Newton's First and Second Laws

In this unit we begin to consider the causes of motion.  The motion is explained using Newton's First and Second Laws.  When one begins to examine these laws they seem very simple but the consequences are very powerful and important.  As I have mentioned in class, many times college physics courses include a separate course for each of these laws - statics (the net force is zero) and dynamics (the net force is not zero).

Newton's First Law - The Law of Inertia:

Inertia is a measure of resistance to the change in motion.  We measure this quantity by measuring the mass.  Mass units are either kilograms (metric system) or slugs (English system).  Mass is not measured in pounds.  Newton's first law states that an object will stay in its own motion unless acted on by an outside force.  Or more simplified an object in motion will continue in that same motion in a straight line or an object at rest will continue at rest unless a net force is applied.  If a net force is applied then Newton's 2nd law comes into effect.

Newton's Second Law - F=ma

Again at first glance this looks very simple but because there are so many forces acting around us all the time, it is easy to get this all mixed up.  We often like to state Newton's Second Law as F=ma but the correct form (the form that Newton described) is a=F/m.  This is a "correct" form because acceleration is caused by the force and the mass.  In the typical form one might be convinced that force is caused by mass and acceleration which leads to many incorrect conclusions.

In this simple equation, force and acceleration are both vectors, meaning they both have a defined direction and they must be in the same direction.  In addition, the force and acceleration must be from the same cause.  Example, using the acceleration of gravity to determine the force of a car engine does not work.  Mass in this equation is scalar, meaning that it is direction independent.  You can push (force) something north, but it doesn't make sense to suggset that 15 Kg is north.

When the acceleration is being measured (as in a=(vf - vi )/t for example) the acceleration is caused by the net force.  The net force is the sum of all the forces.  Because forces can act in different directions at the same time it is often useful to separate the components before doing any calculations.  Add to these components the fact that there is usually more than one force many students get "lost" in the calculations if each part is not carefully considered and included in the determination of the net force.  One technique that helps keep track of all of these forces is called a Free Body Diagram (FBD) or a Force Diagram.  We will get plenty of practice with these diagrams in class.

One very common mistake in considering forces is the confusion of mass and weight.  Mass is a measure of inertia, the amount of matter in an object, the resistance to the change in motion.  Weight is simply the force of gravity.  Mass does not have direction, weight has a direction of down (usually we designate this as the negative y direction).  Mass is measured in kilograms or slugs, weight in newtons or pounds.  Because the acceleration of gravity near the surface of the earth is so constant (9.8 m/s2 or 32 ft/s2) it is easy to calculate the weight (F=ma) if the mass is known or to calculate the mass if the weight is known (m=F/a).  Even thought it is "easy" to do, it is a concept that is difficult for most students to "get".

Unit III Dynamics

PDF Word
Look Out! No Breaks! Notes Free Body Diagrams N1L Quiz 1 N1L Quiz 1
N1L Quiz 2
N1L and N2L HW
N2L Quiz 1 N2L Quiz 1
FBD Quiz 1 FBD Quiz 1
N2L Quiz 2
N2L Quiz 3 N2L Quiz 3
Friction Homework

Circular Motion

CM Free Body Diagrams Circular Motion Quiz 1 Circular Motion Quiz 1
CM and Universal Gravitation Homework Circular Motion Quiz 2 Circular Motion Quiz 2
Circular Motion Quiz 3 Circular Motion Quiz 3
Car Race News Universal Gravitation Quiz Universal Gravitation Quiz