Lecture 3 - Newton's Laws of Motion and Friction

Newton’s First Law

An object at rest remains at rest unless a net force acts on it

An object moving with constant velocity continues to move with same speed and in the same direction unless a net force acts upon it

Free Body Diagrams

a diagram that shows all the forces acting on the object only and not by the object

Newton’s Second Law

When a net force acts on an object it experiences acceleration (F=ma)

If acceleration is 0…

F = 0

Tension

mass x gravity

Tension and Newton’s Second Law Equation

T - mg = ma

A 75kg person is standing on a bathroom scale in an elevator, calculate the scale reading in the elevator when it is stationary, ascending with acceleration of 1.2 m/s² and descending with acceleration of 1.2 m/s²

stationary: N - mg = 0

• N = mg

• N = 75×9.8 =735 N

ascending: N - mg = ma

• N = mg + ma

• (75×9.8) + (75×1.2) =825 N

descending: N - mg = -ma

• N = mg - ma

• (75×9.8) - (75×1.2) =645 N

NOTE: if elevator was free falling, scale would read 0 as person feels weightless

when you push down on the desk, what do you feel

the force of the desk pushing up on you

Whenever one body exerts a force on a second body…

The second body exerts an equal and opposite force on the first body

What is Friction

Whenever two surfaces slide across each other there is a force resisting the motion - called frictional force

Static Friction

There is NO relative motion between the two surfaces

Kinetic Friction

There is relative motion between the two surfaces

static friction vs kinetic friction

static friction is inclined upward until it reaches a peak, then it transitions to kinetic friction where it drops and plateaus

Frictional Force

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