Along the tangent

The idea

In circular motion the velocity at a point is along the tangent to the circle in the direction of motion, so its direction changes continuously; an object released from the circular path moves in a straight line in that direction.

Let go of the string

A stone tied to a string is whirled in a circle. Suddenly the string is let go, and the stone flies off.

Which way do you think the stone flies?

• In a straight line, in the direction it was moving
• Straight towards the centre of the circle
• Round the circle once more

Release it

Choose A, B, C or D, press Play and watch the red arrow and the path after release.

Tangent and release

On the circle, the red arrow touched the circle at the object and pointed the way the object was moving: along the tangent. At each point the tangent points in a different direction, so the direction of the velocity keeps changing. When the object was released, it kept going in that direction as a straight line.

In circular motion the velocity at a point is along the tangent to the circle in the direction of motion, so its direction changes continuously. An object released from the circular path moves in a straight line in that direction. (A tangent is a straight line that meets the circle at one and only one point.)

Notes

The velocity in circular motion is along the tangent, so its direction keeps changing. A released object moves in a straight line in that direction.

Check yourself

In circular motion, the velocity of the object at a point is directed…

An object moving in a circle is released at a point. It then moves…

Why does the direction of the velocity change continuously in circular motion?

A straight line that meets a circle at one and only one point is called…

• along the radius, towards the centre. The velocity is along the tangent, not along the radius.
• along the tangent to the circle at that point, in the direction of motion — correct. Yes!
• along the radius, away from the centre. The velocity is along the tangent, not along the radius.
• along the circle again. Without the circular constraint it no longer follows the circle. It moves in a straight line.
• towards the centre of the circle. It continues in the direction it was moving, which is along the tangent and not towards the centre.
• in a straight line in the direction it was moving at the instant of release — correct. Yes!
• The tangent points in a different direction at each point of the circle — correct. Yes!
• Because the object moves in a straight line. The object moves along a circle. Its velocity direction changes because the tangent direction changes.
• Because the speed keeps changing. In uniform circular motion the speed is constant. It is the direction that changes.
• a radius. A radius joins the centre to a point on the circle. A tangent meets the circle at only one point.
• a tangent — correct. Yes!
• a revolution. A revolution is going round the circle once. It is not a line.