# Newton’s Laws

12PHYS - AS91171

2020

## Newton’s First Law

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). Source

### 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). Source

## 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. Source

### 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) Source

## 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. Source

### 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. Source

## 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.
• Draw a diagram each time!