Newton’s 1st Law of Motion

Inertia

Ch. 4.5-4.8

Review

• Aristotles concept of natural vs. violent motion

• Galileo’s concept of motion

• Newton’s 1st Law of motion

• Some review questions . . .

• HW questions review: p.56 (1, 3, 4, 6-8)

Objectives

1. Distinguish among mass, volume, & weight

2. Convert kilograms to Newtons

3. Calculate a net force

Mass

• Let’s kick some stuff• Drop your pen on the floor and kick it.• Now, kick your desk.• Which one has more mass?• What is Mass?• Mass:

– The measure of inertia in an object.

– Quantity of matter in an object

Mass vs. Volume

• Which has more mass? (show example)

• Which has greater inertia?

• Which has a greater volume?

• Volume:– A measure of space

(cm3, m3)

• MASS IS NOT VOLUME!

Mass vs. Weight

• Remind me what mass is?– The measure of the amount of material in an

object.

• What is weight?

• Weight:– The measure of gravitational force on an object

• Can an objects weight or mass change depending on where it is? For example, outer space? Check your neighbor . . .

Mass vs. Weight

• Will the mass of this satellite be the same in outer space as it is on earth? – What about the weight?

• The amount of material in the satellite will always be the same no matter where it is located --> an object’s mass always stays the same, no matter where it is!

• Shake your book back and forth. It will feel the same in space. The mass, or amount of inertia, will not change!

• Weight is different. Less gravity --> less weight

Mass vs. Weight

Does the man’s mass stay the same? What about his weight?

Check your neighbor…

• Review with your neighbor the difference between mass, volume, and weight.

Quick Review

• Mass is. . .– the amount of matter in an object– not affected by gravity

• Volume is . . .– the amount of space that matter occupies

• Weight is. . .– the force of gravity on an object.

Hewitt vs. The Anvil• Why doesn’t this hurt Mr. Hewitt?

Demo with weight and stringPart 1

• I am going to gradually pull harder on the bottom string. Based on the questions below, make a prediction with your neighbor:

1. Is the string tension greater in the upper or lower string?

2. Which string is likely to break?3. Which property, mass or weight, is

important here?

Demo with weight and stringPart 1

1. Upper string has greater tension b/c it supports the weight of the object plus your pull. Lower string supports only your pull.

2. The upper string broke.

3. Weight is more important here.

Demo with weight and stringPart 2

• I am now going to QUICKLY pull the string downward. Will anything be different? Make a prediction with your neighbor based on the following questions.

1. Which string is more likely to break?• The bottom string broke! Demonstrates inertia. The

metal weight resists the sudden downward acceleration of the lower string.

2. What is more important here, mass or weight?• Mass is more important here!

Inertia in Action

• Let’s watch some true geniuses as they are confronted with the task of dealing with inertia and snow. . .

Shorter at Night?

• Remember the hammer demo?

• Demo:– Same idea with piece of

wood and dowel

• Wood remains in motion while dowel stops.

• What does this have to do with you being shorter at night?

Shorter at Night?• The bones of your spine

settle together throughout the day.

• Find a place at home just out of reach before you go to bed

• Wake up in the morning and try to touch it

• Astronauts returning from orbit are more than an inch taller!

Newtons and force• In the U.S., the amount of matter is described by

it’s gravitational pull to earth, or its weight• The U.S. uses units called pounds.• The rest of the world uses kilograms.• What about force?• The SI (International Unit) unit we use to measure

force is called a Newton (N).• 1 kg = 9.8 N (2.2 lbs)• Weight (in newtons) = Mass x Acceleration due to

Gravity– Weight = mg

Definition of a Newton

Quick Practice

• How many Newtons is a 3-kg bag of bananas?– 3 kg x 9.8 = 29.4 N

• How many Newtons is Mr. Ward? – I weigh 170 pounds

– There are 2.2 pounds in every kg

• 170/2.2 = 77.3 kg• 77.3 kg x 9.8 N = 757.5 N• I weigh 757 N

More Practice• Does a 2 kg iron block have twice as much inertia as a 1

kg block of iron? Twice as much mass? Twice as much volume? Twice as much weight when weighed in the same location?

