Forces and Diagrams

When a force is applied to an object, it causes change in magtitude and direction.

To study mechanics, we must first know what is force and how to determine the forces on an object. To indicate a force on an object, we usually use vector diagram to represent it. The basic uses of using the force diagram is to show the magnitude and direction of the force acting on the object. Now, we will tell you how to draw the force diagram.

Example 1 (one object)

An object moving with constant speed along a frictionless table in straight line.

From Newston's first law, An object will remain at rest or moving along the straight line if no net force acting on the object.

Thus, there is no net force acting on the object.

Caution: An moving object does not indicate the exist of force acting at the direction which the object travels.

Example 2 (one object)

An object moving with constant speed in circular orbit.

Though the object is moving with constant speed, there is a change in direction indicating that there are net force acting on the object.

Example 3 (one object)

A ball is rolling down a hill with friction.

There are totally 3 forces acting on the object, they are the weight of the object, force by friction and the reaction.

Example 4 (two objects)

A ball is rolling down on a wedge.

You can see that there are 2 groups of forces in the diagram. The green one are the forces acting on the wedge and the white one are the forces acting on the ball. You can notice that the force acting on the wedge by the ball and the force acting on the ball by the wedge are equal in magnitute and opposite in direction.

Example 5 (Objects connected with strings)

We won't explain the forces in detail this time. One point is that there is a string connected between 2 balls. The forces acting in the string is called tension.Not only apply to elastic or inelastic strings, tension can exist in spring.