Physics 1-5
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Fact:
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A statement about the world that competent observers who have made a series of observations agree on
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Law (or principle)
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A general hypo or statement about the relationship of natural quantities that has been tested over and over again and has not been contradicted
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Hypothesis:
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An educated guess; a reasonable explanation of an observation or experimental result that is not fully accepted as factual until tested over and over again
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Theory:
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A synthesis of a large body of information that encompasses well-test and verified hypotheses about certain aspects of the natural world
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Inertia:
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The property of things to resist changes in motion
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Newton's 1st law (law of inertia)
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Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force
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Force:
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A push or pull
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Mechanical equilibrium:
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The state of an object or system of objects for which there are no changes in motion
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Equilibrium rule:
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For any objects or system of objects in equilibrium, the sum of the forces acting equals zero
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Asteroids have been moving through space for billions of years. What keeps them moving?
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Nothing keeps asteroids moving. They are \"happily\" moving through space in a state of dynamic equilibrium. Objects like the Sun and Earth can change their motion but after this brief interaction they again move off at some constant speed
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In tearing a paper towel or plastic bag from a roll, why is a sharp jerk more effective than a slow pull?
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The sharp jerk demands a large acceleration of the paper towel roll and the inertia of the roll resists this change and only one sheet tears off. A slow pull causes a small acceleration - almost none - and hence the inertia doesn't resist the unrolling of the paper towels
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Can you say that no force acts on a body at rest? Or is it correct to say that no net force acts on it?
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We know that if a body is at rest the sum of forces (net force) acting on the body is zero. This is a much different statement than saying that there is no force acting on a body at rest.
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When you push downward on a book at rest on a table, you feel an upward force. Does this force depend on friction?
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The force you feel when pushing down on a book on a table has nothing to do with friction - it is the support force of the table pushing up on the book that you feel. It would make no difference if the book was on a smooth or rough surface. You would feel the force due to friction if you pushed the book along the surface.
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If you pull horizontally on a crate with a force of 200 N, it slides across the floor in dynamic equilibrium. How much friction is acting on the crate?
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Since the crate you are pulling is in dynamic equilibrium it means that the net force (sum of forces) acting on the body must add to zero. This means if you pull with a force of 200 N friction must be opposing the motion (in the opposite direction to your pull) with a force of 200 N
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Crate @ rest on factory floor. As a pair of workmen begin lifting it, does the support force on the crate provided by the floor increase, decrease or remain unchanged? What happens to the support force on the workman's feet?
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The support force on the crate reduces as the men lift it since the force of the crate pushing on the floor is reduced. The support force on their feet increases since there is now more force pushing down on their feet - the \"load\" has been transferred from the crate to the men.
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speed:
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how fast something moves; the distance traveled per unit of time (speed= distance/time)
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average speed:
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the total distance traveled divided by the time of travel (av speed=total distance/time interval)
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velocity:
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the speed of an object and a specification of its direction of motion
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vector quantity:
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quantity in physics that has BOTH magnitude and direction
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scalar quantity:
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quantity that can be described by magnitude without direction
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acceleration:
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the rate at which velocity changes with time; the change in velocity may be in magnitude, or direction, or both (acceleration= change of velocity/time interval)
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free fall:
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motion under the influence of gravity only
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velocity aquired in free fall (from rest):
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v=gt
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distance fallen in free fall (from rest):
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1/2gt^2
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One airplane travels due north at 300km/h while another travels due south at 300 km/g. Are theirs speeds the same? Are their velocities the same?
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The speed of each airplane is 300 km/hr, regardless of their direction. The airplane velocities are not the same because velocity is the speed AND direction of the body. Their velocities are of the same magnitude, or size, but in opposite directions.
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Can a car with a velocity toward the north simultaneously have an acceleration toward the south?
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If your car is travelling North and you apply the brakes to slow it down it will have an acceleration in a direction to the South - so yes you can have a car travelling North and accelerating to the South.
