Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.

TLDR: A non-zero net force will induce a change in velocity (an acceleration).

Examples

Car Crash: A person not wearing a seatbelt during a crash will continue their motion forwards as the car suddenly stops.

Ball: A ball rolling on a soccer field will come to a stop because of the friction force acting upon it, creating a negative acceleration (deacceleration).

Newton’s Second Law

Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration. \(F=m \times a\)

TLDR: The acceleration created by a force depends directly upon the mass of the object.

Examples

A student pushing a trolley can cause a great deal of acceleration

A student pushing a car will cause very little (or no acceleration)

Newton’s Third Law

For every action, there is an equal and opposite reaction.

TLDR: Objects exert an opposite and equal force as a kind of support.

Examples

Rockets: Gas is ignited causing it to heat and expand. It is expelled out a narrow nozzel at a high velocity. An equal and opposite force is exerted on the spacecraft causing it to move forwards.

Bouncing Balls: A ball thrown on the ground exerts a force on contact, the ground exerts an equal and opposite force which causes the ball to bounce.

Pātai

With the person next to you, write a paragraph describing how Newton’s 1st and 2nd Laws relate to projectile motion.

Whakatika

Newton’s 1st Law

A non-zero net force will induce a change in velocity (an acceleration)

In the x-direction there are no forces acting (friction is negligible), and in the y-direction there is only the weight force of the object. This means the resultant force is in the downwards direction.

An acceleration is therefore induced in the downwards direction (constant).

Newton’s 2nd Law

The acceleration created by a force depends directly upon the mass of the object.

In Earth’s gravitational field at sea level, \(g=9.81ms^{-2}\) at all times, and therefore the object is accelerated uniformly down.

Whakawai/Practise

Try Q1-3 of Activity 10A: Using Newton’s Laws in your textbook.