Newton's First Law

Newton's First Law and Inertia

Newton's First Law states that an object will remain at rest or continue to move at a constant speed in a straight line unless acted upon by a resultant force.

This means:

• Objects at rest stay at rest.
• Objects in motion keep moving at the same speed and in the same direction.
• Any change in this state requires a resultant (unbalanced) force.

Inertia is the tendency of an object to resist changes to its motion. The greater the mass of an object, the greater its inertia.

For example, a heavy lorry has more inertia than a bicycle, so it is harder to start or stop the lorry moving.

This law explains why seat belts are essential in cars: if the car suddenly stops, your body tends to keep moving forward due to inertia, so the seat belt applies a force to stop you safely.

For instance, if a hockey ball is sliding on ice, it will keep sliding at the same speed unless friction or another force slows it down.

Resultant Force and Motion

The resultant force is the overall force acting on an object when all the individual forces are combined.

- If the forces acting on an object are balanced (resultant force is zero), there is no change in the object's motion. It will either stay at rest or continue moving at constant speed in a straight line.

- If the forces are unbalanced (resultant force is not zero), the object will accelerate (change speed or direction).

This directly relates to Newton's First Law: only a resultant force can change an object's motion.

For example, a book resting on a table has balanced forces acting on it: gravity pulling it down and the table pushing it up. Since these forces balance, the book stays still.

Examples of Newton's First Law in Everyday Life

• Stationary objects remain still: A parked car will not move unless a force acts on it, like the engine starting or someone pushing it.
• Moving objects keep moving at constant speed: A football rolling on grass will eventually stop because of friction, but if it were on a frictionless surface, it would roll forever at the same speed.
• Effect of friction and other forces: Forces like friction, air resistance, or gravity often act to change an object's motion, meaning the resultant force is not zero.

For example, when you slide a book across a table, it slows down and stops because friction between the book and table acts as a force opposite to the motion.

If you push a trolley and then stop pushing, it will eventually slow and stop because of friction and air resistance acting on it.