S P H 3 U – U N I T # 2 DY N A M I C S

NEWTON’S FIRST LAW OF MOTION

LEARNING GOALS

After this topic, I will be able to…

• Define inertia

• Solve problems on Newton’s First Law of Motion

SUCCESS CRITERIA

I am successful if I know how to…

• Determine the net force acting on an object, and

• Can solve problems on Newton’s First Law of Motion

GALILEO’S VIEW OF FORCE & MOTION

Prior to the 1600’s, early scientists held a simplistic view of motion. They noticed that moving objects would spontaneously slow down for no apparent reason. Since they did not know about friction, they concluded that a constant net force was needed to keep an object moving. They thought – based upon the teachings of Aristotle – that larger net forces made things move faster and smaller net forces made things move slower.

GALILEO’S VIEW OF FORCE & MOTION

Galileo questioned this view. He performed real experiments with a ball rolling down and up sloped ramps. He also performed virtual experiments to try to explain his ideas.

INERTIA

Galileo concluded that once an object starts moving, it will continue to move at a constant velocity if there is no friction present. Galileo used the concept of inertia to help explain his conclusion. Inertia is the property of matter that causes it to resist changes in motion. The inertia of an object depends on the mass of the object.

Inertia:• The property of matter that causes it to resist changes in

motion• Is directly proportional to the mass of the object

EXAMPLES OF INERTIA

Note:Since an object’s mass is a measure of its inertia, and vice versa, there are many everyday examples of objects resisting a change in their motion, including the following:

EXAMPLES OF INERTIA

When a car stops suddenly, a passenger in the front seat not wearing a seat belt continues to move forward (due to the passenger’s inertia) and collides with the car’s windshield.

EXAMPLES OF INERTIA

A magician pulls a smooth tablecloth quickly out from under a place-setting of expensive china. Due to the inertia, the dishes remain at rest where they were, and are not broken.

EXAMPLES OF INERTIA

A Baffling Balloon

NEWTON’S FIRST LAW OF MOTION: THE LAW OF INERTIA

Although Galileo discovered the notion of “inertia” it was Newton who coined the term when he summarized Galileo’s work in his book Principia Mathematica. Because it was included with Newton’s other laws of motion, it is often referred to as Newton’s first law of motion.

Newton’s First Law of Motion:• also known as the law of inertia• If the net external force on an object is zero, the object will

remain at rest or continue to move at a constant velocity. Internal forces have no effect on an object’s motion.

NEWTON’S FIRST LAW OF MOTION: THE LAW OF INERTIA

We can summarize the first law of motion by stating four important results:

(a) Objects at rest remain at rest unless acted upon by a net force

(b) Objects in motion remain in motion unless acted upon by a net force

(c) If the velocity of an object is constant (or zero), the net external force acting on it must be zero

(d) If the velocity of an object is changing either in magnitude, direction, or both, the change must be caused by a net external force on the object

PRACTICE

1. Suppose you are helping a friend move. The friend asks you to stand in the back of a pickup truck to hold a piano because there is no rope available to tie it down. Explain why you should refuse this request.

Inertia – objects in motion want to stay in motion

PRACTICE

2. A friend carries their metal lunchbox on the ledge between the rear windshield and rear set of their car. Is this safe?

No – if the car stops suddenly the metal lunch box continues moving forward because of inertia

PRACTICE

3. Why must an object at rest have either no force or a minimum of two forces acting on it?

A minimum of two forces are needed because one force is needed to balance the other – one force would not be able to balance itself

HOMEWORK

Read: pg. 32 – 37Practice problems: Pg. 35 #2, 3 and Pg. 37 #1-3