Regents Physics Exam Preparation Guide

Regents Physics Exam Preparation Guide This guide outlines the course, points out most key concepts and skills, and includes practice questions throughout. Make sure to study the whole guide. Preparation for Regents exam should NOT be limited to this guide. You should use this guide to help you focus on the essential areas you must review in the materials and resources received throughout the year. 1. Units a. The MKS (meter-kilogram-second) system is also known as the SI system. b. Base units: meters, kilograms, seconds, Amperes c. Derived units are combinations of base units for example: m/s, Newton (kgm/s2), Joule (Nm), Watt (J/s)… i. Practice: Express the following in terms of base units. (Hint: Use equations.) 1. Newtons 2. Joules 3. Watts d. When estimating you should know that i. 100=1, 10-1=.1, 101=10, etc. ii. A nickel has a mass of 5 grams. iii. A Newton is roughly a quarter pound. iv. A meter is approximately a yard. v. Practice: 1. The mass of a shoe is closest to: 10-1kg, 100kg or 101kg? 2. The length of a desk is closest to: 10-1m, 100m or 101m? 3. The weight of a large football player, on earth, is about: 101N, 102N or 103N? (answers: 1. 10-1kg 2. 100m 3. 103N) e. You should be able to convert between units using conversion factors. i. Convert 100mg to kg (ans=1x10-4kg)(hint: convert mg to g, then g to kg) ii. convert 1000MeV’s to Joules: (ans=1.6x10-10J) 2. Graphing a. Always start the axes at (0,0) b. Use a consistent scale, i.e., make each box worth the same amount. c. Never use “breaks” on either axis. d. Never connect the dots with line segments. i. Draw the best-fit straight line with a straight edge if the data is practically linear. (y=kx) ii. Draw the best fit smooth curve for non-linear data (usually parabola (y=kx2) or hyperbola (y=k/x), or inverse-square which also looks like a hyperbola y=k/x2). iii. Practice: State the shape of the following graphs. Choose from straight line, parabola or hyperbola: 1. Force on spring vs. change in length: _________________ 2. D vs. t for accelerated motion: _________________ 3. Energy of a photon vs. wavelength: _______________ 3. Vectors vs. Scalars a. Vectors have both magnitude (amount) and direction while scalars have only magnitude. b. If you can draw the quantity with an arrow, it is a vector. Examples are: force, velocity and momentum. Scalars are not drawn. Some scalars are mass, energy, and power. c. Speed is a scalar, but speed with direction is a vector called velocity. d. Distance is to displacement as speed is to velocity. e. When adding vectors like forces or velocities, arrange them head to tail without rotating them. f. Practice: Add the following vectors head to tail roughly to scale, and draw the resultant: i. Vx=10m/s + Vy=20m/s ii. D1=100m Northeast plus D2=200m South. iii. Fx=100N plus Fy=-100N g. To get the components of a vector, e.g. velocity, use Vx=Vcos(angle) for the adjacent side, and Vy=Vsin(angle) for the side opposite the angle. 4. Acceleration: a. ..is the amount that velocity changes by every second. So, an acceleration of 4m/s2 means an object is speeding up by 4m/s each second. b. If an object has a velocity of +100m/s, and accelerates at -10m/s/s, it is slowing down by 10m/s each second. If it maintains that acceleration for c. d. e. f. longer than 10s, it reverses at t=10s and then speeds up in the NEGATIVE DIRECTION by 10m/s/s. A ball thrown vertically upward at 40m/s with g=-10m/s/s, would reach the peak at t= _________, then return to its starting position with a velocity of -40m/s after a total time of _____________. (ans: 4,8) Motion graphs: You should be able to graph position, velocity and acceleration vs. time for constant acceleration and constant velocity. If velocity is constant, predict what the graphs would look like. (The position graph is a diagonal line, velocity graph is flat and acceleration is zero.) If acceleration is constant and forward, …think….(the position graph will be parabolic, concave up. The velocity graph is a diagonal line and the acceleration graph is flat.) 5. Projectiles a. Keep horizontal (x) and vertical (y) quantities SEPARATE!! Make an x-y chart and never mix these quantities in the same equation. The only exception is if you know the final Vx and Vy, and you need the resultant final V. Then you use Vx and Vy in the Pythagorean theorem. b. If a projectile is launched horizontally, Vyi=0. c. If a projectile is launched at an angle, get Vx and Vyi as discussed above in part 3-g. d. You are free to choose the pos/neg directions, but usually up is taken as positive and down is negative. If a ball was thrown upward, but is now on the way back down, the displacement is positive because the ball is above the start point, but the velocity is negative because it is moving downward. e. In the X-direction, use only dx=vxt. (accelx =0 so no other equations work.) f. Use the Y-direction to