Arrows That Show Speed

In the picture

1. 1The red ball at the left end of the track, the slowest of the three.
2. 2The short arrow above that ball, matching its small speed.
3. 3The long arrow above the ball at the right end, matching its large speed.
4. 4The tick-marked strip along the track, the scale the arrow lengths are read against.

Examples

• A speedometer needle swings further round the dial the faster the car goes.
• On a weather map, wind arrows are drawn longer where the wind blows harder.

Quiz: Arrows That Show Speed (Age 15)

1. 1

   In a diagram where every moving object carries an arrow whose length stands for its speed, one object’s arrow is drawn twice as long as another’s. What does that tell you about it?

   • It is moving twice as fast.
   • It has travelled twice as far.
   • It has been moving for twice as long.
2. 2

   A ball rolls along a track and gradually slows down. Arrows standing for its speed are drawn at several points along the way. How do those arrows change?

   • They stay the same length, because it is the same ball.
   • They get shorter as the ball slows down.
   • They get longer, because the ball has covered more distance.
3. 3

   You see two speed arrows in a diagram that has no marked scale to read lengths against. What can you still work out?

   • The exact speed of each object in real units.
   • Nothing at all, because arrow lengths are meaningless without a scale.
   • Which object is faster, but not their speeds in real units.

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.