FORCES AND MOTIONChapter 12

Page 356-362

12.1 FORCES

MAY THE FORCE BE WITH YOU YODA

Can cause a resting object to move. It can accelerate a moving object by

changing the object’s speed or direction.

SI unit = N (for newton)

NEWTON (NOT THE COOKIE)

One N is the force that can cause a 1-kg mass to accelerate at a rate of 1 m/s2

N = 1kg x m/s2

BALANCED FORCES

Has a net force of 0 No change in

object’s motion.

IE: Two Sumo wrestlers of equal weight. Locked head to head and neither one is moving.

UNBALANCED FORCES

Net force acting on an object is not equal to 0

Object is put into motion and accelerates.

IE: Two sumo wrestlers not equal in weight.

WHAT THE FRIC….

3 types Static Sliding Rolling

tion

These 2 symbols mean just listen and look no need to take notes as it will not be tested or it will be shown in the lecture at a later time.

STATIC FRICTION

Force that acts on an objects that are NOT moving.

Acts in the direction opposite to that of the applied force.

IE: You pushing on a world’s largest watermelon that weight 2 tons.

SLIDING FRICTION

Force that opposes the direction of motion of an object as it slides over a surface.

IE: You pushing on a really big box of watermelons.

ROLLING FRICTION

Force that acts on rolling objects.

Object is bent slightly out of shape.

IE: You pushing a shopping cart full of watermelons….or a little kid you want to terrorize.

GRAVITY

Force that acts between any 2 masses.

Attractive force that pulls objects together.

Can act over large distances.

LET’S QUIZ YOUR LEARNING

WHAT AMOUNT OF FORCE ACCELERATES A 1-KILOGRAM MASS AT 1 M/S2?

1. 0.1 newton 2. 1 newton 3. 10 newtons 4. 100 newtons

WHAT AMOUNT OF FORCE ACCELERATES A 1-KILOGRAM MASS AT 1 M/S2?

1. 0.1 newton

2. 1 newton 3. 10 newtons 4. 100 newtons

WHAT IS THE NET FORCE ACTING ON THE ROPE?

1. The answer depends upon the mass of the two groups of people.

2. The answer depends upon the mass of the rope.

3. The answer depends upon the length of the rope.

4. zero

WHAT IS THE NET FORCE ACTING ON THE ROPE?

1. The answer depends upon the mass of the two groups of people.

2. The answer depends upon the mass of the rope.

3. The answer depends upon the length of the rope.

4. zero

WHAT IS THE NET FORCE OF A PAIR OF BALANCED FORCES?

1. the sum of the forces

2. the product of the forces

3. zero 4. 1 newton

WHAT IS THE NET FORCE OF A PAIR OF BALANCED FORCES?

1. the sum of the forces

2. the product of the forces

3. zero 4. 1 newton

BECAUSE THE BOULDER IS AT REST, WHAT DO YOU KNOW ABOUT THE NET FORCE ACTING ON IT?

1. The net force is positive.

2. The net force is negative.

3. The net force is decreasing.

4. The net force is zero.

BECAUSE THE BOULDER IS AT REST, WHAT DO YOU KNOW ABOUT THE NET FORCE ACTING ON IT?

1. The net force is positive.

2. The net force is negative.

3. The net force is decreasing.

4. The net force is zero.

Figure

Is Net Force

0?Effect on Motion

Plant

Scale

Rope

Plant 2

CAN YOU ANSWER THESE QUESTIONS?

PLANT 1

Figure

Is Net Force

0?Effect on Motion

Plant Yes none

Scale

Rope

Plant 2

SCALE

Figure

Is Net Force

0?Effect on Motion

Plant Yes none

Scale Yes none

Rope

Plant 2

ROPE

Figure

Is Net Force

0?Effect on Motion

Plant Yes none

Scale Yes none

Rope Yes none

Plant 2

PLANT 2

Figure

Is Net Force

0?Effect on Motion

Plant Yes none

Scale Yes none

Rope Yes none

Plant 2 No Potted plant accelerates

Page 356-362

END OF - - - 12.1 FORCES

Page 363-369

12.2 NEWTON’S 1ST AND 2ND LAW OF MOTION

NEWTON'S FIRST LAW OF MOTION

The state of motion of an object does not change as long as the net force acting on the object is zero

VOCABULARY

inertiathe tendency of an object to resist a change in its motion.

In other words, an object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed.

