A marker on a straight path has position x = -6 m. Which description matches?
You've already seen that in one dimension a position's sign picks out a side of the origin, and that a displacement and an average velocity keep their signs through the arithmetic. The sign of a velocity has an exact meaning of its own.
So v = −3 m/s means moving in the negative direction at 3 m/s.
An object moving along a straight line has v = -7 m/s. What does this velocity say?
Read one with a named frame. A straight cycle path takes east as the positive direction, and a cyclist has v = +6 m/s. The sign is positive, so she is moving in the positive direction: east. The number tells how fast: 6 m/s. She is moving east at 6 m/s.
A model train runs on a straight track where east is the positive direction. Its velocity is v = -4 m/s. Which description matches?
Along a fence line where north is positive, a bird flies with v = −2 m/s. The sign is negative, so the bird is moving in the negative direction: south. The bird is moving south at 2 m/s.
On a straight trail, north is the positive direction. Which velocity describes a runner moving south at 5 m/s?
One caution. The velocity's sign says nothing about where the object IS. A cart can sit at x = +2 m, on the positive side of the origin, while moving with v = −3 m/s. The position's sign gives the side; the velocity's sign gives the direction of motion.
An elevator shaft takes up as the positive direction. At some moment the elevator's velocity is v = -4 m/s while the elevator is above the ground floor. Which statement is correct?
Two carts roll on the same straight track. Cart A has v = +5 m/s and cart B has v = -5 m/s. What do the two velocities share, and how do they differ?
A student reads 'v = -3 m/s' and writes: 'the object is behind the starting point.' What is wrong with the student's reading?