# Forces and Motions.

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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

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. Warning: Doesn’t always mean the object is at rest; Example: Tug of war, Pushing piano, car traveling at constant velocity

Results when net force acting on an object is NOT equal to zero
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 *(moves in direction of net force)

Force A push or pull on an object Units: Newton (N) = 1 kg x m/s2
Net Force (Fnet) : sum of all of the forces that are acting on an object Balanced Forces: Fnet = 0 Object is at rest or moving with constant velocity Unbalanced Forces: Fnet is not equal to 0, Object is accelerating or decelerating

Types of Forces Applied – contact force in the direction the object is moving Tension- caused by a rope, cable, etc., directed away from the object Friction- opposes the motion of objects, must have contact Normal- caused by a surface Force of gravity- force at a distance, caused by attraction between two objects

Two Types of Friction Static Friction of an object at rest
Greater than kinetic friction (harder to get an object moving than to keep it moving) Friction is greatest when surfaces are rough ex. the force that is keeping this block from sliding downhill Kinetic Friction of a moving object

Types of Kinetic Friction
Sliding Two surfaces moving past each other Rolling Movement of a circular object on a surface Ex. Cart on wheels Fluid Friction Movement of an object through a liquid or gas Ex. Air resistance

FRICTION STATIC SLIDING ROLLING FLUID
is defined as & an example is The 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

How can we decrease friction?
Watch this demo and see… Can you think of a situation in which you would want to increase friction?

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.

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 gt2

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

Drawing Free Body Diagrams
Identify the types of forces acting on the object Use a dot or a box to represent the object Use arrows to represent the direction and size of the force -change the size of the arrow if more force is being applied in one direction versus the other Label the arrows with the type of force

Free Body Diagrams How will the object move in the diagram to the right? Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero

Free body diagram practice

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

“Science and the Consumer”
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.