View 1:52 clip

. NEWTON'S SECOND LAW OF MOTION

The acceleration of an object is equal to the net force acting on it divided by the object's mass

Video clip to “get it” run time 1:45

NEWTON'S SECOND LAW

  An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car's acceleration?What information are you given?M= 1000kgF= 4000N (in the forward direction)A = ?

USE FORMULA AND REPLACE

What information are you given?M= 1000kgF= 4000N (in the forward direction)a = Net force then, 4000N = 4N = 4m/s2

Mass 1000kg kg

WEIGHT AND MASS

Weight (W) is substituted for force (F) Acceleration due to gravity (g) is substituted for acceleration (a). In other words, W = mg is a different form of

So when the equation is solved for force, F = ma. The value of g in the formula is 9.8 m/s2.

WHICH OF THESE ASTRONAUTS “WEIGHS” MORE?

AN ASTRONAUT WITH A MASS OF 88 KG WEIGHS 863 N ON EARTH.

AN ASTRONAUT WITH A MASS OF 88 KG WEIGHS 141 N ON THE MOON.

THEY ARE THE SAME!

REMEMBER: WEIGHT IS A MEASURE OF THE FORCE OF GRAVITY ACTING ON AN OBJECT!!

AN ASTRONAUT WITH A MASS OF 88 KG WEIGHS 863 N ON EARTH.

AN ASTRONAUT WITH A MASS OF 88 KG WEIGHS 141 N ON THE MOON.

LET’S QUIZ YOUR LEARNING

HOW DOES A ZERO NET FORCE AFFECT AN OBJECT'S MOTION?

1. The velocity increases. 2. The velocity decreases. 3. The direction changes. 4. The motion does not change.

HOW DOES A ZERO NET FORCE AFFECT AN OBJECT'S MOTION?

1. The velocity increases. 2. The velocity decreases. 3. The direction changes.

4. The motion does not change.

WHICH OBJECTS AND DEVICES ABSORB THE ENERGY OF THE CRASH?

1. the crushing of the car's body

2. the air bag 3. the seatbelt 4. all of the

above

WHICH OBJECTS AND DEVICES ABSORB THE ENERGY OF THE CRASH?

1. the crushing of the car's body

2. the air bag 3. the seatbelt 4. all of the

above

IF THE ASTRONAUT IN THE PHOTO STOOD ON MARS WHERE THE ACCELERATION DUE TO GRAVITY IS ABOUT 3.7 M/S2, HOW MUCH WOULD THE ASTRONAUT WEIGH?

1. 33.0 N 2. 330 N 3. 33.0 kg 4. 330 kg

IF THE ASTRONAUT IN THE PHOTO STOOD ON MARS WHERE THE ACCELERATION DUE TO GRAVITY IS ABOUT 3.7 M/S2, HOW MUCH WOULD THE ASTRONAUT WEIGH?

1. 33.0 N 2. 330 N 3. 33.0 kg 4. 330 kg

HOW WOULD THE ACCELERATION OF A CHAIN OF TWO CARTS COMPARE WITH THE ACCELERATION OF A SINGLE CART IF THE SAME FORCE ACTED ON BOTH?

1. The two-cart chain would accelerate at half the rate of the single cart.

2. The two-cart chain would accelerate at twice the rate of the single cart.

3. The single cart would accelerate at half the rate of the two-cart chain.

4. They would accelerate at the same rate.

HOW WOULD THE ACCELERATION OF A CHAIN OF TWO CARTS COMPARE WITH THE ACCELERATION OF A SINGLE CART IF THE SAME FORCE ACTED ON BOTH?

1. The two-cart chain would accelerate at half the rate of the single cart.

2. The two-cart chain would accelerate at twice the rate of the single cart.

3. The single cart would accelerate at half the rate of the two-cart chain.

4. They would accelerate at the same rate.

Page 363-369

END OF - - - 12.2 NEWTON’S 1ST AND 2ND LAW OF MOTION

12.3 NEWTON’S 3RD LAW OF MOTION & MOMENTUM

NEWTON’S 3RD LAW

When one object exerts a force on a second object, the second object exerts and equal and opposite force on the fist object.

Video Clip (3:00)

ACTION AND REACTION FORCES

Example of the 3rd law

Hammer pushes on stake.Stake pushes on hammer.

The hammer acts, thestake re-acts.

WHAT’S THE REACTION?

WHAT’S THE REACTION?

MOMENTUM

Definition: the product of an object's mass and its velocity

SI Units : kilogram x meters per second.

(kg x m/s)

Keep in mind: An object has a large momentum if it has large mass and velocity is also large.

MOMENTUM

You can calculate momentum by multiplying an object's mass (in kilograms) and its velocity (in meters per second).

