Torrey Pines High School
College Prep Physics
Book Problem Solving Guide
2010-2011
Unit 1:
Chapter 2 CQ:
9. Find which vehicle has the greatest average speed. To do this, divide the total distance traveled by the time. To find the distance traveled, subtract the lower numbered milepost from the higher numbered milepost.
10. See #9
19. Think about a real truck. The truck goes, stops, speeds, up and slows down. The driver stops for breaks. You can compute his average speed but this tells you nothing about his speed at any particular time.
23. Speed is a scalar because it has no direction. Velocity is a vector because it has direction.
30. Acceleration is change in velocity over change in time. Do the division for both cars.
Exercises:
1. Multiply the miles by the conversion factor. This cancels miles and gives km.
2. Multiply by the conversion factor.
3. Subtract the two mile markers to get distance traveled. Divide distance by time to get speed.
4,5: Speed = distance/time
6. Convert the time to seconds, then divide distance by time
7. Distance = speed multiplied by time
8. Convert hours to seconds. Multiply by speed
9. Average speed = distance/time. The time here is 3 hours
10. See #9
11. Time = Distance/speed
12. See #11
13. Find the speed needed to do the marathon in 24 hours. Compare it to the runners maximum speed
14. See #11
Workbook
Chapter 2:
7. Do not average the two speeds. Find the total distance and divide it by the total time.
8. See #7
11. Find the total distance and divide it by the distance traveled in a day
12. see 11 for a similar problem
31. The problem is in two parts. Part 1 is from 0 to 10 seconds. This part uses speed times time = distance. The second part is from 10s to 20s. This part uses x = vot + 1/2at2
32. See #31 for a similar problem
Unit 2:
13 and 14. The V stands for velocity, the a for acceleration. The object speeds up when the vectors add, slow down when they subtract. The object will turn when it accelerates to one side.
30. The velocity is such that the ball follows the red trajectory. The acceleration is always straight down.
31. Same. The acceleration is the same for all objects when we disregard air resistance. There is no air on the moon anyway. The objects will always accelerate downwards.
36. The bike must be launched at an angle or it will fall off the cliff. As soon as it leaves the cliff it will start to fall. Try drawing a picture.
Exercises:
1. Set either east or west as negative. I set west as negative. Subtract 10 from 5. That is a change of 5. For part c subtract -5 from 10. That is a change of 15.
2. See #1
3. Make a triangle with vectors
11. Horizontal speed does not change. Vertical speed changes by -9.8m/s2.
13. To find vertical displacement (height) find how far an object freefalls in 5s. To find how far the object goes horizontally multiply the horizontal speed by the time.
14. Divide how far it lands from the edge by the time it takes to fall from that height.
15. Think about what velocity the ball has in the vertical direction when it hits the ground. Calculate how long this change in velocity will take when a = -9.8m/s2. Then multiply this time by the horizontal velocity to get horizontal displacement.
Workbook:
1-6 See book #1
7-8 Plug in to formula
13-14 Horizontal velocity does not change. Vertical velocity changes by -9.8m/s2
15-20: Always split into separate x and y directions.
15. Find the time to fall 19.6m. Multiply time by velocity to get displacement
17. Find the vertical velocity at 3s and do vector addition of the x and y velocities
19. Find the range because this is how far the ball will go and how far the receiver needs to be. To find range find the time of flight of the ball and then multiply it by the horizontal velocity.
Unit 3
CQ:
1. Straight down because you, the snacks, and the plane all have the same velocity. The net force on the snacks is gravity which pulls them straight down.
3. No because there is a net force.
4. Friction
17. If they act together the force is 130N if opposed it is 50N.
18. Together is 1400N, opposed is 0N. At some angle between 0 and 180 the force must be 1000N.
38. At terminal speed a = 0 so the forces must be equal and sum to 0.
45. 250N to overcome friction
49. Same, Newton’s 3rd law
Exercises:
4. Draw a diagram. Draw in the resultant of the two forces. You should see that a 5N force would cancel the resultant.
5-18: F= ma
19. Remember that Fnet = ma so find the net force. Add 90N to it because the net force must overcome the frictional force as well.
20-24 Draw pictures and remember that net force is what makes acceleration happen
Workbook:
11. We use the equation F = ma. Make up values. Say that the red has a mass of 1kg and plug in.
13. This is a two part problem. In part 1 use F = ma to find a. In the second part use kinematics where a is your a from the first part
15. It must be greater or the crate would not slow down. Up. Yes.
19. Find the upwards force needed to accelerate the man. The net force = 85kg times 1.2m/s2. This force should be added to gravity because the upwards force must overcome gravity. And still have the net force needed to accelerate him upwards.
33. Remember that frictional force = normal force times the coefficient of friction
34. Divide the force needed by the number of dogs.
Unit 4:
Chapter 7
8. Make up numbers and plug them into the formula
22. The total energy stays the same but the amounts of potential and kinetic energy change. When the satellite is close to the earth the potential is low so the kinetic is high and when it is far from the earth the potential is high so the kinetic is low.
23. Same because A is bigger and when multiplied by cos theta it will equal B
24. Different because when A is multiplied by cos theta it is smaller than B
31. The student can set 0 anywhere they want
32. Turns to heat because friction does work
Exercises:
5. Think about what is happening. The cars have energy before so calculate their energies separately and add them. After the collision, they are together so add their masses. Calculate the energy they have together. Subtract after from before.
