Newtons first law

By Randy HedlundSouth High School

9/231. Think of a time when you got in an accident

(bike, running, car). Briefly describe with your neighbor what happened to the motion of your body. (2 min)

• 2. What do you think causes a change in motion?

• 3. Does force keep objects moving?

Forces cause changes in motion.

Seems obvious right? Things that are obvious now were not always.

Learning Target: I can apply Newton's first law of inertia to a situation I have experienced.

Natural motion on Earth was thought to be either straight up or straight down.

Objects seek their natural resting places: boulders on the ground and smoke high in the air like the clouds.

Heavy things fall and very light things rise.

Circular motion was natural for the heavens.

These motions were considered natural–not caused by forces.

3.1 Aristotle on Motion

Nicolaus Copernicus proposed that Earth moved around the sun.

Controversial ideas

“Earth is not the center of the solar system”

Gravity is a force that keeps planets circulation

3.2 Copernicus and the Moving Earth

Galileo, the foremost scientist of late-Renaissance Italy, was outspoken in his support of Copernicus.

-One of Galileo’s great contributions to physics was demolishing the notion that a force is necessary to keep an object moving.

Biggest misconception – a force is required to keep an object in motion.

3.3 Galileo on Motion

• How was Galileo’s views on motion different from Aristotle?

NotesNewton’s first law – an object in motion will stay in motion unless acted on by an unbalanced force.

An object at rest will stay at rest unless acted on by an unbalanced force

Inertia – an objects resistance to change in motion

Friction – _____

Friction is the name given to the force that acts between materials that touch as they move past each other.

Friction is caused by the irregularities in the surfaces of objects that are touching.

Even very smooth surfaces have microscopic irregularities that obstruct motion.

Friction - force that opposes motion caused by surface irregularities

What would happen in you pushed something that had no friction?

<iframe src="http://phet.colorado.edu/sims/html/friction/latest/friction_en.html" width="800" height="600" allowfullscreen></iframe>

3.3 Galileo on Motion

• friction What does it look like?

3.4 Newton’s Law of Inertia

3.4 Newton’s Law of Inertia

Water cup race

Observations:

When did the least amount of water seem to spill?When did the most water seem to spill?What can we say about the waters inertia?

Observations: When did the least amount of water seem to spill? When the students were moving at a constant speed the water did not spill.

When did the most water seem to spill?When the students where changing speed or accelerating. Also when the students were changing direction.

What can we say about the waters inertia?When the water was still, it had a tendency to remain still because when the student moved the cup, some water spilled because it had rest inertia. When the cup slowed down the water had a tendency to stay moving so it spilled over the cup. When the cup slowed down the water kept moving.

Welcome to inertia cafe

• 2. Inertia cafe• Why is it possible to pull a cloth from under a

plate?

• Why does this only work with a smooth cloth?

• 3. Balloon walk• Procedure – hold the balloon still then start walking,

the turn a corner. Keep walking then stop abruptly • What happens to your balloon if you stand still then start

walking? • What happens to your balloon if you are walking and you

turn?

• What happens to your balloon if you are walking and you stop?

• 4. Coin card trick-• a. Is it possible to remove the card without

moving the coin?

• b. Would this trick be easier or harder with a dime? Why? (Hint which has more inertia?

• c. How does this relate to Newton’s first law?

• Coin trick

------------exit slip (last 5 minutes)------------• On exit slip rate your explanation and

understanding of Newton’s first law (1-5)

9/24

1. Why does the first law say that an unbalanced force is needed to change motion?

• 2. What are all the forces acting on the stapler?

• Net force – the combinations of all forces on an object

• Vector – arrow that shows magnitude and direction, examples velocity, force

• Mechanical equilibrium – zero net force

• A mechanical equilibrium is a state in which a momentum coordinate of a particle, rigid body, or dynamical system is conserved. Usually this refers to linear momentum.

All the forces in the universe come from

Gravity - force between any two objects with mass Looking ahead f=Gm*m /d2

Strong nuclear force - holds atoms together, short range

Electromagnetic force - force between positive and negative particles

Weak force - caused by some types of radioactivity

3/4

• 1. If I push hard on a wall what are all the forces acting on the wall?

• 2. Create a drawing showing all the vectors that are happening in your “smart rope” set up holding a 5 N weight

• 3. If gravity is acting on all the objects in this room why aren’t they moving?

Free-body diagrams

Free-body diagrams are pictures that show the size and direction of all forces acting on an object.

• Gravity – f=Gm*m /d2

• Normal force – perpendicular to the floor

• Friction – the force that opposes motion• Drag – air friction

• F = G M1M2 / D2

• What is the average gravitational force between the moon and the earth?

• Mass of earth = 6.0x1024kg• Mass of moon = 7.0 x1022kg• average earth moon distance = 4.0 x108 m

• G = 6.66*10-11

Problem 1

A book is at rest on a table top. Diagram the forces acting on the book.

Problem 1

In this diagram, there are normal and gravitational forces on the book.

Problem 1

The forces are balanced (they cancel each other out)

Problem 2

An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

Problem 2

Gravity is the only force acting on the egg as it falls.

Problem 2

The forces are unbalanced, so the egg will accelerate downward.

Problem 3

A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this squirrel looks like…

Problem 3

Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.

Problem 4

A rightward force is applied to a book at rest, in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram for the book.

Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there are still gravity and normal forces involved.

Problem 5

A skydiver is falling with a constant velocity. Consider air resistance. Draw a free-body diagram for the skydiver.

Gravity pulls down on the skydiver, while air resistance pushes up as she falls.

Problem 6

A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram of the forces acting on the sled.

The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Since there is not information that we are in a blizzard, normal forces still apply as does gravitational force since we are on planet Earth.

Problem 7

A football is moving upwards toward its peak after having been booted by the punter. Neglect air resistance. Draw a free-body diagram of the football in mid-air.

The force of gravity is the only force described. It is not a windy day (no air resistance).

Problem 7

A car runs out of gas and coasts to a stop on flat ground. Draw a free body diagram of the forces acting on the car.

Quick response 1,2,3

Which force is acting on the football

A. The force of the kick onlyB the force of gravity onlyC. The force of the kick and gravity

Even though the car is coasting down the hill, there is still the dragging friction of the road (left pointing arrow) as well as gravity and normal forces.

9/25

• Construct a Free Body Diagram of an orangutan and a sumo wrestler in a tug of war match. (your drawing should predict the winner)

• Why does one team always fall over when the other team lets go of the tug of war rope?

Super moon at 10:47 p.m Sunday night

Find the net force

• Quick review – you drop your keys from a tall building, how fast will they be going after 3 seconds

• https://www.youtube.com/watch?v=YFMpWm6ECgQ

3/5

1. Construct a FBD of a car going at a constant speed

2. A 200 kg filing cabinet experiences a friction force of -300 newtons and an applied force of 500N.a. Construct a free-body diagram of this situationb. Is the cabinet going to move, explain why or why

not.

Agenda: FBD praticeFinish smart ropes