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. You need to know the time when the speed is 0 m/s. You can find this in the table. And you also need to know the time when the speed is 20 m/s. There are two of these times - one for the upward motion and one for the downward motion. When you go to tap on the answer field, only one of these options will be offered as a choice; the other is removed from the list of options.