Conceptual Physics

Chapters 4,5, & 6

Newton’s Laws of Motion

Conceptual Physics

Chapter 4

Newton’s First Law

Conceptual Physics

Newton’s First LawThe Law of Inertia

An object at rest tends to stay at rest. An object in motion tends to stay in motion.

Conceptual Physics

Inertia: A resistance to change, a property of all matter.

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Mass: A Measure of Inertia Mass is measured in kilograms Mass is not Weight

Mass is a “built in” property of matter Just because you leave earth, you

don’t change your mass, but you do change your weight

Weight is an force caused by the acceleration due to gravity on the mass

Conceptual Physics

Weight

Mathematically Weight = mass x gravity

OrFw = mg

So the force of weight on one kilogram of mass on planet earth is given by:

Fw = (1 kg)(9.8 m/s2) = 9.8 Newtons

Conceptual Physics

Sample Problem

What is the weight of a 10 kg object a) on earth b) on the moon (g = 1/6 that of

earth)

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Solution

a) Fw (on earth) = mg

= (10 kg)(9.8 m/s2) = 98 Newtons

b) Fw (on moon) = mg

= (10 kg)(1.6 m/s2) = 16 Newtons

Conceptual Physics

Net Force and Equilibrium

Net force is the sum of all the forces acting on an object

When the net force on an object equal zero – the object is said to be in equilibrium.

Conceptual Physics

Net Force and EquilibriumWhen a statue sits on

the ground, the weight of the statue pushes downward and a support force* from the ground pushes upward, so the statue is in equilibrium

* The support force is also called the Normal Force

Fw

FN

Conceptual Physics

Sample ProblemIf a box with a weight of 20

Newton sits on a table, with what normal force does the table push back?

A second, 10 Newton force is added to the top of the first box. Now what is the normal force exerted by the table on the 20 Newton box? What normal force is exerted by the 20 Newton box on the 10 Newton box?

20 N

20 N

10 N

Conceptual Physics

Chapter 5

Newton’s Second Law of Motion

Conceptual Physics

Newton’s Second LawThe Law of Acceleration

Forces cause acceleration Net force must be greater than zero

Masses resist acceleration due to inertia This is, in part, why it is harder to start

something moving than to keep it moving Hence we say that acceleration is

directly proportional to net force and inversely proportional to mass

Conceptual Physics

Newton’s Second LawThe Law of Acceleration The acceleration of an object is in the

direction of the force applied. Acceleration is directly proportional to the

force applied. The harder you push an object the faster it

goes Acceleration is inversely proportional to the

mass of the object. The heavier the object, the less affect a push

has.

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Sample Problem

What force is required to accelerate this object horizontally at 6 m/s2?

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Solution

F = ma = (10 kg)(6 m/s2) = 60 Newtons

Conceptual Physics

Friction Forces always come in pairs, hence the

Normal force, which is perpendicular to the contact surface, has a companion force that is parallel to the contact surface, this force is friction

Friction always opposes motion Friction depends upon two things:

The nature of the contact between two objects

How strong the force of contact is (The Normal Force)

Conceptual Physics

Friction

Friction also occurs in gases and liquids both of which are referred to in physics as fluids.

In fluids we call friction drag and in air we refer to it specifically in air as air resistance.

Conceptual Physics

Sample Problem

If a 200 Newton force is applied to a box that undergoes a 100 Newton resistive force (friction). What is the net force on the box? If it is a 30 kg box, what is its acceleration?

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Solution

Fnet = 200 N – 100 N = 100 N

Fnet = ma so a = Fnet/m

= 100 N/30 kg = 3.3 m/s2

Conceptual Physics

Pressure Pressure is defined as the force exerted

divided by the amount of area over which the force is spread

Mathematically this is expressed as:P = F/A

P – Pressure F = Force – in lbs or newtons A – area – usually in meters2 or centimeters2

Conceptual Physics

Chapter 6

Newton’s Third Law

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Newton’s Third LawAction - Reaction

For every action there is an equal and opposite Reaction.

Conceptual Physics

Conceptual Physics

Sample Problem

If a 0.4 kg shotgun shell undergoes a 100 Newton force when it is fired, what is its acceleration?

If it was fired from a 2 kg shotgun what is the recoil acceleration of the shotgun?

Conceptual Physics

Solution

Since F = ma; a = F/m So:a = 100 N/0.4 kg = 250 m/s2

Since the recoil force is equivalent to the firing force according to Newton’s Third Law the same equation applied however now you use the mass of the shotgun:

a = 100 N/2 kg = 50 m/s2

Demonstrating that it is the mass of the shotgun that keeps it from doing the same damage as the bullet