Speed, velocity and distance-time graphs

Answer Key - Grade 8 Science Worksheet

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

1. 1.

   East is positive. Ari walks 90 m east in 30 s, then 30 m west in 10 s. What is the average velocity for the whole trip?

   1.5 m/s
2. 2.

   East is positive. Bo rides 90 m east in 30 s, then 10 m west in 10 s. What is the average velocity for the whole ride?

   2 m/s
3. 3.

   East is positive. Esme's robot rolls 75 m east in 15 s, then 15 m west in 5 s. What is its average velocity?

   3 m/s

4. Read this first

   Object on a straight track (example data)

   time (s)	position (m east of the start)
   0	0
   2	10
   6	26
   8	12

5. 4.

   East is positive. Use the table. During which interval is the object's speed greatest, Jun?

   • 2 s to 6 s, since the object is farthest from the start, at 26 m, at the end of it
   • 6 s to 8 s, at a velocity of −7 m/s, so the object moves west fastest there
   • 0 s to 2 s, since +5 m/s is the greatest velocity in the table
   • 2 s to 6 s, since the position changes by 16 m, the greatest change in any interval
6. 5.

   East is positive. Use the table. What was the object's velocity from 3 s to 6 s, Kira?

   • −12 m/s: the position falls by 12 m between the readings at 3 s and 6 s
   • 4 m/s east: the position changes by 4 m each second, so the object moves east
   • −4 m/s: the object moves 4 m toward the west each second, so the velocity is negative
   • −4 m/s, so the object moves more slowly than an object that is at rest

7. Scooter on a straight path (example data)

   time (s)	position (m east of the start)
   0	0
   3	15
   6	27
   9	18

8. 6.

   East is positive. Which statement does the table support, Luz?

   • The velocity is positive until 6 s and negative after, so the scooter turned around at 6 s
   • The scooter slows down after 6 s, because −3 is a smaller number than +4
   • The scooter keeps moving east after 6 s, at a speed of 3 m/s, since it moves at a steady rate
   • The scooter moves fastest from 3 s to 6 s, since it is farthest from the start at 6 s
9. 7.

   Noah graphs two buses, P and Q. What does the point where the lines cross at 10 s show?

   • P and Q have the same speed at 10 s, because the two lines meet at that point
   • Both are at 30 m from the start at 10 s, and P is moving faster than Q
   • Q is faster, since it started 20 m from the start while P started at 0 m
   • Both buses stop at 10 s, because the lines end at the point where they meet
10. 8.

    Pia graphs two trains, A and B, over 20 s. Which statement does the graph support?

    • A and B both travel 40 m in 20 s, but B moves more slowly at first and faster later
    • B slows down after 10 s, because the steeper piece of its line means a slower train
    • A and B move at the same speed the whole time, because their lines meet again at 20 s
    • B climbs a hill that gets steeper after 10 m, while A is on flat ground
11. 9.

    0. Ren times a cart over 2 m three times. Which is the best estimate of the cart's average speed?

    • About 1.0 m/s, from the 2 m distance and the mean time of 2.0 s
    • About 2.0 m/s, since the mean time of the three trials is 2.0 s
    • About 1.05 m/s, from the 2 m distance and the fastest time of 1.9 s
    • About 0.33 m/s, from the 2 m distance and the total of 6.0 s for the three trials together
12. 10.

    0. Tala wants to know whether ball A is faster than ball B. Which change makes the test fair?

    • Roll ball A over 5 m on tile and ball B over 3 m on carpet, so the distances are swapped
    • Roll ball A on carpet over 3 m and ball B on tile over 5 m, so the surfaces are swapped
    • Time each ball three times and compare the means, keeping tile for A and carpet for B
    • Roll both balls over the same distance on the same surface, so only the ball differs