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Constant speed, but varying velocity? How can this be achieved?
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An object moving in a circle (eg. moon in orbit or a car going around a track) can have a constant speed but it will not have a constant velocity since it is changing direction.
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Something that undergoes acceleration while moving at constant speed. What about something that accelerates while traveling at a constant velocity?
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a car moving around a track has a constant speed and since it is turning it is accelerating (changing direction and hence velocity implies acceleration). Since acceleration is defined as a change in velocity it is not possible to have a constant velocity AND be accelerating.
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Which ball reaches the end first?
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The ball in the middle (see below) reaches the bottom first. The reason is that the ramp for this ball is steepest at the start and hence the acceleration - due to gravity - will be greatest. This leads to a greater average speed, even though the other ramps are steeper at the end
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Which is greater, an acceleration from 25 to 30 or one from 96 to 100 if both occur during the same time?
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Acceleration is defined as the change in speed (or velocity) divided by the time interval of the change. The 25 km/hr - 30 km/hr change is 5 km/hr compared to the 4 km/hr change for the higher speeds and since the time interval is the same, the 25-30 km/hr change results in the greatest acceleration
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If air resistance can be neglected, how does the acceleration of a ball that has been tossed straight upward compare with its acceleration if simply dropped?
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The acceleration for the thrown ball and the dropped ball are the same (10 m/s2).Once the ball leaves the thrower's hand the only force, and hence acceleration, the ball experiences is die to gravity.
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While rolling balls down an inclined plan, Galileo observes that the ball rolls 1 cubit (the distance from elbow to fingertip) as he counts to 10. How far will the ball have rolled from its starting point when he has counted to 20?
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From our discussion of distance traveled under acceleration due to gravity (free fall) we found that d is proportional to t2. Since the time Galileo counted was twice as much (20 seconds instead of 10) the distance traveled will be 4 times as much (2x2= 4).
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As speed increases for an object in free fall, does acceleration increase also?
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: No, the acceleration due to gravity (the only acceleration a body in free fall experiences) is the same. The increased speed comes from the constant acceleration
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Two balls are release simultaneously from rest at the left end of equal-length tracks A and B. Which ball reaches the end of the track first?
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The ball released along track B (see below) reaches the end of the track first as it is moving at a faster speed than the ball along track A for more of the trip (i.e. its average speed is greater).
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volume=
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the quantity of space an object occupies
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Netwon's second law
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The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object
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Netwon:
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Si unit of force, one N is the force that will give an object of mass 1kg an acceleration of 1 m/s2
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Kg
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fundamental unit of mass, 1 kg=mass of 1 L of water at 4 Celcius
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free fall:
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motion under the influence of gravitational pull only
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terminal speed:
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the speed at which the acceleration of a falling object terminates because air resistance balance its weight. when direction is specified, then we speak of terminal velocity.
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weight=
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mg
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acceleration=
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fnet/m
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force=
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ma
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A 400kg bear grasping a vertical tree slides down at constant velocity. What is the friction force that acts on the bear?
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The bear is sliding down the tree at a constant speed (dynamic equilibrium) so the acceleration of the bear (and net force on the bear) is zero. This means that the force on the bear due to friction balances the force on the bear due to gravity. Ffriction = Fgravity = ma = (400 kg)*(10 m/s2) = 4000 N.
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Does the mass of an astronaut change when he or she is visiting the International Space Station?
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No, the astronaut's mass is a property of their body. The astronaut's weight will change because the force acting on them due to gravity is vastly different on the space station than on Earth but their mass will remain the same.
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What weight change occurs when your mass increases by 2 kg?
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Weight = mass * gravity = 2 kg * 10 m/s 2= 20 N.
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Explain how Newton's first law of motion can be considered to be a consequence of Newton's second law?
