Newton’s 2nd and 3rd Laws of Motion Physics: Chapter 4 Section 3

Newton’s 2nd Law

• Force is proportional to mass and acceleration

• The unbalanced force acting on an object equals the object’s mass times the object’s acceleration

• If the same force is applied to 2 objects of different mass, the less massive object will accelerate more quickly

Newton’s 2nd Law

It obviously takes less force to make the ball accelerate because the ball has less mass.

Newton’s 2nd Law

F = m•a• SI units for force = Newton

***(1 N = 1 kg·m/s2)***

F = Vector sum of all external forces

acting on a body

m = mass of the object (in kilograms)

a = acceleration of the object (in m/s2)

Newton’s 2nd Law

• Example: Think of the force required to push an empty shopping cart, and the force required to push a full shopping cart. What is the difference?

• More force applied = greater acceleration

Newton’s 2nd Law

•Weight ≠ Mass

Weight – the measure of gravity pulling on an object

Mass – the measure of the amount of matter in an object

Newton’s 2nd LawWeight influences shape

On land a supporting skeleton is required

In water less body support required because the water helps lift the mass

*Animals can be larger if they live in the water

Newton’s 2nd Law

•Free fall – motion of a body when the only force acting on it is gravity•Terminal velocity – maximum velocity reached when air resistance pushes up as much as gravity pulls down

Force of gravity pulling down = force of air resistance pushing up

Newton’s 3rd Law•For every action there is an equal but opposite reaction

***Action and reaction forces are applied to different objects, but act in pairs (both contact and field forces)

These forces occur in pairs at the same time, but do not cancel out! Example: rocketry

Newton’s 3rd Law

• Hammer and Nail example

Newton’s 3rd Law

Actions and Reactions

Newton’s 2nd and 3rd Laws Homework• Page 138 #1-5