find time, then use dx=vxt to find the horizontal distance, or “range” of the projectile. g. Practice: i. A ball is kicked from ground level with a velocity of 20m/s at 30 degrees above horizontal. Determine the horizontal displacement as it lands: (t=2s, dx=34m) ii. Determine how high the ball went: (t=1s, dy max = 5m) 6. Newton’s First Law a. An object’s velocity will not change unless and unbalanced force makes it change. This is known as Newton’s first law, the law of inertia. b. Example 1: A car traveling at constant velocity. Which force is greater – the force of traction making it go forward, or the force of friction resisting? They are EQUAL!! If they weren’t the velocity would change. More traction force means going faster, more friction means slowing down. c. Example 2: A bullet fired in outer space (no gravity) will continue in a straight line at constant speed until it hits something. Also, in a head on car crash, passengers without seatbelts tend to go forward into the window because they are objects in motion who tend to stay in motion. Wear your seatbelt!! d. Inertia is measure by mass. Which has more inertia: a train at rest or you on a bike at top speed? It is the train. It has more mass and is hard to get moving. 7. Newton’s Second Law a. Fnet=ma. This means if there is a net (unbalanced) force there will be an acceleration (change in velocity). b. Example: If the forward force on an 8kg object is 12N, and the opposing force is 8N, determine the net force and acceleration. (4,.5) c. Example: If a 100N object is in freefall, and there is a 40N drag force, what is the acceleration? (100N-40N=10kg(a) so a=6m/s/s) d. You also need to be able to draw force diagrams. Practice with a mass on an incline with friction, then a mass hanging from a clothesline. Look them up in the notes or review book if you are not an expert! e. Remember that the coefficients of friction depend only on the type of surfaces, and NOT on the surface area in contact. Kinetic=sliding, static=no sliding. 8. Newton’s 3rd Law is the law of action-reaction. Whenever any two bodies interact they ALWAYS apply equal and opposite forces to each other. This includes the earth and you (gravity), a brick breaking a window, a bullet fired from a gun, a baseball bat hitting a ball,…) 9. The Law of Universal Gravitation a. If two planets move twice as far apart, what happens to the force of gravity they apply to each other? Of course it would be quartered since the law of universal gravitation is an inverse square law. b. If the masses of two stars both are halved, what happens to the force of gravity between them? Same answer (quartered) since both m’s are in the numerator. c. Where does gravity’s strength rank among the 4 natural forces? The four forces are gravity, electromagnetic, strong nuclear and weak nuclear. Gravity is by far the weakest of these. (Gravity-Weak-EM-Strong) 10. Circular Motion a. During circular motion, the net force and acceleration are toward the center. That means they are “centripetal”. b. The centripetal force could be tension if swinging a mass in a circle at the end of a string. It could be gravity (satellites) or it could be friction (car around track). c. Use Fc=mac = mv2/r. If r doubles what happens to F? a? (both halved) If v doubles, what happens to F? a? (both quadruple). If m doubles with no change in r or v? (a=no change, F=doubles) 11. Impulse and Momentum a. Momentum is conserved in all collisions and recoils, and when two bodies pull together with a spring or gravity. b. Usually the system is at rest either before or after the collision/recoil which means the total momentum is ZERO and the 2 bodies have EQUAL and OPPOSITE momentums. c. For more complicated problems, use m1v1i + m2v2i = m1v1f + m2v2f (not on reference table.) d. Impulse = change in momentum. J=mv is not on the reference table either. Set m v = Ft to solve most impulse problems. e. The area under an F-t graph is impulse. 12. Work, Power and Energy a. Work is the energy given to an object. We use W to stand for potential or kinetic energy in equations like V=W/q and W=Pt. b. Work = Fd. The force and displacement must be IN THE SAME DIRECTION when using this equation. c. While carrying a pizza at constant speed you are NOT doing work on the pizza since your force is UP but displacement is FORWARD. Also, the pizza gained no PE since it didn’t go higher, and didn’t gain KE since it didn’t go faster. d. The area under what type of graph would equal the work done? (F vs. d) e. If you are using a pulley and do 100J of work to lift a 10N box 8m higher (80J of PE), how much work was done against friction? (20J) f. Power is not how much work is done, but how FAST. Power is the RATE at which energy is produced/delivered. P=Fv gives you the average power if you use average velocity, and instantaneous power if you use the velocity at that instant. 