Plus and minus on a line

The idea

In straight-line motion an object can move only forward or backward, so direction is shown by plus and minus signs; positions to the right of the reference point O are generally positive and to the left negative.

Forward or backward

On a straight track an athlete can only run forward or backward. We want one short mark that tells the direction.

What do you think can do this job?

• A plus sign or a minus sign
• A different colour for each number
• The shape of the track

Slide along the track

Move the athlete to the places asked for and read her position.

A sign for direction

When the athlete was 40 m to the right of O, her position was written +40 m. When she was 30 m to the left, it was written −30 m. The number is the distance from O and the sign is the direction.

In straight-line motion an object can move only forward or backward, so the direction is shown by plus and minus signs. Positions to the right of the reference point O are generally positive and to the left negative.

Notes

In a straight line the direction is a plus or minus sign: right of O is +, left of O is −.

Check yourself

In motion in a straight line, how is the direction shown?

A position 25 m to the left of O is written…

An athlete is at +60 m. Where is she?

An athlete is at −30 m. How many metres is she from O?

Answer: 30 m

She is 30 m from O, to the left of it.

• With a number without any sign. Without a sign, two places on opposite sides of O could not be told apart.
• With a plus or a minus sign — correct. Yes!
• With a curved arrow. In a straight line there are only two directions, so a plus or minus sign is enough.
• +25 m. A plus sign is generally used for positions to the right of O.
• 0 m. 0 m is the position of O itself, not of a place 25 m away from it.
• −25 m — correct. Yes!
• 60 m to the right of O — correct. Yes!
• 60 m to the left of O. Left of O is negative, so a left position would be written with a minus sign.
• At O. At O the position is 0 m, not 60 m.