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Chapter 12 Forces and Motion

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Force Force is a push or a pull.

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**A resting object to move **

Force A force can cause: A resting object to move Accelerate a moving object By changing the object's Speed Direction

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Measuring Force

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lbs - Pound N - Newton Measuring Force

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**Connecting Motion and Forces**

Force (newton, N): A push or pull one body exerts on another. Earth

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Balanced Forces Balanced Forces: Forces on an object that are equal in size and opposite in direction.

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Unbalanced Forces Unbalanced Forces: Forces on an object are not equal resulting in a Net Force. 5 N 3 N

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Unbalanced Forces Unbalanced Forces: When an unbalanced force acts on an object, the object accelerates. 5 N 3 N

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**A Net Force on an object always changes the velocity of the object.**

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Balanced Forces Balanced Forces: When the forces on an object are balanced, the net force is zero and there is no change in the object's motion.

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Friction Force that opposes the motion of objects that touch as they move past each other.

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Static Friction A friction force that acts on objects that are not moving.

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Sliding Friction A friction force that opposes the motion of an object as it slides over a surface.

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Rolling Friction

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Rolling Friction A friction force that acts on rolling objects, caused by the change in shape at the point of rolling contact.

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Fluid Friction Air Resistance

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Fluid Friction

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**A friction force that opposes the motion of an object through a fluid.**

Fluid Friction A friction force that opposes the motion of an object through a fluid.

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Gravity Gravity: Every object in the universe exerts a force on every other object. This force is Gravity!!!

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Gravity Gravity causes objects to accelerate downward, whereas air acts in the direction opposite to the motion and reduces acceleration.

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Gravity Air Resistance

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Projectile Motion

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Projectile Motion

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Projectile Motion Horizontal Component Vertical Component

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

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Physical Science: 12-1 Worksheet: 12-1 Due:3/4/04

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**Force was required to keep an object moving at a constant speed.**

Aristotle

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**Object will move indefinitely**

Aristotle Object will move indefinitely if no force is applied.

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Newton

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Newton

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Newton

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Inertia Inertia (mass): The tendency of an object to resist any change in its motion. 25 kg 1kg

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**The more mass an object has, the greater its inertia.**

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**stay in motion at a constant velocity unless a net force acts on it.**

Newton’s First Law The Law of Inertia An object in moving tends to stay in motion at a constant velocity and an object at rest remain at rest unless a net force acts on it.

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Newton’s First Law

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**Newton’s Second Law of Motion**

The Law of Acceleration A net force acting on an object will accelerate that object in the direction of that force F a

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**FE FE - engine FNet FNet – net force FNet = FE - FF FF FF - friction**

Net Force Accelerates FNet FE FF FE - engine FF - friction FNet – net force FNet = FE - FF

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**FE=25N FN FN = FE - FF FN = 25N – 5N FN = 20N FF =5N**

Net Force Accelerates FN FE=25N FF =5N FN = FE - FF FN = 25N – 5N FN = 20N

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**Newton’s Second Law Acceleration**

Force = mass x acceleration F = ma

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ma F = F F a m= m a F m a=

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**(Assume that there is no friction against the car.)**

Example #1 A car with a mass of 2000kg is parked on the side of the road. How much force is needed to accelerate the car at 3m/s²? (Assume that there is no friction against the car.)

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Example #1 F

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Given: m = 2000kg a = 3m/s² Find: F = ? Equation: F = ma F = (2000kg) x (3m/s²) F = 15,000N

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Weight Weight: The measure of the force of gravity on an object.

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Mass Mass: The amount of matter making up an object.

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Weight Moon Earth Jupiter (16.7 lb) (100 lb) (254 lb)

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Falling

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**{ Falling All objects fall at the same rate!! 9.8m/s² Acceleration**

due to Gravity!!! g 32ft/s² 22mph/s

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**W = mg Acceleration due to gravity. g Force due to gravity. W (weight)**

Falling Acceleration due to gravity. g Force due to gravity. W (weight) W = mg

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Falling W W = Fair Terminal Velocity Fair

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Homework: 12-2 Worksheet: 12-2 Due: 3/10/09

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**Math Practice Page: 367, 1-4 Due: 3/10/09**

Homework: 12-3 Math Practice Page: 367, 1-4 Due: 3/10/09

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**A boy pushes forward a cart of groceries with a total mass of 40. 0 kg**

A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? Given: Find: Equation: Solve:

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What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction? Given: Find: Equation: Solve:

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**Given: Find: Equation: Solve:**

An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? (Hint: Solve the acceleration formula for force.) Given: Find: Equation: Solve:

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**Given: Find: Equation: Solve:**

A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair? (Hint: Solve Newton's second law for mass.) Given: Find: Equation: Solve:

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**Newton’s Third Law Action and Reaction**

When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction.

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**For every action there is an equal and**

Action and Reaction For every action there is an equal and opposite reaction.

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Action - Reaction Pair

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**Rocket Propulsion Reaction Force Rocket Accelerates Action Force**

Escaping Gases

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**Momentum - Mass on the Move**

Momentum ( p unit:kg•m/s ) A property a moving object has because of mass and velocity.

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v v

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**momentum = mass X velocity**

Momentum Equation momentum = mass X velocity p = m x v

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**Example: A car with a mass of 2000 kg is moving with a velocity of 40 m/s. What is its’ momentum?**

Given: m = 2000 kg v = 40 m/s Find: p = ? Equation: p = m x v Solve: p = 2000 kg x 40 m/s p = 80,000 kg•m/s

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**Conservation of Momentum**

The total momentum of a group of objects does not change unless outside forces act on the objects. The momentum before a collision is equal to the momentum after the collision.

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**Total Momentum Before = Total Momentum After**

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**Total Momentum Before = Total Momentum After**

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Momentum

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Homework: 12-3 Worksheet: 12-3 Due: 3/12/08

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Universal Forces d

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**Electromagnetic Forces**

Electromagnetic forces are associated with charged particles.

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Electric Forces

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Magnetic Force Magnetic Poles

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**Unlike Poles (North-South)**

Attract!!!

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**Like Poles (North-North South-South)**

Repel!!!

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**Two forces that act in the nucleus to hold it together.**

Nuclear Force Two forces that act in the nucleus to hold it together. Strong Nuclear Force Weak Nuclear Force

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**Strong Nuclear Force Strongest Force Holds the nucleus together**

Short Range

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Weak Nuclear Force Weaker than the Strong Force Short Range

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Gravitational Forces

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**Any object with mass exerts a gravitation force. Long Range**

Gravitational Forces Any object with mass exerts a gravitation force. Long Range

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Gravitational Forces

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Tides

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Tides

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Circular Motion

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**Circular Motion V - Velocity Direction of Motion Centripetal Force (g)**

Acceleration

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Sending up Satellites V = 29,000km/h (18,000mph)

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Sending up Satellites Geostationary

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Homework: 12-4 Worksheet: 12-4 Due: 3/13/09

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