Forces and Motions

Forces and Motions

ForcesWhat is a Force? Anything that changesthe state of rest or motion of an object It’s what causes ACCELERATION

has magnitude and direction Therefore force is a vector

Which of the following lines shows acceleration?

A B Both

Neither

14%10%10%

67%

1. A2. B3. Both4. Neither

Force cont.A force can cause a

resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

Units = Newtons (N)

Forces can be COMBINED:Net Force: the combination of all of the forces

acting on the object

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.

Example: Tug of war, Pushing piano

Unbalanced Forces Results when net force acting on an

object is NOT equal to zero When an unbalanced force acts on an object,

the object accelerates in the direction of the net force

Fig 14, p.332

http://www.darvill.clara.net/enforcemot/friction.htm

*(moves in direction of net force)

Friction is a force: Friction is a force that opposes motion between two

surfaces that are in contact

There are two main types of friction: Static- keeps things “static”

Ex- the force that is keeping this block from sliding downhill

Kinetic- friction of movementthree types: Sliding,Rolling, Fluid

Complete the concept map for the 4 types of friction! Use your textbook (pages 332-334)

FRICTION

STATIC SLIDING ROLLING FLUID

Thre friction between surfaces that are

stationary

force that exists when objects slide

past each other

force that exists when a round object

rolls over a flat surface (usually less than sliding friction)

force that exists when an object

moves through a fluid (air, water)

a book sitting on a table

-hockey puck on ice-child going down a

slide-a sled down hill

-a roller blade on a sidewalk

-bowling ball on bowling alley

-a car driving down the road

-swimmer swimming through pool

is defined as is defined as is defined as is defined as

& an example is & an example is & an example is & an example is

Can you think of a situation in which you would want to increase friction?

How can we decrease friction?Watch this demo and see…

http://www.scottso.net/examples_encfri.htm

Gravity is a force: natural phenomenon in which objects that have mass are

attracted to one another

Gravity is an attractive force pulls objects together

Earth’s gravity acts downward toward the center of the Earth.

There is an upward force that balances gravity AIR RESISTANCE

Falling Objects have two forces acting on them:

Gravity causes objects to accelerate downward air resistance acts in the direction opposite to

the motion & reduces acceleration.

Terminal Velocity:

when something falls with a constant velocity

(no net force = no acceleration= constant velocity) Formal Definition: the constant velocity of a

falling object when the force of air resistance is equal in magnitude & opposite in direction to the force of gravity

Terminal Velocity (cont) In other words…If something falls for a long

time,the upward force of air resistance becomes equal to downward force of gravity.

1) Example: Sky divers reach terminal velocity after a period of time(Fig 10, p.356)

Free Fall: the motion of a body when only the force of gravity is acting on the body

Free fall acceleration of an object is directed toward the center of Earth

Because free fall acceleration results from gravity, its symbol is g

Acceleration due to gravity on Earth = 9.8 m/s2

Formula for objects in free fall: d = ½ at2

In a vacuum, two objects would accelerate at the same rate because both are in free fall (Fig 8, p.354)

Question: What other force is not present in vacuum that would affect acceleration?

Answer = air resistance

II.) Newton’s First Law of MotionA) Historical Development

1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving

2) Galileo (1564 – 1642): Concluded that moving objects not subjected to friction or other force, would continue to move indefinitely; Disproved Aristotle

3) Newton (1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion

C) Newton’s 1st Law of Motion 1) According to Newton’s 1st Law, the state of

motion of an object does not change as long as the net force is zero. a) Basically saying that an object at rest will stay at rest unless a force acts on it

1) Example: Soccer ball will remain (at rest) on the grass unless a force is

acted on it

2) Sometimes called the “Law of Inertia”a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object

1) Car crash: You continue forward because of inertia

“Science and the Consumer”

p.348

How is inertia related to mass?P 347 Mass is a measure of inertia.

Who would you rather be tackled by…a toddler or a defensive lineman?

What is easier to move? An empty garbage can or a garbage can full of lead? Why? The empty garbage can has less mass= less inertia=

less resistance to being accelerated.