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The first law states that an object will stay at rest or at constant velocity until a force acts on it. The second law states that the acceleration of a body is equal to the force acting on the body divided by its mass. If no force acts on a body it will have no acceleration and hence either remain at rest or remain at a constant velocity. This shows that for the case of zero force, the 1st and 2nd laws are equivalent, or that the first law is a consequence of the 2nd law
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When blocking in FB, a defending lineman often attempts to get his body under the body of his opponent and push upward. What effect does this have on the friction force between the opposing lineman's feet and the ground?
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When the opposing lineman is lifted, less of his weight is on his feet, and the supporting force is reduced. The result is that the force due to friction is reduced since it depends on the supporting force.
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Free mall is motion in which gravity is the only force acting. a) Is a skydiver who has reached terminal speed in free fall? b) is a satellite above the atmosphere that circles Earth in free fall?
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Remember that free fall means only gravity is acting the body: (a) The skydiver is not in free fall since they are experiencing air resistance (terminal speed); (b) the satellite is in free fall since it is falling toward Earth - as it is orbiting.
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As you leap upward in a standing jump, how does the force that you exert on the ground compare with your weight?
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The force exerted on the ground as you jump is greater than your weight, which is why you are able to move upward. If the jump wasn't greater than your weight there would be no net upward force and you wouldn't leave the ground. (
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What is the acceleration of a rock at the top of its trajectory when it has been throwin straight upward?
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The acceleration of a rock at the top of its trajectory is 10 m/s2 in a downward direction. This is consistent with Newton's second law since at the top of the trajectory the rock is changing direction from going up to coming down. For an instant the velocity there is still a constant rate in the change in velocity
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What is the net force acting on a falling 1 kg ball if it encounters 2N of air resistance?
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A 1 kg ball has a weight of (1 kg * 10 m/s 2), which is equal to 10 N. Air resistance is a force in the opposite direction to a falling weight so the net force acting on the ball is 10 N - 2 N = 8 N.
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Newton's 3rd law:
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Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first
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Vector quantity
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a quantity that has both magnitude and direction. Examples: force, velocity, acceleration
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scalar quantity
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a quantity that has magnitude but not direction. Examples: mass, volume, speed
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vector:
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an arrow drawn to scale used to represent a vector quantity
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resultant:
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the net result of a combination of two or more vectors
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components:
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mutually perpendicular vectors, usually horizontal and vertical, whose vector sum is a given vector
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How does the terminal speed of a parachutist before opening a parachute compare to terminal speed after? Why is there a difference?
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The terminal speed of a parachutist prior to opening their chute is higher than after. The reason is that the larger area of the chute will experience more resistance to the air because of its larger area.
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Why is it that a cat that accidentally falls from the top of a 50 story building hits a safety net below no faster than if it fell from the 20th story?
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The cat reaches terminal velocity at or before falling 20 stories and therefore falling from a higher point doesn't cause the cat to go faster.
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Does a baseball bat slow down when it hits a ball?
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A baseball bat will slow down when it hits a baseball since Newton's third law states that the ball will act on the bat with the same force as the bat imparts on the ball. There will be a slow down due to this force.
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If a Mack truck and Honda Civic have a head-on collision, upon which vehicle is the impact force greater? Which vehicle experiences the greater deceleration?
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Both vehicles will feel the same force due to Newton's third law. Due to the smaller mass of the Honda Civic, the acceleration (deceleration) for the car will be greater than that for the more massive truck
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What can you correctly say about two vectors that add together to equal zero?
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Two vectors that add together to be equal to zero must have the same magnitude (length) and the opposite direction.
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Why does vertically falling rain make slanted streaks on the side windows of a moving automobile? If the streaks make an angle of 45 degrees, what does this tell you about the relative sped of the car and the falling rain?
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Driving a car in the rain results in slanted streaks on the side window of a car because of the relative motion of the car with respect to the rain. We saw in class that a right angle triangle formed with two equal sides will give a hypotenuse angle of 45° (see below). Since the streaks are at a 45° angle it means that the car and rain are moving at the same speed (the car in a horizontal direction, and the rain downward in a vertical direction
