into twotime zones: t =

0 s to t = 2 s and t = 2s to t = 4 s.

From t = 0 s to t = 2 s the following happens:

The ball starts to move at an initial velocity of 19,6 m·s−^1 and de-

creases its velocity to 0m·s−^1 at t = 2 s. At t = 2 s thevelocity of

the ball is 0 m·s−^1 and therefore it stops.

From t = 2 s to t = 4 s the following happens:

The ball moves from a velocity of 0 m·s−^1 to 19,6 m·s−^1 in the op-

posite direction to the original motion.