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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 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.

III. Defining Force

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

N

- 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

Same = addition

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.

- 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

F = ma

F:force (N)

m:mass (kg)

a:accel (m/s2)

1 N = 1 kg ·m/s2

F

a

m

- 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

- 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

more mass

less distance

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

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

- 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)

elephant

feather

- 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

- 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.

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

- 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

- 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