• A: Yes to all questions!• Does a 2 kg bunch of oranges have twice as much

inertia as a 1 kg loaf of bread? Twice as much mass? Twice as much volume? Twice as much weight when weighed in the same location?

• A: Yes to all questions EXCEPT volume.– The density of oranges is much greater than that of bread -->

oranges occupies less volume

Let’s Practice

• Pass out worksheet 4-1

• Please work in pairs. THIS MEANS TWO PEOPLE ONLY!

Net Force• Volunteer?• Pushing chair across floor with 10 N• Neglecting friction, if I push this chair with

10 N, then the net force, is 10 N.• Volunteer come up and push in opposite

direction with equal force. What will the net force be?

• What if student pushed on it in opposite direction with a force of 4 N? What would net force be?

Net Force• Net Force:

– The combination of all forces acting on an object• Let’s draw this out together.

Equilibrium

• What forces are acting on the cat while it’s motionless on the table?

• Gravity AND the support force of table

• Table pushes up with a force equal to books weight

• We call this equilibrium.• Equilibrium:

– The net force equals zero.

Equilibrium• Hanging from a rope is another

example.

• Rope pulls you up and gravity pulls you down

• Tension in rope equals your weight

• What if you are hanging from two ropes? What will the tension of each rope be?

• 1/2 your weight

• When you step on a bathroom scale, the downward force supplied by your feet and the upward force supplied by the floor compress a calibrated spring. The compression of the spring gives your weight. In effect, the scale measures the floor’s support force.

Check Question

• If you weigh 50 kg, what will each scale read if you stand on two scales with your weight equally divided between them? What happens if you stand with more of your weight on one foot than the other?

Vertical vs. Horizontal lines• Vertical lines can

support your weight much better than a horizontal line.

• Why?• Tension in horizontal

line is much greater• Look at figure 4.15 in

text

Vertical vs. Horizontal Lines• For any pair of ropes, scales, or wires supporting a

load, the greater their angle from the vertical, the larger the tension force is.

• What does this mean?• The more horizontal a line is, the less it can support.• A can support more than B• Demo with chain and weights (p. 53) (birds on a wire)

AB

The Moving Earth

• The earth moves around the sun at 30 km/s. If this is so, than how can the bird catch the worm?

• Everything is moving at 30 km/s; the bird, tree, air, worm, grass, etc!

The Moving Earth

• If the plane is traveling at a high speed, then how can we flip a coin in the air and catch it? Shouldn’t it fly over our head at the speed of the plane?

Review Questions

1. Your empty hand is not harmed if it bangs lightly against a wall, but is harmed if it bangs against the wall while carrying a heavy load. Why?

A: A heavy load has a lot of mass, and once in motion has a lot of tendency to remain in motion. More force is required to stop the mass so your unfortunate hand is squashed.

Review Questions

2. Does a person diet to lose mass or to lose weight?

A: A person diets to lose mass. One loses weight whenever gravity is reduced, like being on the surface of the moon. Even with less weight on the moon, an obese person is still obese.

Review Questions

3. How does a car headrest help to guard against whiplash in a rear-end collision?

A: In a rear end collision your head tends to stay put while your body is pushed forward by the seat. Relative to your body, your head “whips” back. The headrest extends the seat up to your head and keeps your head and body together.

Review Questions

4. A car at a junk yard is compressed until its volume is less than 1 cubic meter. Has its mass changed? Has its weight changed?

A: Neither its mass nor its weight has changes when it has been compressed, because the same quantity of matter is present. Only its volume is less.

Worksheet

• Let’s give worksheet 4-2 a try.