Download FORCES and MOTIO BENCHMARK REVIEW Section 5

FORCES and MOTIO BENCHMARK REVIEW
Section 5-1 “MEASURING MOTION” pages 118-123
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An object is in motion if it changes position over time in relation to a reference point.
Speed is the distance traveled by an object divided by the time the object takes to ravel that distance.
Velocity is speed in a given direction.
Acceleration is the rate at which velocity changes.
An object can accelerate by changing speed, direction, or both. Acceleration = Final velocity – Initial Velocity / Time
Speed can be represented on a graph of distance vs time.
Acceleration can be represented by graphing velocity versus time.
Section 5-2 “WHAT IS A FORCE? pages 124-127
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A force is a push or a pull. Forces have size and direction and are expressed in newtons.
Force is always exerted by one object on another object.
Net force is determined by combining forces.
Forces in the same direction are added. Forces in opposite directions are subtracted.
Balanced forces produce no change in motion. Unbalanced forces produce a change in motion.
SECTION 5-3 “FRICTION: A FORCE THAT OPPOSES MOTION” PAGES 128-133
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Friction is a force that opposes motion.
Friction is caused by hills and valleys on the surfaces of two objects touching each other.
The amount of friction depends on the following two factors: (1) the roughness of the surfaces and (2) the force pushing the
surfaces together. Friction does not depend on the surface area of the object.
Two kinds or friction are kinetic friction (friction between moving surfaces) and static friction (a force is applied but the object
does not move because static friction balances the force applied)
Section 5-4 “GRAVITY: A FORCE OF ATTRACTION” pages 134-139
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Gravity is a force of attraction between objects that is due to their masses.
The law of universal gravitation states that all objects in the universe attract each other through gravitational force.
The strength of gravity depends on the masses of the objects and the distances between the objects.
G-force increases as mass increases.
G-force decreases as distances increases.
Weight and mass are NOT the same. Mass is the amount of matter in an object. Mass does not change unless the amount of
matter in an object changes.
Weight is a measure of gravitational force on an object’s mass. Weight = Mass x Acceleration due to gravity
Weight changes when mass or gravitational force changes. Since each planet and moon have a different mass, gravity is
different on each planet and moon. An astronaut’s weight on the moon is about one-sixth of his weight on earth, but his mass
remains constant.
Section 6-1 “GRAVITY: A FORCE OF ATTRACTION” pages 150-153
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Gravity causes all objects to accelerate toward earth at a rate of 9.8m/s/s. So for every second that an object falls, the object’s
downward velocity increases by 9.8 m/s. After two seconds, an object’s velocity is 19.6 m/s downward.
Objects fall to the ground at the same rate, regardless of their mass. A heavier object experiences a greater gravitational force
than a lighter object to keep acceleration the same.
Objects falling through air experience a type of friction called air resistance.
Air resistance is not the same for all objects. The greater the surface area
Section 6-2 “NEWTON’S LAWS of MOTION” pages 158-164
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Newton’s first law of motion states that the motion of an object will not change if no unbalanced forces act on it.
Objects at rest will not move unless acted upon by an unbalanced force.
Objects in motion will continue in motion at a constant speed and in a straight line unless acted upon by an unbalanced force.
Inertia is a property shared by all types of matter. Inertia is the tendency of matter to resist a change in motion. Mass is a
measure of inertia. An object that has less mass has less inertia than an object that has a greater mass.
Newton’s second law of motion states that the acceleration of an object depends on its mass and on the force exerted on it.
Newton’s second law is represented by the following equation: F = m x a. An object’s acceleration can be increased by
decreasing the mass of the object or by increasing the force applied on the object.
Newton’s third law states that if one object exerts a force on another object, then the second object exerts a force of equal
strength in the opposite direction on the first object.
Section 8-1 “WORK AND POWER” pages 208-213
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In scientific terms, work is done when a force causes an object to move in the direction of the force.
The amount of work you do depends on both the amount of force you exert and the distance the object moves.
Work is calculated as force times distance (W = f x d). If you exert a force of 1 newton to move and object a distance of 1
meter, you have done one joule of work. The unit of work is the newton-meter, or joule.
Section 8-2 “WHAT IS A MACHINE” pages 216-219
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A machine makes work easier by multiplying either force or distance, or by changing the direction of the force.
A machine can increase force or distance, but not both. A machine does not do more work than you do when trying to
move an object.
The force you exert on the machine is called the input force, or effort.
The force exerted by the machine is the output force, or resistance.
Section 8-3 “TYPES OF MACHINES” pages 222-228
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There are six types of simple machines: the lever, the pulley, the wheel-and-axle, the inclined plane, the wedge, and the
screw.
A lever is a rigid bar theta pivots on a fixed point called a fulcrum. There are thee classes of levers, depending on the
position of the load, fulcrum, and input force 9effort).
