Forces 8th Grade Science Chapter 10 Forces 8th Grade Science

Force _______- a push or a pull One object exerts a _______ on another Force is described by its ______ and by the ________ in which it acts Example – if you ______ on a door, you exert a _____ in a different _______ than if you ____ on the door. Force Force strength direction push Force direction pull

Static Friction _____________ - the friction that acts on an object that is not moving. You must use________ to start motion of a stationary object. If you use a force that is less than force of ___________, then the object that is stationary will not move. Example: pushing a desk on the floor Static Friction Extra Force Static Friction

Sliding / Rolling Friction ____________ - occurs when two surfaces slide over each other. Example: a ballet dancer puts sticky powder on the bottom of ballet shoes so that they do not slip on the floor. _____________ - when an object rolls across a surface Example: skateboard – rolling friction acts in the _________ direction of the skateboards motion. Sliding Friction Rolling Friction opposite

Fluid Friction ___________ - occurs when a solid object moves through a liquid. Example: parts of a machine are bathed in oil so that they move smoothly over each other. Example 2: a bike rider experiences fluid friction between the individual and the air. When an object ____________, fluid friction occurs Fluid Friction Pushed fluid aside

Gravity _______ - a force that pulls objects towards each other __________ - concluded that force acts to pull objects straight downward towards the center of the Earth. Newton realized that ________ acts everywhere in the _______, not just on Earth = ___________________ Gravity Isaac Newton gravity universe Law of Universal Gravitation

Law of Universal Gravitation Law of Universal Gravitation states that _____________________________________ Any two objects in the universe, _______________, attract each other. We are attracted to the _______ and all the other objects around us. Objects all around us are _________ to us as well – we don’t know it because the forces are small compared to the force of Earth’s attraction. The force of gravity acts between all objects in the universe Without Exception Earth attracted

Factors Affecting Gravity Two factors that affect gravitational attraction between two objects are ____ and ________ The more _____ an object has, the greater the ______________. Because the ______ mass is so great, it exerts a large gravitational force on the ________ ________ - the farther the two objects are, the _________ the gravitational force between the objects mass distance mass Gravitational force Sun’s planets distance lesser

Weight and Mass ________ - the measure of the amount of matter in an object _________ - the measure of the gravitational force exerted on an object When you step on a scale, you are determining the amount of gravitational force the Earth is exerting on you! _________ varies but _______ does not mass weight Weight Mass

Gravity and Motion On Earth, gravity is a ________ force that affects __ objects. __________ - when the force of gravity is an ____________ force, which causes the object to _________. Near the surface of the Earth, the acceleration due to gravity is _____ - regardless of the object’s ______. downward all Free Fall Unbalanced accelerate 9.8 m/s mass

Math / Analyzing the Data Page 346 (1-4)

Air Resistance ____________ - fluid friction that objects encounter while falling through the air. Air resistance is an _______ force that is exerted on an object falling _________. Falling objects with a greater ____________ experience ______air resistance than objects with less surface area. Air resistance increases with ________ Air Resistance upward downward Surface area more velocity

Air Resistance Continued As a falling object gains speed, ______________ increases also – eventually the velocity of the falling object will be fast enough that the ________ force of air resistance is equal to the ________ force of gravity = _______________ When forces are ________ there is no _________ - velocity remains constant Air resistance upward downward Balanced force balanced acceleration

Projectile Motion __________________ - the greatest velocity of a falling object ________ - an object that is thrown When a projectile is thrown at an ________ angle, the force of gravity ________ its vertical velocity. Eventually the ______ motion of the projectile will stop and ________ will pull it ________ Terminal Velocity projectile upward reduces upward gravity downward

Section Assessment Page 348 (1-3)

Newton’s Laws Newton proposed three basic laws of motion in the 1600’s Newton’s first law restated _________ idea about force and motion. Newton’s first law of motion states: An object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force. Galileo’s

1st Law An object that is not moving, will not move until a ___________. If an object is moving, it will continue to move at a constant velocity until ________________ _________________________ On Earth, ________ and _________ are _______________ that often change an objects motion. Force acts on it force acts to either Change either speed or direction gravity friction Unbalanced forces

Inertia _______ - is the tendency of an object to resist a change in motion - ______________ Newton’s first law of motion is also called the _________. Inertia explains why a person moves forward in their seat when a car stops suddenly. ________ moves you forward in your seat while a seat belt ________________ inertia Galileo’s concept Law of inertia inertia Changes your direction

Inertia depends on Mass The _________ the _______ of an object, the __________ its ______ and the _________ the ________ required to ______________. greater mass greater inertia greater force Change its motion

Second Law of Motion _________ depends on an objects ________ and the ____________ acting on the object. Acceleration = Net Force Mass acceleration mass Net force

Acceleration Acceleration is measured in _______________ Mass is measured in __________ According to Newton’s Second Law – force is measured in __________ = _____________ ________ = SI unit of force Math Sample Problem on page 351 Math Practice page 351 (1 and 2) Meters per second² Kilograms (kg) kg times m/s² Newton (N) Newton

Newton’s Third Law To increase __________ you must either increase _____ or decrease _____ Newton’s Third Law states: if one object exerts a ________ on another object, then the second object exerts a ________ of _____________ in the ____________ on the first object. FOR EVERY ACTION THERE IS AN EQUAL BUT OPPOSITE REACTION acceleration mass force force force Equal strength Opposite direction

Why don’t action and reaction forces cancel each other? Read Examples on pages 354-355

Momentum ____________ is a characteristic of a moving object that is related to the mass and velocity of the object Momentum of a moving object is calculated by __________________________ The more momentum a moving object has, the more difficult its is to stop Objects with greater ______ and ________ have greater ____________ momentum Multiplying mass times velocity (m x v) mass velocity momentum

Conservation of Momentum ____________ - conditions before and after an event An amount that is conserved is the same amount ______ the event as it was ________ The total amount of momentum objects have is _________ when they ________ Momentum can be __________ but not ____ conservation after before conserved collide transferred lost

Law of Conservation of Momentum _____________ of any group of objects remains the same, or is ___________, unless ___________ act on the objects. Examples on page 358-359 - three different types of collisions Total Momentum conserved Outside forces

Section Assessment Page 359 (1-5)