# Chapter 12 Forces and Motion.

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Chapter 12 Forces and Motion

Force Force is a push or a pull.

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

Measuring Force

lbs - Pound N - Newton Measuring Force

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

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

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

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

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

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.

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

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

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

Rolling Friction

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

Fluid Friction Air Resistance

Fluid Friction

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.

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

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

Gravity Air Resistance

Projectile Motion

Projectile Motion

Projectile Motion Horizontal Component Vertical Component

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

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

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

Object will move indefinitely
Aristotle Object will move indefinitely if no force is applied.

Newton

Newton

Newton

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

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

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.

Newton’s First Law

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

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

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

Newton’s Second Law Acceleration
Force = mass x acceleration F = ma

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

(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.)

Example #1 F

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

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

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

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

Falling

{ Falling All objects fall at the same rate!! 9.8m/s² Acceleration
due to Gravity!!! g 32ft/s² 22mph/s

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

Falling W W = Fair Terminal Velocity Fair

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

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

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:

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:

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:

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:

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.

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

Action - Reaction Pair

Rocket Propulsion Reaction Force Rocket Accelerates Action Force
Escaping Gases

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

v v

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

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

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.

Total Momentum Before = Total Momentum After

Total Momentum Before = Total Momentum After

Momentum

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

Universal Forces d

Electromagnetic Forces
Electromagnetic forces are associated with charged particles.

Electric Forces

Magnetic Force Magnetic Poles

Unlike Poles (North-South)
Attract!!!

Like Poles (North-North South-South)
Repel!!!

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

Strong Nuclear Force Strongest Force Holds the nucleus together
Short Range

Weak Nuclear Force Weaker than the Strong Force Short Range

Gravitational Forces

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

Gravitational Forces

Tides

Tides

Circular Motion

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

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

Sending up Satellites Geostationary

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