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Newton’s Laws of Motion describe how objects move as a result of the forces acting on the object. Newton’s First Law looks at the case of balanced forces on the object.
Newton’s First Law
An object at rest will remain at rest and an object in motion will continue to move at a constant speed in a straight line, unless a resultant force acts on it.
There are really two parts to this law – the motion and the force.
Understand that actually the motion part could be written most simply as objects moving with ‘constant velocity’ – ‘constant speed in a straight line’ means ‘constant velocity’, also ‘at rest’ is just a special case of a ‘constant velocity’ (velocity of zero). Another way to think of this is objects that are not accelerating.
We know that we can apply this law in situations where the resultant force on the object is zero. Of course this can be when there are no forces on the object at all, but is more likely the situation of the forces cancelling out – balanced forces.
Some textbooks refer to ‘external forces’ being applied. Imagine squeezing your hands together. Is this going to make you move? Of course not, the force is an internal force. Now consider someone else pushing you, would that get you moving? Of course you would move, because the force applied was an external force.
Note that the Law should be considered from both situations. If an object is not moving (or moving with constant velocity) then we can imply that their is no resultant force acting on the object.
Conversely, if we know there is no resultant force on an object then we know the object cannot be accelerating and must be moving with a constant velocity (or at rest).
Physics questions about Newton’s First Law are frequently descriptive in nature. Look for keywords in the question such as ‘object at rest’, ‘stationary object’, ‘moves with a constant velocity’ – these are usually clues that you will be applying the Newton’s First Law to solve this problem.
Newton’s First Law is also referred to as the Law of Inertia.
- The inertia of a body is its tendency to continue with its state of rest or motion.
- The mass of a body is a measure of its inertia. The greater the mass, the greater its inertia.
- As inertia depends only on mass an object’s inertia does not vary as the object travels to different parts of the universe.
Lifting and pushing a car on the Moon!
A car would be just as difficult to push horizontally if it were on the Moon as it is on earth. In both cases the inertia is the same.
The same car would, however, be much easier to lift up off the ground if it were on the Moon as its weight would be less.
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