5.6.8 Newton’s First Law

Newton's First Law states that an object at rest will remain at rest, and an object in motion will continue to move at a constant velocity, unless acted upon by an external force. This fundamental principle helps us understand the behaviour of objects when forces are applied to them.

Newton’s First Law

• If the resultant force acting on an object is zero and the object is initially at rest, it will remain stationary.
• If the resultant force acting on a moving object is zero, the object will continue to move with the same speed and in the same direction. So the object continues to move at the same velocity.

Balanced Forces in Steady Speed

When a vehicle travels at a steady speed, the resistive forces (such as air resistance, friction, and drag) balance the driving force provided by the engine. This balanced condition allows the vehicle to maintain a constant velocity without acceleration or deceleration.

Role of Resultant Force in Changing Velocity

The velocity of an object (which includes speed and direction) will only change if a resultant force acts upon it. A non-zero resultant force is required to cause a change in the object’s state of motion.

Application of Newton's First Law

• Uniform Velocity: Newton's First Law can be applied to explain the motion of objects moving with a uniform velocity. In the absence of any external force, these objects will continue to move at a constant speed and direction.
• Changing Speed and/or Direction: Objects that experience changes in speed and/or direction require a resultant force to initiate these changes. Newton's First Law helps us understand that altering the state of motion of an object requires an unbalanced force acting upon it.

Inertia (HT only)

The tendency of objects to resist changes in their state of rest or motion is known as inertia. Inertia is an inherent property of all objects and is quantified by their mass. Objects with greater mass have greater inertia, making it more difficult to change their motion.

Conclusion

Newton's First Law provides the foundation for understanding the behaviour of objects in the absence of external forces. It explains why objects at rest remain at rest and objects in motion continue to move without any changes in their speed or direction. The concept of inertia helps us comprehend why objects resist changes in their state of motion unless acted upon by an unbalanced force.