9. The increase in ke is 22J. That is the amount of work done to the object. Work = force times distance
15. Work done by gravity = the change in potential energy
17. gravitational energy is converted to kinetic
19. Do mgh for h = 12
21. Power = work/ time = energy/time
Workbook
1-4, 11-14
11. The kinetic energy of the ball must equal the work done by the hand
Unit 5:
Chapter 6:
CQ:
1. It is not created or destroyed
3. Toby because momentum is a vector and adds to zero before the collision
9. Padding increases the time of a collision. Impulse = forceXtime so if time increases, force decreases.
16. If it doesn’t bounce the change in v is 4. If it bounces the change in v is greater than 4. The a and therefore the force is greater when it bounces.
18. See #16
23. Multiply them out
41. Same before and after. Any explosion involves forces that come from within the object and therefore momentum cannot change. In order for momentum to change there must be a net outside force.
Exercises:
7. Impulse = momentum
11. The change in speed is 16m/s and this must be plugged into the formula
14. Momentum forward must = momentum backward
17. Calculate for both and do a vector addition.
Workbook:
1. The change in velocity is 10.
9-10. Calculate the weight using m and 9.8. In 9 you can make up the mass of the passenger as 1kg. Plug this in as force into mv = Ft
11. The change in v is 90m/s
13-16: Momentum forward must = momentum backward
17-22: Calculate p before the collision. This must = the p after the collision
Unit 6:
1. Same because they both turn through the same number of degrees per second.
2. Same, see #1
18. When the people are holding the ladder anywhere other than the center of mass, we do not think about it in terms of force, we think about torque. Also, there is one person on one side of the center of mass and another on the other side. If one person were holding the ladder it would be easier to hold it by the middle. On the other hand, if two people are holding it, we think of it like a seesaw. It is easier to generate torque farther from the center so Dana can exert less force than Loren.
49. With spin, an object has angular momentum that keeps the object travelling in a straight path. This is why good quarterbacks throw spirals and bad quarterbacks throw ducks.
Exercises: 1-14
1. Divide 1000 by 5 for per minute, then divide this by 60 for per second
3. The minute hand goes around once each hour
5. The change in velocity is 7490 and it happens over 3 seconds so 7490/3.
7. Torque = forceXdistance
13. Add the torques on one side and subtract the torques on the other side. If they equal 0 they balance.
Workbook: 11-12
1. Torque = forceXdistance and they must balance
11. Multiply acceleration by time to get velocity
12. Use the analogous kinematic formula. Assume initial velocity is zero and plug in to x = v0t + 1/2at2
Chapter 13:
CQ:
9. Yes because they may not have been in contact for enough time
16. Gallon is more
20. Shape
33. Steam has more potential energy. A gas releases energy when it turns into a liquid. This energy is heat that does damage
38. Heat takes longer to transfer into rug than into tile. What feels cold to you is heat leaving your body.
46. To do this question you must know that hotter things lose heat at a higher rate. The hot coffee will lose heat faster than coffee that has cream added so add the cream first to slow down the rate of heat loss.
Chapter 14 21
CQ
9.
The tube is needed to bring cold water from the deep ocean because any heat engine needs cold and hot.
16. They operate at a lower Th
20. Make up two sets of data, for example 100K and 200K and 300K and 400K and see which is best.
21. Hotter hot source
38. Each flip is independent of the other flips
46. Any example of disorder increasing
Exercises:
1,2,3,4,
Electricity Unit:
Ch. 20
E:
4. Divide 1 by the number of coulombs in one electron
5. Plug in to Coulomb’s law
6. Coulomb’s law
7. In this problem there is one electron and three protons separated by .018X10-9m
19. Force = fieldXcharge
21. The change in energy is 50J. The potential energy must be -10J since its overall energy cannot change. 40 = 50 + -10
22. Energy = voltageXcharge
23. See 22
24. See 22
Ch.21
CQ:
1. The voltage is the same for all batteries except 9 volts which are six 1.5 volt batteries in series. The current depends upon what circuit it is hooked up to. The larger a battery is the longer the lifetime
3. In series, add the voltages.
4. In parallel the voltage is the same as each battery. Never hook batteries of different voltages in parallel
7-10: A and C light but B and D do not. In D there is no way for current to flow from positive to negative, in B the current will flow between the batteries and avoid the light bulb which is a resistor if we assume the wires have no resistance. If we assume the wires have resistance, then the two batteries are trying to push current through the light in opposite directions so they just push current through the wires connecting the two of them. In C the voltage is double that of A and since resistance is the same, the current will be double. The power is quadruple since P = IV. Since current is double in C the batteries last half the time of one battery alone and one fourth the time of A because is A each battery provides half the current.
19.
23.
34.
35-38: Try making up a voltage and a resistance for each bulb. The resistance of each bulb must be equal. Solve for current.
41. More current flows where there is less resistance
47-52: See 35-38
E: V = IR and P = IV and P = I2R
Assume any unknown wall socket is either 110V or 120V whichever you like
Remove 25 and 26.
Workbook:
7-39 all
9. Draw all possibilities. Calculate to make sure they are not the same.
10. Draw all possibilities and calculate to make sure none are the same
11. Calculate the resistance in the first case (2 Ohms). Calculate it in the second case (8 Ohms) To increase resistance we need series. To get from 2 to 8 we add 6.
13. When there is only one set of resistors and they are parallel the voltage of each resistor is the same as the voltage of the circuit
19. Assume a 120V circuit
21. If it is not apparent then assign a voltage and assign the same resistance to each bulb. Calculate the power of each bulb and that gives the brightness