Chapter 2 Forces

Section 1 The Nature of Force

• *Describe what a force is

• *Explain how balanced and unbalanced forces are related to an objects motion

• What is a force?

• Force—a push or a pull

• -described by its strength and the direction in which it acts

Newton—SI unit that measures the strength of a force

-direction and strength represented by arrows

-Arrow points in the direction of the force-Length of arrow tells the strength

-longer arrow—more strength

• Net Force —combination of all forces acting on an object

-determines if an object moves AND the direction

-forces in the same direction—add forces together

5N + 5N = 10N

• -Forces in opposite direction towards each other—subtract the smaller from the larger

5N + 10N = 5N

moves in the direction of the larger force

5N + 5N = 0

0 net force—no movement

• Unbalanced force—Net force acting on an object that causes it to:

1. Start moving 2. Stop moving 3. Change directions

**Causes a change in the objects motion***

Balanced Force—Equal forces acting on an object in opposite directions

-balanced forces acting on an object do not change the object’s motion

--no motion in either direction

Section 2

Friction and Gravity

*Describe friction and identify the factors that determine the friction force between two objects

*Identify the factors that affect the gravitational force between two objects

*Explain why objects accelerate during free fall

Friction—force exerted when 2 objects rub against each other

Strength of the Force of Friction 1. How hard surfaces push together

2. Types of surfaces-smooth surface has less friction then a rough surface

**Friction acts in a direction opposite to the objects movement**

-without friction life would be VERY difficult

Static Friction—friction that acts on objects that are NOT moving

-friction between a chair and the floor—chair won’t move

Sliding Friction—friction where 2 solid surfaces slide over each other

-we need this to function-our feet on the floor have sliding friction so we can walk

Rolling Friction-when an object rolls across a surface

-things on wheels that roll across the ground

Fluid Friction —a solid object moves through a liquid

-water—oil—air -easier to overcome then sliding friction-why we use oil in cars to lubricate moving parts-why we wear bike helmets—

-to cut through air easier—-to make us more aerodynamic

Gravity—force that pulls objects to the earthLaw of Universal Gravitation —Force of gravity acts between all objects of the universe

-any 2 objects in the universe attract each other—NO EXCEPTIONS!!!

-you to the earth and the earth to you—but some forces we don’t realize because the attraction is so small compared to the earth

Gravity affected by 2 forces

1. Mass—amount of matter in an objectThe more mass the more gravity an object has

—the sun has greater gravity then the earth—it has much more mass—that is why we orbit the sun!!

2. Distance—the farther apart the objects are the less the gravitational attraction

Weight –measure of the gravitational force exerted on an object

-can vary depending on where you are-less on top of a mountain-less on the moon

-mass is the same-weight is less

Free fall –when the only force acting on an object is gravity

-it is an unbalanced force-causes the object to accelerate to the earth

-gravity = 9.8 m/s2 this is acceleration due to gravity****All objects fall at the same rate no matter what the mass!!******

Air Resistance –type of fluid friction objects falling through the air experience

-air acts upward against falling object-objects with more surface area have more resistance-in a vacuum ALL objects fall at the same rate-increases with velocity—as an object falls it gets faster and faster—UNTIL gravity

Air Resistance

Terminal Velocity –greatest velocity a falling object reaches

-reached when the force of air resistance equals the weight of the object

Air = gravity

Projectile Motion –object that is thrown horizontally

-falls at the same velocity as falling objects

-if you drop an object and throw an object from the same height and at the same time they will hit the ground at the same time

-if you throw something straight up—gravity will eventually bring it to a stop – 0 - then it will

accelerate back to the earth

4 s 4 S

Section 3

Newton’s First and Second Laws

Objectives:

State Newton’s First Law

State Newton’s Second Law

Newton’s First Law of Motion –an object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an unbalanced force

-gravity and friction are unbalanced forces that can change an objects motion

Inertia –tendency of an object to resist a change in motion

-another name for Newton’s First Law

-Galileo first came up with the theory about moving objects in 1600

-seatbelts keep you from continuing to move forward if you happen to stop quickly

-**the greater the mass the greater the inertia**-semi truck has more inertia then a bicycle

Newton’s Second Law of Motion –acceleration depends on the object’s mass and on the net force acting on the object

Formula: acceleration = Net Force / Mass

-acceleration measured in meters per second per second = m/s2

-mass measured in –kg

-force measured in kg x m/s2 so they call it a Newton for short

To increase acceleration:

1. increase the force if the mass stays the same 1 m/S 2 m/S

2. Decrease the mass if the force stays the same

1m/s 4 m/s

1000 KG

100 KG

1000 KG

1000 KG

Section 5

Rockets and Satellites

Objectives:

Explain how a rocket can lift off the ground

Explain what keeps a satellite in orbit

Rockets can rise into the air because of the gas it expels with a downward action force—it exerts an equal but opposite reaction force on the rocket

-as long as the force pushing the rocket up is greater then the downward force of

gravity the rocket will accelerate into space

Satellite –any object that orbits another object in space

Circular motion –

Centripetal Force –any force that causes an object to move in a circular path

-means “center” seeking

Satellite motion-they have a greater horizontal velocity then a ball that is thrown-satellites continually fall towards the earth but because the earth is curved they travel around the earth

-see page 66 picture-object must be moving at 7,900 m/s to orbit the earth-object is always accelerating because it is constantly changing directions