The closer the load is to the fulcrum, the more the force is multiplied by the machine, resulting in a greater output force
over a shorter distance. The farther the load is from the fulcrum, the more distance is multiplied by the machine,
resulting in a greater input force over a shorter distance.
A wheel-and-axle consists of two circular objects of different sizes. The greater the ratio between the radius of the wheel
to the axle, the more force is multiplied by the machine, resulting in a greater output force over a shorter distance..
A pulley consists of a rope or chain that is wrapped around a grooved wheel. There are three types of pulleys: a fixed
pulley, a movable pulley, and a pulley system.
The number of times that a pulley multiples force depends on the number of rope segments supporting the load.
An inclined plane is a flat, slanted surface. Using an inclined plane to load a piano into a truck is easier than lifting the
piano into the truck. Rolling the piano along an inclined plane requires a smaller input force (effort) than is needed to lift
the piano into the truck. The same work is done on the piano, just over a longer distance.
The greater the ratio of an inclined plane’s length to its height, the more it multiplies force.
A wedge is a pair of back-to-back inclined planes that move. The longer and thinner the wedge is compared to it’s width,
the greater the output force.
A screw is an inclined plane wrapped around a cylinder. The longer the spiral is and the closer together the threads, the
greater the output force.
Section 9-1 “WHAT IS ENERY? pages 240-243
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The ability to do work or cause a change is energy.
The two kinds of energy are potential and kinetic. The more work you do on an object, the more energy you give that
object.
The more mass an object has, the more kinetic energy it has. Kinetic energy also increases when velocity increase.
Energy that is stored and held in readiness is potential energy.
The potential energy associated with objects that can be stretched or compressed is called elastic potential energy.
When you lift an object you give it gravitational potential energy.
Gravitational potential energy depends on height above earth’s surface and an object’s weight. (GPE = Wt x Ht)
Mechanical energy is the energy associated with the motion or position of an object. Mechanical energy can be all
potential, all kinetic, or both.
Section 9-1 “ENERGY CONVESRIONS AND CONSERVATION OF ENERGY” pages 240-243
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An energy conversion is a change from one form of energy to another.
On a roller coaster, PE is greatest at the top of the hill, but KE is greatest at the bottom of the hill.
Because of friction, some energy is always converted into thermal energy during an energy conversion.
Thermal energy is wasteful energy because it is not used to do work.
A closed system is a group of objects that transfer energy only to each other.
Energy is conserved within a closed system. According to the law of conservation of energy, energy cannot be created or
destroyed but can be changed from one form to another.
MOTION REVIEW QUESTIONS
True or False (Correct the false statements to make them true)
____ 1. The graph of distance versus time for an object moving at a constant speed is a curve. _________________
_____2. You are accelerating if you ride your bike in a straight line at a constant speed of 10km/h. ______________
_____3. When you say that a race car travels northward at 100km/h, you are talking about its
velocity. ___________
_____4. The graph on the right plots a car’s speed for 8 seconds. The car’s acceleration during
this time is 7 /s2..
_____5. When an unbalanced force acts on an object, the force changes the motion of the
object. __________________
_____6. If you increase the force on an object, its acceleration stays the same. ______________
_____7. 16 N north and 16 N south are a pair of balanced forces because they cancel out. __________________
_____8. The net force illustrated in the diagram on the right is 3N to the right.
__________________________________________________
_____ 9. The strength of friction depends on the force applied and the surface area of the objects. __________________
_____10. If Earth’s mass doubled without changing its size, your weight would increase because gravitational force
increases. ___________________________
_____11. Mass is always constant for an object no matter where the object is located in the universe. _____________
_____12. The feather and the rock on the right are accelerating at different
rate(s)._____________________________
_____13. Gravitational force between two objects depends on the mass of the
bodies
and the distance between them. ___________________________________
_____14. To accelerate two objects at the same rate, the force used to push the object that has more mass should be
larger than the force used to push the object that has less mass. _____________________
_____15. If none of the force on an object is in the direction of the object’s motion, no work is done. __________
_____16. A machine can make work easier for you is by increasing force, increasing distance, or changing direction of the
force. ________________________
_____17. The increase the force applied by a lever, move the fulcrum closer to the load. _____________________
_____18. An inclined plane requires a smaller force to accomplish the same amount of work, by applying the force over a
shorter distance. ______________________________________
_____19. A pulley can be thought of as an inclined plane wrapped around a cylinder. __________________
_____20. Energy of motion is called potential energy. ____________________________
_____21. The skier’s gravitational potential energy is greatest at Point a. _____________
_____22. As the skier races down the slope, kinetic energy increases. ________________
_____23. The skier has equal amounts of potential and kinetic energy at point A. _______
_____24. The ski slope represents a closed system because energy is not created or
Destroyed, just transferred between different forms.
_____25. When energy changes from one form to another, some of the energy always changes into light energy. _____