forces in one dimension

Chapter 4

Forces in One Dimension

FIRST LAW OF MOTIONLaw of inertia

INERTIAAll objects resist changes in their state of motion - they tend to "keep on doing what they're doing."

NOTE• Inertia is directly proportional to the

mass of an object• Inertia is present to both objects at rest

and moving

Which object has inertia, a bullet train at rest or a moving bicycle? Why?

What is an external force?

UNBALANCED FORCES CHANGE MOTION

SYSTEMS AND EXTERNAL WORLD

System – identifying the forces interacting on the object of interest

SYSTEMS AND EXTERNAL WORLD

1. Contact Force – when an object from the external world touches the system exterting force on it (example: touching or tying a string)

SYSTEMS AND EXTERNAL WORLD

2. Field Force – field forces are exerted without contact (example: gravitational force)

COMBINING FORCESNet Force – Sum of vectors of all forces

SECOND LAW OF MOTION

Law of acceleration

• The heavier objects require more force to move the same distance as lighter objects

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PROBLEM 4• Supposing you were in space in In a weightless environment, would it

require a force to set an object in motion?

In a weightless environment, is force needed to keep an object

moving?

THE BIG MISCONCEPTION

PRACTICE PROBLEM

Free-fall - When a falling object is free of all restraints – no friction, air or otherwise – and falls under the influence of gravity aloneNonfree fall - The acceleration of an object depends not only on the force acting on the object but also the object’s resistance to motion

Free-fall - When a falling object is free of all restraints – no friction, air or otherwise – and falls under the influence of gravity aloneNonfree fall - The acceleration of an object depends not only on the force acting on the object but also the object’s resistance to motion

ANALYSIS ON FREE FALL

• In a vacuum, a coin and a feather fall side by side. Would it be correct to say that in a vacuum equal forces act on both bodies?

During each second of fall, the object gains a speed of 10 m/s.

Free-fall acceleration is approximately 10m/s/s or in short hand = 10m/s2.

Time of fall (s) Instantaneous speed (m/s)0 01 102 203 304 405 506 60

The acceleration of freely falling objects is customarily referred to as “g” because the acceleration is solely due to gravity. Thus, the velocity of a falling object is:

V = gt where:

V = velocity of falling objectg = gravitational constant (10 m/s2)t = time

2. Without considering air resistance, describe the velocity of the ball thrown straight upward. (5pts)– What is the acceleration of

the ball? – What is the velocity of the

ball when it gets to its highest point (10 seconds after being thrown)?

– What is its velocity 1 sec before it reaches its highest point?

– What is its velocity 1 sec after it reaches its higher point?

– At what speed was the ball thrown upward?

NONFREE-FALL ACCELERATION

BRAIN TEASERBRAIN TEASER• A sky - diver jumps from a highflying A sky - diver jumps from a highflying

helicopter. As she falls faster and helicopter. As she falls faster and faster through the air, does her faster through the air, does her acceleration increase, decrease or acceleration increase, decrease or remain the same? remain the same?

Why do heavier objects fall Why do heavier objects fall faster than lighter objectsfaster than lighter objects

THIRD LAW OF MOTION

Law of interaction

• The rocket's action is to push down on the The rocket's action is to push down on the ground with the force of its powerful engines, ground with the force of its powerful engines, and the reaction is that the ground pushes the and the reaction is that the ground pushes the rocket upwards with an equal force. rocket upwards with an equal force.

UP, UP, and AWAY!

• A fat man sat on a monoblock chair and the chair collapsed due to his heavy weight. Would it be correct to say that the chair failed to exert an equal and opposite force against the weight of the man?

• If we are going to total these action – reaction forces that are present in a system the net force will be equal to zero