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. 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

3. 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

4. 3.

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

   • 2 s to 6 s, since the position changes by 16 m, the greatest change in any interval
   • 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 object is farthest from the start, at 26 m, at the end of it
5. 4.

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

   • 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
   • −12 m/s: the position falls by 12 m between the readings at 3 s and 6 s
   • −4 m/s, so the object moves more slowly than an object that is at rest

6. Scooter on a straight path (example data)

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

7. 5.

   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 moves fastest from 3 s to 6 s, since it is farthest from the start at 6 s
   • The scooter keeps moving east after 6 s, at a speed of 3 m/s, since it moves at a steady rate
   • The scooter slows down after 6 s, because −3 is a smaller number than +4
8. 6.

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

   • 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
   • Both are at 30 m from the start at 10 s, and P is moving faster than Q
   • P and Q have the same speed at 10 s, because the two lines meet at that point
9. 7.

   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
   • A and B move at the same speed the whole time, because their lines meet again at 20 s
   • B slows down after 10 s, because the steeper piece of its line means a slower train
   • B climbs a hill that gets steeper after 10 m, while A is on flat ground
10. 8.

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

    • About 1.05 m/s, from the 2 m distance and the fastest time of 1.9 s
    • About 2.0 m/s, since the mean time of the three trials is 2.0 s
    • About 0.33 m/s, from the 2 m distance and the total of 6.0 s for the three trials together
    • About 1.0 m/s, from the 2 m distance and the mean time of 2.0 s
11. 9.

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

    • Roll ball A on carpet over 3 m and ball B on tile over 5 m, so the surfaces are swapped
    • Roll ball A over 5 m on tile and ball B over 3 m on carpet, so the distances 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
12. 10.

    Each cart was timed once. Zee claims heavy carts roll faster. Which statement about the claim does the table support?

    • The result shows mass has no effect, because the two times are so close together
    • The heavy cart is three times faster, since its mass is three times greater
    • The 0.1 s gap is small, and with one timing each it cannot show that heavy carts are faster
    • The result shows heavy carts are faster, because the heavy cart's time of 1.7 s is shorter