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**Forces 8th Grade Science**

Chapter 10 Forces 8th Grade Science

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**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

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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

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**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

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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

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**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

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**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

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**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

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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

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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

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**Math / Analyzing the Data**

Page 346 (1-4)

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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

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**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

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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

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Section Assessment Page 348 (1-3)

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**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

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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

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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

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**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

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

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**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

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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

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**Why don’t action and reaction forces cancel each other?**

Read Examples on pages

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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

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**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

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**Law of Conservation of Momentum**

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

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Section Assessment Page 359 (1-5)

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