← Components and relative motion

Component signs from direction

A gull wheels over a harbor. One moment its velocity points up and to the right; the next it points down and to the left. The components of those two velocities differ by more than length. They differ in sign.

Keep the convention you have used since Unit 1's number lines: right is the positive horizontal direction, and up is the positive vertical direction.

Read each component's sign straight from the direction words. Rightward means a positive horizontal component; leftward means a negative one. Upward means a positive vertical component; downward means a negative one.

The gull flies up and to the right. Rightward makes its horizontal component positive. Upward makes its vertical component positive.

right is positiveup is positivehorizontal +, vertical +
Up and to the right: both components are positive.

Now the same gull flies down and to the right. One direction word changed. Downward makes the vertical component negative; the horizontal component stays positive.

right is positiveup is positivehorizontal +, vertical −
One word changed, down: the vertical sign flips to negative.

Now the same gull flies down and to the left. The horizontal word changed too. Leftward makes the horizontal component negative.

right is positiveup is positivehorizontal −, vertical −
Down and to the left: both components are negative.

Now the same gull flies up and to the left. Leftward keeps the horizontal component negative, and upward makes the vertical component positive.

right is positiveup is positivehorizontal −, vertical +
Up and to the left: horizontal negative, vertical positive.

Last, the gull folds its wings and drops straight down. Straight down has no rightward or leftward part at all. So its horizontal component is zero, and its vertical component is negative.

right is positiveup is positivehorizontal 0, vertical −
The boundary case: straight down has no horizontal word, so the horizontal component is zero.
A sign comes from a direction word. If the direction has no horizontal word, the horizontal component is zero; if it has no vertical word, the vertical component is zero.

A paraglider's velocity points down and to the right. Right and up are positive. State the sign of each component of the velocity.

The direction has two words: down, and to the right.

Rightward makes the horizontal component positive.

Downward makes the vertical component negative.

The horizontal component is positive; the vertical component is negative.

Now make the read quick. Six movers follow, and each asks the same question: is the named component positive, negative, or zero?

Right and up are positive. A gull climbs up and to the right. Is the horizontal component of its velocity positive, negative, or zero?

Right and up are positive. A squirrel runs down a branch, down and to the left. Is the vertical component of its velocity positive, negative, or zero?

Right and up are positive. A skier glides down and to the left. Is the horizontal component of her velocity positive, negative, or zero?

Right and up are positive. A balloon drifts up and to the left. Is the vertical component of its velocity positive, negative, or zero?

Right and up are positive. A stone falls straight down. Is the horizontal component of its velocity positive, negative, or zero?

Right and up are positive. A hawk soars up and to the left. Is the horizontal component of its velocity positive, negative, or zero?

Your turn

Right and up are positive. A drone's velocity points up and to the left. Which pair gives the signs of its horizontal and vertical components, in that order?

Right and up are positive. A kite's velocity has a negative horizontal component and a negative vertical component. Which way does the kite move?

Right and up are positive. A diving pelican's velocity has a positive horizontal component and a negative vertical component. Which way does the pelican move?