One line or a flat surface?

The idea

Motion in a straight line is motion in one dimension, while motion in a plane is motion in two dimensions.

Straight or sideways too

A ball falls straight down. Another ball is kicked and flies along a curved path over the ground.

Do you think we can describe both motions along one straight line?

• No, the kicked ball needs a flat surface (a plane)
• Yes, both can be described along one line
• Neither ball moves

Line or plane?

Tap the box where each motion belongs.

One dimension, two dimensions

Motion along a straight line is one dimension: forward and backward. Motion in a plane, such as a vehicle overtaking another or the path of a kicked ball, is two dimensions.

Motion in a straight line is motion in one dimension, while motion in a plane, such as a vehicle overtaking another, the path of a kicked ball or a satellite in a circular path, is motion in two dimensions.

Notes

Straight line motion is one dimension. Motion in a plane (overtaking, a kicked ball, a satellite in a circular path) is two dimensions.

Check yourself

Motion in a straight line is also called motion in…

Which of these is motion in a plane (two dimensions)?

Which of these is motion in one dimension?

• two dimensions. Motion in two dimensions is motion in a plane, not in a straight line.
• one dimension — correct. Yes!
• a plane. Motion in a plane is motion in two dimensions. A straight-line motion is one-dimensional.
• A vertically falling ball. A vertically falling ball moves along a straight line, so it is one-dimensional.
• A train on a straight track. A train on a straight track moves along a straight line, so it is one-dimensional.
• The path of a kicked ball — correct. Yes!
• A car moving along a straight stretch of a highway — correct. Yes!
• A satellite moving in a circular path. A circular path lies in a plane, so this is motion in two dimensions.
• A vehicle overtaking another vehicle. The book gives overtaking as an example of motion in a plane, which is two-dimensional.