Newton’s Laws of Motion

First and Third Laws

In 1666 (at the age of 23!), had an annus mirabilis, or "miracle year,” in which he developed his three laws of motion and his Law of Universal Gravitation, conceived ideas of calculus and developed ideas on light and color.

Terms Force –

Contact Force –

Field Force –

A force is a and therefore has both

and . It should be

represented with an in drawings.

Sir Isaac Newton (1643 – 1727)

There are two parts to Newton’s First Law

Newton’s First Law

Newton’s First Law is sometimes also referred to as the

.

means .

Examples of Newton’s First Law:

Newton’s First Law, cont.

Mass –

Weight –

The relationship between mass and weight is given by the following formula:

Mass Versus Weight

The units (and abbreviations) for mass and weight are as follows:

Mass Weight Metric

English (ours)

1 N is equal to 1 kg∙m/s2.

The conversion factor from newtons to pounds is:

lb = N

1 kilogram is roughly equivalent to 2.2 pounds; however, we do not talk about “converting” between pounds and kilograms. Why not?

Units for Mass and Weight

Newton’s First Law is about one object and the forces acting on it.

Newton’s Third Law will talk about the interaction between two objects and their forces.

Newton’s Third Law –

Newton’s Third Law

You are driving down the highway and a bug splatters on your windshield. How do the forces that the bug and windshield exert on each other compare in size?

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1. The bug exerts more force on the car

2. The car exerts more force on the bug

3. The forces each exert are the same

If you step off a ledge, you accelerate noticeably toward the earth because of the gravitational interaction between you and earth. Does the earth accelerate toward you as well?

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1. No; the earth is too big

2. No; you do not exert a force on it

3. Yes; the acceleration is too small to measure

4. Yes; the earth moves as much as you do

A horse refuses to pull a cart. The horse reasons, “According to Newton’s third law, whatever force I exert on the cart, the cart will exert an equal and opposite force on me, so the net force will be zero and I will have no chance of accelerating the cart.” What is wrong with this reasoning?

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1. The cart does not pull back on the horse, so there is nothing to slow the horse down

2. The cart moves because of the forces acting on it, not forces it exerts on other objects

3. Nothing is wrong; the horse is correct and the cart will not move.

To determine the “net external force” acting on an object, we draw what are called free body diagrams.

In a free body diagram, there are several things to consider.◦ Forces should be drawn roughly to scale.

You do not have to use rulers and make a scale, but if force 1 is two times as large as force 2, it should be drawn two times longer.

◦ Forces should be drawn as coming from the object’s center of mass

◦ We represent objects as boxes There is no environment drawn – even if it is on a table, you

only draw the object

Free Body Diagrams

Free Body Diagrams ChecklistMnemonic :

Abbrev.

Force Definition

One student pulls a box down a hall with a rope at an angle of 30 degrees. Another student pushes the box from the other side. There is friction between the box and the floor. Draw the free body diagram.

Free Body Diagram Example