There is at least two things worth knowing that will help with every question in this activity:
- A free-falling object accelerates downward with an acceleration of approximately -10 m/s/s.
- The vertical velocity of the object is changing by -10 m/s for every second of motion.
Make a Table
The first two sentences tell you the speed of the ball at a given moment after launch. Use this information to create a velocity-time table. For instance, suppose it says the ball's speed is 20 m/s two seconds after launch and that the ball is moving upward. You then know that at 2 seconds, the object has a velocity of +20 m/s. Create a table that begins with that as one of the rows of the table. Fill in the rest of the table knowing that the velocity changes by -10 m/s for each second of motion. So v = 10 m/s at 3 seconds, v = 0 m/s at 4 seconds, v = -10 m/s at 5 seconds. Moving upward in the table from 2 seconds, we can say that v = 30 m/s at 1 second and 40 m/s at 0 seconds.
Answer the Question
Now that the table is complete, you can find the answer to the question ... in the table. For instance, if it asks at what time is the object moving upward at 20 m/s, then you want to know the time in the table when the velocity is +20 m/s. Always look for negative velocities if the object is moving downward and positive velocity values if the object is moving upward.
The second speedometer is a bit odd. It provides a speed reading that isn't possible. The value they give you is a value that is faster than the original speed. This isn't possible. Fortunately, one of the possible answer options for this question is Not Possible. Pick this as the answer for the second speedometer.