Chapter 12

Forces and Motion

Force

Force is a push or a pull.

Force

A force can cause:

A resting object to move

Accelerate a moving object By changing the object's

Speed

Direction

Measuring Force

Measuring Force

lbs - Pound

N - Newton

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

Net Force

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

2 N

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

AirResistance

Fluid Friction

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

AirResistance

Projectile Motion

Projectile Motion

Projectile Motion

Vertical Component

HorizontalComponent

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: 12-1Due:3/4/04

Ari

stotl

e

Force was required to keep an object

moving at a constant speed.

Ari

stotl

e

Object will move indefinitely if no force is applied.

New

ton

New

ton

New

ton

Inertia

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

1kg 25 kg

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

Newton’s First LawThe Law of Inertia

An object in moving tends to

stay in motion at a constant velocityand

an object at resttends to

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 forceFa

Net Force Accelerates

FE

FE - engine

FF

FF - friction

FNet

FNet – net forceFNet = FE - FF

Net Force Accelerates

FE=25NFF =5N

FN

FN = FE - FF FN = 25N – 5N

FN = 20N

Newton’s Second Law

Acceleration

Force = mass x acceleration

F = ma

F

m a

F = ma

m= Fa

a=Fm

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.

MassMass: 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

32ft/s²

22mph/s{Acceleration due to

Gravity!!!g

Falling

Acceleration due to

gravity.g

Force due to

gravity.W (weight)

W = mg

Falling

W

Fair

W = Fair

Terminal Velocity

Homework: 12-2

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

Homework: 12-3

Math PracticePage: 367, 1-4Due: 3/10/09

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:

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:

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:

Action and Reaction

Newton’s Third Law

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

opposite reaction.

Action and Reaction

Action - Reaction Pair

Rocket Propulsion

Reaction ForceRocket Accelerates

Action ForceEscaping 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 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/sFind: 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: 12-3Due: 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!!!

Nuclear Force

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

Strong Nuclear Force

Weak Nuclear Force

Strong Nuclear ForceStrongest Force

Holds the nucleus together

Short Range

Weak Nuclear Force

Weaker than the Strong Force

Short Range

Gravitational Forces

Gravitational Forces

Any object with mass exerts a gravitation force.

Long Range

Gravitational Forces

Tides

Tides

Circular Motion

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

Sending up Satellites

Geostationary

Homework: 12-4

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