# bell ringer - what a force is? 2. ... what is a balanced force? 4. what is an unbalanced force?...

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• Bell Ringer:

Who was Isaac Newton and what did he do?

Who was Galileo Galilei and why is famous?

What is a force?

What do Newtons three laws pertain to?

• 4.1 Newtonian Mechanics: ForcesNewtons 1st & 2nd Law

Physics Honors I

• Objectives:

Define Force.

Understand the difference between balanced and unbalanced forces

Explain the meaning of Newtons 1st Law.

Apply Newtons 2nd Law to solve problems.

Describe how the weight and mass of an object are related.

• Vocabulary:

Force Net Force

Inertia Equilibrium

Newtons 1st Law Newtons 2nd Law

Free-Body Diagram System

• Active Physics Reference:

Chapter 2 - Section 1: p. 132- 153 (1st Newton's Law) Chapter 3 Section 2: p. 268 - 272

Chapter 2 - Section 3: p. 157 - 240 (2nd Newton's Law) Chapter 3 - Section 4: p. 294 - 297 (2nd Newton's Law)

Chapter 2 - Section 6: p. 198 (3rd Newton's Law)

• Further Learning:

Physics: Principles and Problems - the red book we have in class

Chapter 4 - Newton's LawsChapter 5 - Friction and 2-Dimensional Forces

(3) Physics Classroom:

Newton's Laws

Newton's Laws Balanced vs Unbalanced Forces

• What are Forces?

• What do we see forces being used?

Newtons 1st Law (Biology): Animals with large body mass are generally unable to change direction quickly when in motion. Smaller animals can often elude their larger predators by making sharp turns while moving quickly.

Newtons 2nd Law (Biology): A flea is able to exert tremendous force for its size, allowing it to jump up to 13 in. (33 cm), or 200 times the length of its body.

Newtons 3rd Law (Earth Science): When a hurricane strikes land, the wind exerts tremendous force on topographical features and objects on the land. The reaction force of these objects on the moving air causes it to slow down, which is why hurricanes eventually dissipate as they move over land.

• What is a Force?

A force is any type of push or pull.

Contact Forces: When we physically make contact with an object to push or pull it a certain direction.

Non-Contact Forces: Gravitational Force, Magnetic Force, Electric Force.

SI Unit for a force is a Newton.

1 N = 1 kg m

s2

• What is a Force? (Continued)

A net force causes an acceleration, which is a change in velocity.

Whenever an object in motion changes direction, speeds up, slows down, or stops, it

does so because a force is acting on it.

A FORCE CAUSES AN ACCELERATION.

A force is measured by the acceleration it produces.

There may be many forces acting on a body, but if their net force is zero, the body

cannot acceleration.

• Inertial Reference Frame:

An inertial reference frame is one in which Newtons laws hold.

A non-inertial reference frame is one in which Newtons laws do not hold.

• Inertial Reference Frame:

For example, we can assume that the ground is an inertial frame provided we can neglect Earths astronomical motions (such as its rotation).

Basically, we more or less imagine that your frame of reference isnt moving. We are sitting in a classroom now and imagine the classroom is still. When in truth our planet is hurling around the Sun at a speed of 30 km/s (18.5 miles/s), while our Solar System is rotating around the center of the Milky Way galaxy at 486,000 mph. So we basically, we dont have to account for the spin of the planet, the rotation of the planet around the Sun, or the rotation of the Sun around the Milky Way galaxy to do most physics problems.

• Newtons Laws of Motion:

Newtons 1st Law of Motion:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm#balanced

• Newtons Laws of Motion:

Newtons 2nd Law of Motion:

An acceleration is produced when a force acts on a mass.

Or a force is produced whenever a mass is accelerated.

• Newtons Laws of Motion:

Newtons 3rd Law of Motion:

When two bodies interact, the forces on the bodies from each other are always equal in magnitude but opposite in direction.

• Comprehension Check 1:

1. Explain what a force is?

2. Explain what an inertial reference frame is?

3. What is the SI Unit for Force?

4. Is a force a vector or a scalar? Why is that important?

• Newtons 1st Law:

Newtons 1st Law of Motion:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

What does this mean?

http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm#balanced

• Newtons 1st Law - An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

THREE DISTINCT PARTS TO NEWTONS FIRST LAW:

Part 1: If an object is at rest, it stays at rest.

Part 2: If a body is moving, it continues to move with the same velocity (same

magnitude and direction). That is, objects in motion stay in motion in a straight line

with constant speed.

Part 3: The first two parts are only true when the net force on the object is zero. An

object may have forces acting on it and still have no change in its motion. (Net force

is the sum of all forces acting on an object).

http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm#balanced

• Newtons 1st Law Balanced Forces vs Unbalanced Forces

PHET Stimulation:

PHET - Forces and Motion - Acceleration

Go to: Acceleration

Try it: With and Without Friction. What happens to the box?

Does it take longer to get to maximum speed if it weighs more?

• Newtons 1st Law Balanced Forces vs Unbalanced Forces

PHET Stimulation:

PHET - Tug-O-War

Go to: Net Force Tab (The tug of War)

Try it: What happens when you put a small blue guy on one end?

Now put a small red guy on the opposite end. Why does blue team still win?

• Newtons 1st Law Balanced Forces vs Unbalanced Forces

From the stimulations we see that forces can be summed together to find the net force on the object.

= 1 + 2 + 3 + +

If = 0 , the forces are balanced.

If 0 , the forces are unbalanced.

• Newtons 1st Law - An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Newtons first law is sometimes called the law of inertia.

Is inertia a force?

No. Inertia is the tendency of an object to resist change.

Forces are results of interactions between two objects; they are not properties of single

objects, so inertia cannot be a force.

http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm#balanced

• Newtons 1st Law - An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Inertia is a measure of mass The more mass an object has the more inertia,

without respect to what the object is composed of.

Mass is an intrinsic characteristic of a body that is, a characteristic that

automatically comes with the existence of a body. The SI unit for mass is kg.

Equal amounts of mass imply equal amounts of inertia.

Inertia is a measure of the objects mass only. Size, shape, volume, and speed do not

determine its inertia.

http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm#balanced

• Newtons 1st Law - An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Example:

You are standing still on a train. What happens when the train starts?

You get jerked back because an object at rest wants to stay at rest. That is your inertia

at play.

Now the train is coming to a stop. What happens to your body as your standing there?

Your body wants to keep moving forward. That is your inertia again, because an

object in motion desires to stay in motion until acted on by an unbalanced force.

http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm#balanced

• Comprehension Check 2:

1. Why is Newtons first law sometimes called the law of inertia?

2. Is inertia a force?

3. What is a balanced force?

4. What is an unbalanced force?

• Newtons 2nd Law The net force on a body is equal to the product of a bodys mass and its acceleration.

So this is the biggest Newtonian Law. We can represent it in equation form as:

= = force (a vector)

m = mass

a = acceleration.

RECALL: SI Unit is 1 N or 1 kg times m s2

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