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# Ch. 12 Motion & Forces - PowerPoint PPT Presentation

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. 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 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 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 of Motion

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

Force

Newton’s First Law

Friction

• Force

• a push or pull that one body exerts on another

• What forces are being exerted on the football?

Fkick

Fgrav

• Balanced Forces

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

• no change in velocity

N

A. Force

• Net Force

• unbalanced forces that are not opposite and equal

• velocity changes (object accelerates)

Fnet

Ffriction

Fpull

W

• 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).

Opposite = subtract

8N – 3N = 5 N

5N + 2N = 7 N

2 N + 5N – 8N = 1 N

7N + 2N – 3N = 6N

• 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.

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.

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

a

m

Newton’s Second Law

F = ma

F: force (N)

m: mass (kg)

a: accel (m/s2)

1 N = 1 kg ·m/s2

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

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

• Gravity

• force of attraction between any two objects in the universe

• increases as...

• mass increases

• distance decreases

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

• When gravity is the only force acting on an object

• Represented by the letter g

• Near Earth’s surface g = 9.8 m/s2

• 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)

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

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

• 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

• 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.

• 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.

• Friction

• is the force that opposes motion between 2 surfaces

• depends on the:

• types of surfaces

• force between the surfaces

• Friction is greater...

• between rough surfaces

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

• Pros and Cons?

• 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 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 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 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 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 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