Balanced force

Newton’s First Law of Motion

When two or more external forces acting on a body produce no net resultant force, i.e., vector sum of forces is zero, we say that the forces are balanced. The lack of resultant force produces no net acceleration and hence, the body will remain at rest or moves at its original constant velocity in a straight line.

• This is Newton’s First Law of Motion
• An external force is one whose source lies outside of the body being considered. e.g. weight of a body and friction
• The resultant force is the vector sum of all external forces exerted on a body.

Newton’s First Law of Motion is also known as the law of inertia. Inertia is a property of mass resisting any change from its original state of rest or motion. The greater the mass of a body, the greater will be its inertia and the greater will be its resistance to changes to its state of rest or motion.

A body at rest implies that the net resultant force exerted on the body is zero.

• However, it is not necessarily that there is no forces acting on the body.

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    For instance, a box resting on a table has zero net resultant force. But there are two forces acting on the box! One of the force is the gravitational force due to the weight of the box, while the other is the normal force.
• The normal force and weight arrows are of the same length (same magnitude) but in opposite directions.
• Normal force (or reaction force) is an external force exerted perpendicularly by the surface in reaction to any body placed against it. The normal force arrow starts from the base of the box (contact between the box and the table top).
• The weight arrow starts from the centre of mass of the box (labelled as C.M.), as indicated by the black dot.

Other Laws of motion

Newton’s 3 laws of motion are:

1. 1st law: Newton’s first law of motion states that a body will continue in its state of rest or uniform motion in a straight line unless an external resultant force acts on it.
2. 2nd law: Newton’s second law states that the rate of change of momentum of a body is proportional to the resultant force acting on it and the change takes place in the direction of the force.
3. 3rd law: Newton’s third law states that: If body A exerts a force on body B, then body B exerts a force of equal magnitude but in the opposite direction on body A.

We will explore the 2nd law of motion in greater details in the subsequent posts.

Newton’s Third Law of Motion

Newton’s third law states that: If body A exerts a force on body B, then body B exerts a force of equal magnitude but in the opposite direction on body A.

Forces always occur in pairs – Action force and reaction force.

Action and Reaction Forces

Some properties of the action and reaction forces are as follows:

• The action and reaction forces are equal in magnitude.
• The action and reaction forces act opposite to one another.
• The action and reaction forces act on different bodies.

Worked Example 1

Ignoring air resistance and upthrust, describe the action-reaction pair for a ball in free-fall.