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Ch. 12 Motion & Forces. I. Newton’s Laws of Motion. “If I have seen far, it is because I have stood on the shoulders of giants.” - Sir Isaac Newton (referring to Galileo). Newton’s First Law. Newton’s First Law of Motion

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Ch. 12 Motion & Forces

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Ch. 12Motion & Forces

I. Newton’s Laws of Motion

“If I have seen far, it is because I have stood on the shoulders of giants.”

- Sir Isaac Newton

(referring to Galileo)


Newton’s First Law

  • Newton’s First Law of Motion

    • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.


Newton’s Second Law

  • Newton’s Second Law of Motion

    • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

F = ma


Newton’s Third Law

  • Newton’s Third Law of Motion

    • When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.


III. Defining Force

Force

Newton’s First Law

Friction


A. Force

  • Force

    • a push or pull that one body exerts on another

    • What forces are being exerted on the football?

Fkick

Fgrav


A. Force

  • Balanced Forces

  • forces acting on an object that are opposite in direction and equal in size

  • no change in velocity


N

N

A. Force

  • Net Force

    • unbalanced forces that are not opposite and equal

    • velocity changes (object accelerates)

Fnet

Ffriction

Fpull

W


Calculating Net Force from Images

  • Net force is the sum of forces acting on an object

    • If forces are acting in the same direction they combine (add together)

    • If forces are acting in opposite directions they reduce (subtract)

  • If net force is zero the motion of the object will NOT change

  • If net force is not zero the object will accelerate (this may be positive or negative acceleration).


What is the net force on the objects below?

Opposite = subtract

8N – 3N = 5 N

Same = addition

5N + 2N = 7 N

2 N + 5N – 8N = 1 N

7N + 2N – 3N = 6N


B. Newton’s First Law

  • Newton’s First Law of Motion

    • “Law of Inertia”

    • Inertia

      • tendency of an object to resist any change in its motion

      • increases as mass increases

  • Newton’s First Law of Motion

    • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.


ConcepTest 1

TRUE or FALSE?

The object shown in the diagram must be at rest since there is no net force acting on it.

FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.


ConcepTest 2

You are a passenger in a car and not wearing your seat belt.

Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door.

Which is the correct analysis of the situation? ...

1. Before and after the collision, there is a rightward force pushing you into the door.

2. Starting at the time of collision, the door exerts a leftward force on you.

3. both of the above

2. Starting at the time of collision, the door exerts a leftward force on you.


C. Newton’s Second Law

  • Newton’s Second Law of Motion

    • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

F = ma


F

a

m

Newton’s Second Law

F = ma

F:force (N)

m:mass (kg)

a:accel (m/s2)

1 N = 1 kg ·m/s2


F

a

m

Calculations

  • What force would be required to accelerate a 40 kg mass by 4 m/s2?

GIVEN:

F = ?

m = 40 kg

a = 4 m/s2

WORK:

F = ma

F = (40 kg)(4 m/s2)

F = 160 N


F

a

m

Calculations

  • A 4.0 kg shot-put is thrown with 30 N of force. What is its acceleration?

GIVEN:

m = 4.0 kg

F = 30 N

a = ?

WORK:

a = F ÷ m

a = (30 N) ÷ (4.0 kg)

a = 7.5 m/s2


D. Gravity

  • Gravity

    • force of attraction between any two objects in the universe

    • increases as...

      • mass increases

      • distance decreases


more mass

less distance

B. Gravity

  • Who experiences more gravity - the astronaut or the politician?

  • Which exerts more gravity - the Earth or the moon?


D. Gravity

  • Would you weigh more on Earth or Jupiter?

  • Jupiter because...

  • greater mass

  • greater gravity

  • greater weight


Free Fall

  • When gravity is the only force acting on an object

  • Represented by the letter g

  • Near Earth’s surface g = 9.8 m/s2


B. Gravity

  • Weight

    • the force of gravity on an object

W = mg

W:weight (N)

m:mass (kg)

g:acceleration due to gravity (m/s2)

W:weight (N)

m:mass (kg)

g:acceleration due to gravity (m/s2)

W:weight (N)

m:mass (kg)

g:acceleration due to gravity (m/s2)

MASS

always the same

(kg)

WEIGHT

depends on gravity

(N)


elephant

feather

D. Gravity

  • Accel. due to gravity (g)

  • In the absence of air resistance, all falling objects have the same acceleration!

  • On Earth: g = 9.8 m/s2


W

g

m

Calculations

  • Mrs. J. weighs 557 N. What is her mass?

GIVEN:

W = 557 N

m = ?

g= 9.8 m/s2

WORK:

m = W ÷ g

m = (557 N) ÷ (9.8 m/s2)

m = 56.8 kg


ConcepTest

  • Is the following statement true or false?

    • An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.

  • False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.


E. Newton’s 3rd Law

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


3rd law: Action / Reaction

  • The action force and the reaction force occur to different objects so the force is not balanced.

  • Not all action and reaction forces produce motion.

  • Unbalanced forces equal changes in motion


Newton’s third law

  • Kicking a soccer ball

    • Action force = your foot hits the ball

    • Reaction force = the ball pushes against your foot.

      • Ball moves b/c action force is larger than reaction force

  • Leaning against a wall

    • Action force = you pushing against a wall

    • Reaction force = wall pushing against you

      • Nothing moves b/c the action force equals the reaction force.


F. Friction

  • Friction is a force that opposes the motion of objects that touch as they move past each other.

  • Friction acts at the surface where object are in contact

  • Four main types of friction: static friction, sliding friction, rolling friction and fluid friction.


C. Friction

  • Friction

    • is the force that opposes motion between 2 surfaces

    • depends on the:

      • types of surfaces

      • force between the surfaces


C. Friction

  • Friction is greater...

    • between rough surfaces

    • when there’s a greater force between the surfaces (e.g. more weight)

  • Pros and Cons?


Static Friction

  • Static friction is the friction force that acts on objects that are not moving.

  • Static friction always acts in the direction opposite to that of the applied force

  • Prevents objects from sliding.

  • Example: pushing a dresser that does NOT move


Sliding Friction

  • Sliding Friction is a force that opposes the direction of motion of an object as it slides over a surface.

  • Occurs when there is enough force to overcome the static friction

  • There will be a net force in the direction of motion

  • Pushing a desk that slides against the floor.


Rolling Friction

  • Rolling friction is the force that acts on rolling objects.

  • Rolling friction is about 100 to 1000 times less than the force of static or sliding friction

  • Skate boarding down a hill


Fluid Friction

  • Fluid friction opposes the motion of an object through a fluid.

  • Fluid examples: air, water, quick sand, and cake batter

  • Air Resistance is the fluid friction action on an object moving through the air.

What are the forces acting on a falling leaf?


Fair

Fgrav

Air Resistance

  • Terminal Velocity

    • maximum velocity reached by a falling object

    • reached when…Fgrav = Fair

  • no net force

     no acceleration

     constant velocity

     still falling


Projectile Motion

  • Projectile motion the motion of a falling object after it is given an initial forward velocity.

  • this is a curved path

  • Objects with different mass fall at the same rate.

  • The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.


Momentum

  • Momentum is the product of an object’s mass and its velocity, an object with large momentum is hard to stop

  • momentum for any object at rest is zero

  • Law of conservation of momentum if not net forces acts on a system, then the total momentum of the system does not change

  • In a closed system the loss of momentum of one object equals the gain in momentum of another object--- momentum is conserved


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