Newton’s Laws of Motion

A scientific law is something that will happen in the physical world each and every time that conditions allow for it.◦ Gravity◦ Conservation of momentum◦ Newton’s laws

1st Law of Motion (Law of Inertia)

An object in motion or at rest will stay in constant motion or at rest forever and ever unless an outside force acts on it.

1st Law

The moon will keep revolving around the earth forever, unless acted on by an unbalanced force.

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force?

• Moon in orbit around earth

1st Law Once airborne, unless acted on by an unbalanced force, it would never stop!

1st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

There are four main types of friction:◦Sliding friction: friction between a sliding

object and the surface it is sliding on.◦Rolling friction: friction between a rolling

object and the surface it is rolling on.◦Static friction: friction that keeps a

stationary object in place ◦Fluid friction (air or liquid): air or water

resistance

What is this unbalanced force that acts on an object in motion?

Sliding◦ +◦ -

Rolling◦ +◦ -

Static◦ +◦ -

Fluid◦ Air

+ -

◦ Water + -

HomeworkOne positive and one negative example of each friction.

Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force.

What force? Sliding Friction In the absence of a force

of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.)

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.The Law of Inertia states that an object in motion stays in motion until acted on by an outside force. Let’s make that outside force the seatbelt and not the windshield or pavement!

Newton’s 1st

Law in Action.An person will remain in motion until acted upon by an outside force!

Can you identify the outside forces?

2nd Law

An object will accelerate in the direction of the greatest force acting on it.

The total force of an object is equal to its mass times its acceleration.

2nd Law

F = m x a◦ Mass in kilograms◦ Acceleration in m/s2 ◦ Force is kg x m/s2 or Newtons

One Newton is equal to the force required to accelerate one kilogram of mass at one m/s2 .

2nd Law

How much force is needed to accelerate a 1400 Kilogram car 2 m/s2 ?

Write the formula F = m x a Fill in given numbers and units F = 1400 kg x 2 m/s2 Solve for the unknown 2800 kg X m/s2 or 2800 N

2nd Law (F=ma)

1. How much force is needed to accelerate a 66-kg skier 1 m/s2 ?

2. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/s2 ?

3. What acceleration will result when a 12 N net force applied to a 3-kg object?

4. A net force of 16 N causes a poodle to accelerate at a rate of 5 m/s2. Determine the mass of the poodle.

  

Check Your Understanding

1. How much force is needed to accelerate a 66 kg skier 1 m/s2 ?

66 N = 66 kg x 1 m/s2 2. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2 ?

9800 N = 1000 kg x 9.8 m/s2

3. What acceleration will result when a 12 N net force applied to a 3 kg object? 12 N = 3 kg X 4 m/s2

  4. A net force of 16 N causes a poodle to accelerate at a rate of 5 m/s2.

Determine the poodle’s mass. 16 N = 3.2 kg x 5 m/s2

 

Check Your Understanding

The fastest speed/velocity a falling object can go.

This occurs when downward gravity is balanced by upward air resistance.

If enough time exists, all falling objects eventually reach a constant speed.

This is the only time on earth that a moving object has balanced forces acting on it.

Terminal Velocity

Gravity = 9.8m/s2

Terminal Velocity of Feather

Air Resistance

Terminal Velocity of Ball

Questions:

What is the only difference between the two objects?

Which object will accelerate for a longer period of time?

Which object will have the greatest terminal velocity? Why?

Occurs when objects move in a circular/orbital path.

The force that keeps these objects moving in that path.

These objects always have an inertia that is perpendicular to the orbit.

Example: The Moon and Earth.◦ What would happen if we had a switch to flip and

turn off Earth’s gravity?

Centripetal Force

Where would it go?

How much force does a 2000 kg meteor hit the earth with?

What is the acceleration of a 55 kg swimmer jumping off a cliff at Raystown?

On Earth, when dropped, why does a bowling ball hit the ground before a feather?

On the moon, when dropped, which one will hit the lunar surface first and why?

http://www.youtube.com/watch?v=5C5_dOEyAfk

Challenge Questions.

3rd LawFor every action, there is an equal force and opposite direction reaction.

Draw a diagram for each of the following slides.

3rd Law The action force is the

arrow pushing backward on the string.

The reaction force the string pushing forward on the arrow.

The mass of the bow + Indian and the acceleration of the string is transferred to the mass of the arrow giving it acceleration.

3rd Law Projectile Motion The action force is the

mass of the bullet accelerating forward.

The reaction force is the mass of the gun accelerating backward.

Therefore, guns kick! The bigger and faster the

bullet, the more kick. The lighter the gun, the

more it kicks.

Pitching a baseball/softball.

The action force is the pitcher pushing backward on the mound.

The reaction is the mound pushing the pitcher toward home plate!

3rd Law

Other examples of Newton’s Third Law The baseball forces

the bat to the left (an action); the bat forces the ball to the right (the reaction).

Newton’s 3rd Law in Nature Consider the propulsion of a

fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

3rd Law

Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

3rd Law Consider the motion

of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

3rd LawThe reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.