Solve for speed, distance or time

Answer Key - Grade 8 Science Worksheet - Challenge Me

Circle the best answer, or write your answer on the line.

1. 1.

   Ari and a friend start 120 m apart and walk toward each other at 2 m/s and 4 m/s. After how many seconds do they meet?

   20 s
2. 2.

   Bo bikes toward a friend who is 240 m away, at 3 m/s. The friend bikes back at 5 m/s. How many seconds until they meet?

   30 s
3. 3.

   Jun walks at 2 m/s. A friend starts from the same spot 10 s later, at 3 m/s. How many seconds after the friend starts do they catch up?

   20 s
4. 4.

   Kira bikes at 6 m/s. A friend leaves 10 s later from the same spot at 9 m/s. How many seconds after leaving does the friend catch up?

   20 s
5. 5.

   Luz rides a scooter at 4 m/s. A friend sets off 15 s later at 5 m/s from the same spot. How many seconds after setting off does the friend catch up?

   60 s
6. 6.

   Milo's robot drives 60 m in 20 s, rests for 10 s, then drives 40 m in 10 s. What is its average speed for the whole trip?

   2.5 m/s
7. 7.

   Noah skates 30 m in 10 s, stops for 4 s to tie a lace, then skates 24 m in 6 s. What is the average speed for the whole trip?

   2.7 m/s
8. 8.

   A sled carries Pia 45 m in 15 s, rests 5 s at a bump, then slides 40 m in 5 s. What is its average speed over the whole trip?

   3.4 m/s

9. Read this first

   Use this

   Investigation card (test results). Changed: none; the same run is repeated. Measured: time to roll 1 m (s). Kept the same: same car, same track. Run 1: 0.8. Run 2: 1.0. Run 3: 1.2.

10. 9.

    The car rolls 1 m each run. Tala says its speed is exactly 1.25 m/s, from run 1. Which speed do all three runs support best?

    • Exactly 1.25 m/s, since the first run is cleanest
    • About 0.8 m/s, from the slowest run
    • About 1 m/s, from the mean time of 1.0 s
    • About 0.33 m/s, from 1 m over a 3.0 s total time

11. Cart on two surfaces (example data)

    surface	distance rolled (m)	trial 1 (s)	trial 2 (s)	trial 3 (s)
    smooth floor	2	1.9	2.0	2.1
    carpet	2	2.4	2.5	2.6

12. 10.

    A cart rolls 2 m on each surface. Which conclusion is best supported by the three trials, Zee?

    • The speeds were the same, since each trial took about 2 s
    • The cart's speed on the floor is 2 m/s, since it rolled 2 m
    • Faster on the smooth floor: about 1 m/s against 0.8 m/s
    • The carpet was faster, since its times were bigger