Forces and Motions

Forces What is a Force? Anything that changes the 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? 1. A 2. B 3. Both 4. 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)

B) Combining Forces 1) Net Force: the combination of all of the forces acting on the object 2) Balanced Forces a) When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion. b) Example: Tug of war, Pushing piano

3) Unbalanced Forces a) Results when net force acting on an object is NOT equal to zero b) When an unbalanced force acts on an object, the object accelerates in the direction of the net force c) Fig 14, p tm *moves in direction of net force

C) Friction a force that opposes motion between two surfaces that are in contact There are two main types of friction: 1. Static- keeps things “static” Ex- the force that is keeping this block from sliding downhill 2. Kinetic- friction of movement 1. three types 1. Sliding,Rolling, Fluid Complete the concept map for the 4 types of friction! Use your textbook (pages )

FRICTION STATIC SLIDINGROLLINGFLUID 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 & 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… m

D) Gravity: natural phenomenon in which objects that have mass are attracted to one another 1) Gravity is an attractive force  pulls objects together 2) Earth’s gravity acts downward toward the center of the Earth. 3) There is an upward force that balances gravity

4) Falling Objects a) Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion & reduces acceleration. b) If something falls for a long time, upward force of air resistance becomes equal to downward force of gravity. c) Terminal velocity: 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 1) Example: Sky diving (Fig 10, p.356)

E) Free Fall: the motion of a body when only the force of gravity is acting on the body 1) Free fall acceleration of an object is directed toward the center of Earth a) Because free fall acceleration results from gravity, its symbol is g b) Acceleration due to gravity on Earth = 9.8 m/s 2 Formula for objects in free fall: d = ½ at 2

c) In a vacuum, two objects would accelerate at the same rate because both are in free fall (Fig 8, p.354) 1) Question: What other force is not present in vacuum that would affect acceleration? Answer = air resistance

II.) Newton’s First Law of Motion A) 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.