Newtonian mechanics
Newton’s first law (inertia)
If the net force on an object is zero, it remains at rest or continues moving uniformly in a straight line.
A ball does not change its state of motion by itself. A net force is needed to speed it up, slow it down, or change its direction.
Definition: Quantities and model
The law is stated in an inertial reference frame. Inertia is the tendency to keep the same velocity; the greater the mass, the harder it is to change that velocity.
Reading the model
If the net force on an object is zero, it remains at rest or continues moving uniformly in a straight line.
Example: Worked example
A trolley rolling on a very low-friction surface continues nearly uniformly in a straight line. Small friction means a small net horizontal force; a continuing push is not needed to maintain velocity.
Solution
A trolley rolling on a very low-friction surface continues nearly uniformly in a straight line. Small friction means a small net horizontal force; a continuing push is not needed to maintain velocity.
Inertia is the tendency to maintain velocity, not a force that pushes an object forward. When a bus brakes suddenly, passengers tend to keep moving and lurch forward; a seat belt exerts a force that changes their velocity. A book resting on a table has zero net force because weight and support balance. Zero net force does not mean that no forces act.
Example: Forces on a book at rest
A book has a 3 N downward force and a 3 N upward force. What is the net force?
Solution
ΣF=3−3=0 N; it remains at rest or continues in uniform straight-line motion.
The law of inertia applies in an inertial reference frame; a frame fixed to Earth is often a good approximation in everyday situations. In an accelerating vehicle, passengers seem to be pushed backward because their bodies resist a change in velocity. Identify the reference frame before describing motion.
Quick check
Which quantity or statement is correct for this motion?
Which statement correctly describes the motion in this topic?
References
- Young, Freedman (2019). University Physics
- Halliday, Resnick, Walker (2014). Fundamentals of Physics