Honors Physics - Chapter 4: Lesson 3

Chapter 4: Lesson 3

Newton's Laws of Motion

Chapter 4: Lesson 3 - Newton's Laws of Motion

Newton's First Law of Motion

This is sometimes referred to as the law of inertia. Inertia is the reluctance of any body to change its state of motion. Mass is the measure of inertia.

All About Inertia

Watch my video to hear a story about inertia and see a demonstration.

Newton's First Law of Motion

Newton's First Law of Motion states that an object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

He bascially says that if something is moving, it will continue to move and if something is stopped it will stay stopped unless you apply an outside force on the object.

Lets say that the car and the ladder were traveling at 40 mph. The car suddenly stops, but the ladder keeps going at 40 mph. Newton's first law in action!

There are many examples of Newton's First Law of Motion. Here are a few. Click on each video to watch the short demonstrations.

Why you should wear a seatbelt!

Newton's first law is a perfect example of why you should ALWAYS wear a seatbelt!

The tablecloth trick.

My version of the tablecloth trick!

A braver teacher than I am...

Newton's Second Law of Motion

Newton's Second Law of motion can be written in both words and with an equation.

Newton's Second Law of Motion states that the acceleration of a body is directly proportional to the net force on it and inversely proportional to its mass.

When we say something is directly proportional to something else, we mean that the same thing happens to both quantities. If A is doubled, then B doubles as well. If A is cut in half, then B is cut in half too.

If something is inversely proportional to something else, then the opposite thing happens to both quantities. If A doubles, then C is cut in half. If A triples, then C in reduced by 1/3. If A is reduced by 1/3, then the C quantity triples in value.

In equation form, Newton's Second Law can be written as a=F/m, or more commonly seen: F=ma. F is the abbreviation for force, m is the abbreviation for mass, and a is the abbreviation for acceleration.

To have a force, you HAVE to have an acceleration. Look at the equation, F=ma. If you are talking about an object, it will always have a mass, so if the acceleration is 0 m/s2 then zero times anything is zero, so you would not have a force. Force and acceleration always point in the same direction.

The difference between mass and weight

Mass and weight are sometimes used interchangeably, but they are two VERY different things.

MASS, m: Mass is what makes you up. Your mass does not change if you travel to the moon or any other planet. The only way your mass can change is if you consume more calories than you burn during the day, or if you burn more calories than you consume. Mass in measured in kilograms (kg.)

WEIGHT, Fgrav or FW: Weight or Gravitational Force, is mass multiplied by the gravitational acceleration where the object is. Since the gravitation acceleration on Earth (9.8 m/s2 ) is different than the moon (1.62 m/s2 ) then your weight, or gravitational force would be different. If you remember from Lesson 2 about free body diagrams, then you know that weight always points STRAIGHT DOWN towards the center of the Earth.

Fgrav=FW=mg

This is an important equation to remember when doing the math in this chapter. If a math problem has a mass and you need a force, then you will have to multiply the mass by g. If you have a force and need to use a mass, then you will have to take the force and divide it by g. We will be using this equation throughout the entire year. If you notice, it is just a version of Newton's Second Law in equation form.

Thinking about Newton's Second Law of Motion

When faced with a question about Newton's Second Law of Motion, just relate that question to a time when you have either pushed something really heavy or really light.

When pushing something really heavy, you know that you have to push really hard, a large force. So, more mass requires more force. When pushing something light, you don't have to push as hard, a smaller force. So, less mass takes less of a force to move it.

You can also think about how fast you can change the speed of the cart, the acceleration. When the cart is very heavy, you have to push really hard and it takes a long time to get the cart moving, (a small acceleration.) So, if you have a large mass, then your acceleration is smaller. When the cart is really light, you don't have to push as hard, and the cart moves easily. The acceleration is much larger. So, when you have less mass, your acceleration is larger.