Sometimes “P” is used for the word momentum

INTERPRETING GRAPHS

Graph shows the speed and momentum of a 0.25-kilogram ball dropped from a bridge.

The graph shows the momentum of the ball from the time it was dropped until the time it hit the river flowing below the bridge.

INTERPRETING GRAPHS

At what time did the ball have zero momentum?

Time = 0 seconds

INTERPRETING GRAPHS

Describe the ball at this point.

The ball has zero momentum at this point. It’s NOT moving.

INTERPRETING GRAPHS

At what time did the ball have the greatest momentum?

Time = 2.5 seconds

INTERPRETING GRAPHS

What was the highest momentum value?

6.5 kg m/s

INTERPRETING GRAPHS

What was the ball’s speed after 1.5 seconds?

16 m/s

CONSERVATION OF MOMENTUM

In a closed system, the loss of momentum of one object equals the gain in momentum of another object—momentum is conserved.

See figures on page 376 of your text.

We will answer the following questions

CONSERVATION OF MOMENTUM

Refer to Fig. A

What is the mass of each train car?

What happened to the momentum of the green car?

What was the sum of the momentum of the cars before and after collision

30,000 kg

The momentum was transferred from the blue car but NOT lost

The sum was 450,000 before and after

CONSERVATION OF MOMENTUM

Refer to Fig. B

What is the mass of each train car?

What happened to the momentum of the green car?

What was the sum of the momentum of the cars before and after collision

30,000 kg

The momentum was transferred from the blue and put it into motion

The sum was 300,000 before and after

CONSERVATION OF MOMENTUM

Refer to Fig. C

What is the mass of each train car?

What happened to the momentum of the green car?

What was the sum of the momentum of the cars before and after collision

30,000 kg

The momentum was transferred from the blue and put it into motion

The sum was 300,000 before and after

CONSERVATION OF MOMENTUM

ReviewIn all cases, the momentum of each train car was conserved. It may have transferred from one car to another but it was NEVER lost.

What type of systems were these examples?

LET’S QUIZ YOUR LEARNING

WHY DON'T ACTION AND REACTION FORCES CANCEL EACH OTHER OUT?

because they act on the same object because they act on different objects because they are not equal because one of the forces is always

zero

WHY DON'T ACTION AND REACTION FORCES CANCEL EACH OTHER OUT?

because they act on the same object because they act on different objects because they are not equal because one of the forces is always

zero

TELL ME WHAT YOU KNOW ABOUT…

mass velocity (kg x m/s)

BUGS AND GUTS

While driving down the road, a bug hits the windshield of a bus and goes “splat”! 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?

TRICK QUESTION!

For every action, there is an equal ... (equal!).

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. Besides, fireflies have guts and bug guts have a tendency to be “splatterable”. Windshields don't have guts..

END OF - - - 12.3 NEWTON’S 3RD LAW OF MOTION & MOMENTUM

12.4 UNIVERSAL FORCES

GRAVITATIONAL FORCE

The weakest universal force Newton's law of universal gravitation

states that every object in the universe attracts every other object.

IN OTHER WORDS: An attractive force that acts between any 2 masses.

GRAVITATIONAL FORCE DEPENDS UPON MASS AND DISTANCE.

A Two masses, X and Y, attract each other.

GRAVITATIONAL FORCE DEPENDS UPON MASS AND DISTANCE.

B The larger mass of X results in a larger gravitational force.

GRAVITATIONAL FORCE DEPENDS UPON MASS AND DISTANCE.

C Increasing the distance between the masses significantly reduces the gravitational force.

CENTRIPETAL FORCE

A center-directed force that continuously changes the direction of an object to make it move in a circle.

LET’S QUIZ YOUR LEARNING

WHAT FACTORS AFFECT GRAVITATIONAL FORCE?

1. mass and distance 2. mass and air resistance 3. distance and air resistance 4. distance and velocity

WHAT FACTORS AFFECT GRAVITATIONAL FORCE?

1. mass and distance 2. mass and air resistance 3. distance and air resistance 4. distance and velocity

NAME THAT LAW!!!

Let’s play…….

WHICH LAW STATES

The acceleration of an object is equal to the net force acting on it divided by the object's mass

2

WHICH LAW STATES

An object at rest tends to stay at rest and an object in motion tends to stay in motion (unless acted upon by an unbalanced force.)

1

WHICH LAW STATES

For every action, their is an equal and opposite reaction.

3

The End……………………………

WITH A COOL CLIP ON MOMENTUM (2:00)