Chapter 8 Section 4 Chapter 8 Section 4 NotesNotes

Newton’s Laws of Motion

Newton’s Laws Newton’s Laws of Motionof MotionNewton’s First Law: An object at rest

stays at rest and an object in motion stays in motion unless acted upon by an outside force.

In other words, the net force acting on the object is zero.

Example: A book sliding on any surface will eventually come to a stop due to friction. If it weren’t for friction, the book would continue to slide.

Newton’s First LawNewton’s First LawInertia: the

tendency of an object at rest to remain at rest OR of an object moving at a constant velocity to continue moving at that speed.

Example of Newton’s First LawExample of Newton’s First LawFamous Tablecloth

Experiment◦ Try It!

Why does it work?◦The law of inertia

says if we pull the tablecloth out really fast, it won’t be able to pull the dishes for long enough to get them moving. The inertia of the dishes wants to keep them standing still.

Newton’s Second LawNewton’s Second LawNewton’s second law states that: the

unbalanced force acting on an object equals mass times acceleration

Mathematically, force = mass x acceleration, or, F = ma.

Newton’s Second LawNewton’s Second Law

Newton’s Second Law: describes the relationship between 3 factors:◦ mass◦ force◦ acceleration

Newton’s Second LawNewton’s Second Law Example: Consider 2

shopping carts: one empty and one full of groceries. If you push both carts with the same force, the one without the groceries will have a higher acceleration because it has less mass. Since the masses are different, the accelerations are different.

If the masses are the same, a larger force gives a larger acceleration

The SI unit for force is: Newton,

Symbolized by: N◦ 1 Newton = 0.225 lbs◦ 1 pound = 4.448 N

Newton’s Second LawNewton’s Second Law

Sample Problem: Newton’s Sample Problem: Newton’s Second Law of Motion:Second Law of Motion:What is the net force necessary for a 1600

kg automobile to accelerate forward at 2.0 m/s2?

Solution:

GravityGravityRemember that gravity depends on 2 factors:

◦Mass: the larger the mass, the larger the gravitational pull; direct relationship

◦Distance: the larger the distance, the smaller the gravitational force; indirect relationship

Think about an apple that falls from a tree. In terms of gravity, explain why the apple falls toward the earth and not toward the tree.◦The gravitational force is much greater between the

apple and Earth than the apple and tree.

Free Fall and Weight Free Fall and Weight Free fall: the motion

of an object when gravity is the only force acting on it

Free-fall acceleration: directed toward the center of the Earth

Abbreviation for free fall acceleration: g

On or near Earth’s surface, acceleration due to gravity is 9.8 m/s2

Free FallFree FallAstronauts in space appear to be

“weightless”. This statement is NOT true because gravity exists everywhere in the universe; it is the force of attraction between 2 objects due to mass.

Astronauts in orbit experience apparent weightlessness because they are in free fall. The astronauts and vehicle are falling toward Earth with the same acceleration.

Free Fall and WeightFree Fall and WeightIn the absence of air resistance, all

objects falling near Earth’s surface accelerate at the same rate regardless of their mass.

Why do all objects have the same free-fall acceleration?◦Newton’s second law states acceleration depends on mass and force. A heavy object has a greater gravitational force, but it is harder to accelerate because the heavy object has more mass.

Acceleration due to gravityAcceleration due to gravity Acceleration due to gravity changes

depending on where you are: ◦ Earth: 9.8 m/s2

◦ Mars: 3.7 m/s2

◦ Venus: 8.8 m/s2

◦ Neptune: 11.8 m/s2

Example: Suppose a person has a mass of 50 kg. What would be their weight on Earth? What about the moon?

Terminal VelocityTerminal Velocity 2 factors are in

affect when an object moves through the atmosphere: air resistance and gravity.

Terminal Velocity: maximum velocity reached by a falling object; occurs when air resistance = force of gravity

Mass and WeightMass and Weight Weight: force on an object due to

gravity◦ Weight equals mass x free fall

acceleration ◦ Equation: w = mg◦ SI unit for weight: Newton

Mass and weight aren’t the same thing:◦ Mass: measure of the amount of

matter in an object◦ Weight: gravitational force an object

experiences because of its mass

Weight and shape of objectsWeight and shape of objects Weight influences

the shape of objects:

On land: animals have strong skeletons to support themselves against gravity

In water: some animals don’t have skeletal systems because the downward force of gravity is balanced by the upward force of water.

Newton’s Third LawNewton’s Third Law Newton’s Third Law:

For every action force, there is an equal and opposite reaction force.

This law of Newton’s implies that forces always occur in pairs, but the forces act on different objects and occur at the same time.

Newton’s Third Law is used in rocketry.

Question #1Question #1While driving down the

road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

Answer #1Answer #1Trick Question! Each

force is the same size. For every action, there is an equalequal and opposite reaction. The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction.

Question #2Question #2 Many people are familiar

with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...

a. greater than the acceleration of the bullet.

b. smaller than the acceleration of the bullet.

c. the same size as the acceleration of the bullet.

Answer #2Answer #2 B. The force on the rifle

equals the force on the bullet. Yet, acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass. Remember: acceleration and mass are inversely proportional.

Newton’s First LawNewton’s First LawUse Newton’s First

Law to describe the motion of the skateboarder and the motion of the person driving the car.

Newton’s Second LawNewton’s Second Law Mike's car, which weighs

1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. Using Newton's Second Law, how much force is Mike applying to the car?

Newton’s Third LawNewton’s Third LawHow does a rocket

work?◦The rocket's action

is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.