Using F=ma

If an object is moving at a constant velocity, there is no acceleration because it it not speeding up or slowing down.

Looking at the equation F=ma, when there is no acceleration, there is no net force. If a=0 m/s2 , then mass times 0 will also equal 0. Your net force is 0 N.

Newton's Third Law of Motion

Newton's third law of motion states that, "for every action, there is an equal and opposite reaction." This just means that if you are touching something, that object is also touching you.

Watch the video for a quick demonstration on Newton's Third Law!

Action-Reaction Forces

Identifying and describing action-reaction force pairs is a simple matter of identifying the two interacting objects and making two statement describing who is pushing on who and in what direction.

Consider the interaction between a baseball bat and a baseball. The baseball forces the bat to the left. This is the action. The bat forces the baseball to the right. This is the reaction.

When writing action-reaction statements, the nouns in the sentence switch places and the direction is reversed.

Check Your Understanding:

An action force is stated. Can you determine the reaction statement for the three questions below? Click on the down arrow when you have your answer to check to see if you are correct.

Action statement: An athlete pushes the bar up. What is the reaction statement?

Reaction statement: The bar pushes the athlete down. The two nouns (athlete and bar) in the action statement are switched and the direction is reversed.

2. Action statement: The bowling ball pushes the pin to the left. What is the reaction statement?

Reaction statement: The pin pushes the bowling ball to the right. The two nouns (pin and bowling ball) in the action statement are switched and the direction is reversed.

3. Action statement: The balloon wall pushes the compressed air inwards. What is the reaction statement?

Reaction statement: The compressed air pushes the balloon wall outwards. The two nouns (compressed air and balloon wall) in the action statement are switched and the direction is reversed.

Check Your Understanding:

Think about the four questions below and click on the down arrow when you have your answer to check to see if you are correct.

While driving down the road, Anna Litical observed a bug striking the windshield of her car, quite obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

Trick Question! Each force is the same size. Remember – for every action there is an equal and opposite reaction. The fact that the bug splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

More explanation on the bug and the car!

Click on the video to hear a more detailed explanation on the bug and the car. Which would have the bigger acceleration?

2. Rockets are unable to accelerate in space because...

a. There is no air in space for the rockets to push off of.

b. There is no gravity in space.

c. There is no air resistance in space.

d. Nonsense! Rockets do accelerate in space.

WATCH THE VIDEO BEFORE CLICKING THE ANSWER!

It is a common misconception that rockets are unable to accelerate in space. The fact is that rockets do accelerate. Rockets are able to accelerate due to the fact that they burn fuel and push the exhaust gases in a direction opposite the direction that they wish to accelerate, the same way the air in the balloon pushed the balloon in the opposite direction.

3. In the top picture, a physics student is pulling upon a rope that is attached to a wall. In the bottom picture, the physics student is pulling upon a rope that is held by an elephant. In each case, the force scale reads 500 Newtons. The physics student is pulling…

a With more force when the rope is attached to the wall.

b. With more force when the rope is attached to the elephant.

c. With the same force in each case.

C – The student is pulling with 500 Newtons of force in each case. The rope transmits the force from the physics student to the wall (or to the elephant) and vice versa. Since the force of the student pulling on the wall and the wall pulling on the student are action – reaction force pairs, they must have equal magnitudes. Inanimate objects such as walls can push or pull.

4. How do you win a tug-o-war game when both teams are pulling in equal and opposite directions?

Tension on the rope is the same throughout the entire rope, so the tension on the rope is the same when playing tug-o-war. The team that pushes off the floor the hardest wins! You push on the earth and the earth pushes back at you. We pull on the rope to make the other team not be able to push off the ground. When playing tug-o-war, it is important to wear shoes that have a lot of friction on them to allow you to push off the ground.

When choosing a tug-o-war team, rowers are your best choice! They use the exact muscle groups in rowing as you would in tug-o-war!

You have finished Lesson 3!

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