13. Conservation of Energy a. When a system is allowed to move on its own – no engine, no applied force – you will want to use conservation of energy: ETi = ETf. b. ET means total energy – the total amount stays constant just changes forms. ET includes KE, PE and Q (heat) when friction is present. c. Work done against friction can be calculated with Wf=Ffd d. PE can be gravitational, (PE=mgh) or in a spring (PE=1/2kx2.) 14. Static Electricity a. Electric fields are drawn from (+) to (-), and don’t cross. b. The electric field between parallel plates is uniform, and puts a constant force on a charged particle at any point in the region. c. “Charge by induction” is when one charged object re-arranges the electrons on another object from a distance. Like when a negative balloon moves toward a wall, the wall’s electrons are repelled leaving a (+) charge near the balloon, causing an attraction. No electrons are transferred. If they are actually transferred during contact, that is called “conduction”. d. If you want eV’s of energy to move a charge through a potential, use W=qV with q in elementary charges (e). If you want Joules of energy use Coulombs of charge instead. 15. Circuits a. Series Circuits i. Ammeters connect in series so current flows through it. ii. Voltmeters are connected across an element in parallel to measure the potential DIFFERENCE between two different points. iii. Calculate the total resistance to get the current. iv. The current in the whole circuit is the same at any point. v. If V=90V, R1=15, R2=30, find the current and voltage drop across R1. (2A, 30V) vi. If one resistor is removed the current stops, everything turns off. b. Parallel Circuits i. The currents in each branch add up to the total current. ii. Each resistor is connected independently to the power source. iii. If one resistor is removed the others operate the same, unchanged. iv. Don’t forget the FLIP when using the 1/R equation! 16. Magnetism a. Magnetic field lines are drawn from N to S poles, and don’t cross. b. Field strength is a.k.a. “flux density”. Measured in Teslas (Weber/m2) c. Webers are the unit for the total # of magnetic field lines. d. Magnetic fields are created by moving charges, usually electrons. e. Domains are areas where electrons in iron spin the same way. When you put a magnet to a piece of iron, the domains align and the iron is temporarily magnetized. f. The earth’s “north pole” is really a magnetic south. Obviously, since the N pole of compass is attracted to it, and opposites attract. 17. Waves a. Amplitude of a wave (height) is a measure of the total energy in the wave. b. Waves transfer energy but not matter. c. Sound is longitudinal and cannot be polarized. d. Light and most other waves are transverse. e. A wave front is a peak. Drop a rock in a pool and see a circular wave front. f. Doppler Effect = the change in frequency due to motion. If the source is approaching, higher frequency is perceived. g. Diffraction is when waves bend around an edge. Particles don’t do this. h. Constructive interference occurs when waves overlap in phase, destructive interference occurs if the waves are 180 degrees out of phase. i. A standing wave is formed by identical waves passing through each other. Nodes are formed at points of complete destructive interference. j. Standing waves are a good example of resonance. Resonance is when waves fit just right and when something vibrates at natural frequency. k. Refraction is the bending of a wave due to a change in speed when going into another medium. Dispersion is the separation of the colors of while light during refraction. Remember violet bends the most because it changes speed the most. l. Remember to measure all angles of incidence, reflection, and refraction from the NORMAL line. 18. Modern Physics a. Mass is a form of energy. If you want energy in Joules, use E=mc2 with mass in kilograms, and c=3x108m/s. b. If you want energy in MeV’s, don’t even use E=mc2, just use the conversion factor: 1 universal mass unit = 931MeV. c. Wave-particle duality i. Light acts like particles during: photon emission from an atom (emission spectrum), photoelectric effect, and photon collisions. ii. Light acts like a wave during diffraction and interference. iii. Particles act like waves, diffract and form an interference pattern when fired through a double slit, but only if the slits are not directly observed. d. The energy of a photon is equal to the difference between the energies of the energy levels of the electron’s transition. e. Lower energy photons make longer wavelength light. E=hc/ f. Quarks are never observed alone, only in triples (baryons, like protons) and paired with an anti-quark (mesons). g. Quarks and leptons (like electrons) are fundamental (elementary) and cannot be broken into smaller pieces. h. Anti-particles have the same mass as particles, but opposite charge. They also annihilate with normal matter to produce pure EM energy according to E=mc2.