Don't forget .... there could be a quiz next class. Ooohhhhhhh.............
1. Finish the Moon/Earth problem from last class.
2. A ball kicked upwards (at the edge of a cliff) with an initial speed of 35 m/s. It goes up, achieves apogee, and comes down along the side of the cliff, landing 20 m below where it was launched. Find the total time in air. I suggest the approach wherein you use the quadratic formula, but any good approach is fine by me.
3. Consider dropping a pebble into a river below you (standing on a bridge). The splash occurs 2.4 seconds after you drop it. How high up is your bridge?
4. A car accelerates at 2 m/s^2 for 5 seconds. Then it maintains its velocity for 10 seconds. Then the driver applies the brakes for 2 seconds to bring the car to a halt.
a. how far did the car travel during the entire trip? (Break it into little chunks of the trip.)
b. draw the 3 related graphs of motion for this trip.
Monday, April 30, 2012
Friday, April 27, 2012
Gravity problems
1. Consider a ball dropped from rest. Find the following:
a. How fast it is traveling after 2.5 seconds of freefall
b. How far it will have fallen in this time
2. You throw a ball straight up into the air with an initial speed of 35 m/s.
a. How long will it take to reach apogee (highest point)?
b. How high will it rise?
c. How long will it take (total) to return to your hand?
d. What will be the velocity immediately before returning to your hand?
3. An object falls to the surface of a new planet from rest, starting at 10 m above the ground. It takes 2 seconds to hit the ground. What is g on this planet?
4. How does the time to fall from a certain height on the Moon compare to the time to fall from the same height on Earth? (g on the Moon is 1/6 that of Earth) Determine a general relationship, if you can. If not, pick a specific height.
a. How fast it is traveling after 2.5 seconds of freefall
b. How far it will have fallen in this time
2. You throw a ball straight up into the air with an initial speed of 35 m/s.
a. How long will it take to reach apogee (highest point)?
b. How high will it rise?
c. How long will it take (total) to return to your hand?
d. What will be the velocity immediately before returning to your hand?
3. An object falls to the surface of a new planet from rest, starting at 10 m above the ground. It takes 2 seconds to hit the ground. What is g on this planet?
4. How does the time to fall from a certain height on the Moon compare to the time to fall from the same height on Earth? (g on the Moon is 1/6 that of Earth) Determine a general relationship, if you can. If not, pick a specific height.
Tuesday, April 24, 2012
Hw for c
Consider a runner, starting from rest. What acceleration would they need to reach 8 m/s in 20 s? Assume that their acceleration is uniform. Also, how far will they go during this run?
Derive the fifth equation of motion - in which the initial velocity is missing.
Keep working on lab.
Write and solve your own motion problem.
Monday, April 16, 2012
Velocity Lab (formal) homework
C - for Wednesday / E - for Thursday
Homework: Read this lab, and come to class with a hypothesis:
How will the speeds compare, ticker tape timer method vs. photogate (LED) timer method?
Here's the lab:
Formal Lab – The Measurement of Velocity
In this experiment, we will determine the velocity of a cart by 2 methods:
• Photogate timer
• Ticker tape timer
Each method can be quite accurate, though what is actually being measured by each is worth some discussion.
Recall that velocity is calculated by knowing the displacement and the amount of time required to traverse it:
v = d / t
Strictly speaking, this is average velocity. In theory, the average velocity is a mathematical average of all (if that were possible) instantaneous velocity points throughout the trip.
Instantaneous velocity is the type of velocity you receive from a speedometer – it is the velocity at that instant. In the case where the object moves at a constant rate, the instantaneous velocity (at all points) is equal to the average velocity. That should be the case (approximately) for this lab. We will determine the extent to which this idea is true in this lab. In this lab, you may work in cm OR m – be consistent.
Procedure
1. Set up a path for your car to travel – 1 meter should be long enough. Place your motorized car on it.
2. Attach a piece of timer tape to the card and ready the cart for motion.
3. Place a photogate timer at some point along the cart’s path. Place a flag on the cart – it must break the light gate fully. Ready the photogate for timing. Measure the width of the flag for future reference.
4. Set the tape timer and note the frequency of operation. Turn on the tape timer.
5. Turn on the car and allow it to run the length of the path.
6. Remove the tape and write the time value from the photogate on the tape for future reference.
7. Repeat for 2 different cart trials, using new tapes each time.
Analysis I – the Ticker Tape Timer
• Examine the ticker tape. If the car is traveling at a uniform velocity, how should the dots appear? Verify that this does occur.
• Starting with the first clear dot, measure the distance that each consecutive dot is from the first dot. Recall the frequency of the timer – this determines the time intervals. For example, if it is set at 10 Hz, the time between each dot is 1/10 of a second. With this in mind, write down the first 30 or so total displacements from the first point. The corresponding times (for 10 Hz) are 1/10, 2/10, 3/10, and so on.
• Plot total displacement versus time on a graph. What type of relationship is it? Does this seem correct?
• Find the slope of the graph. What does this represent?
• What would a (displacement vs. time) graph of an accelerating car look like? How about a decelerating car? How about a car moving backwards with constant velocity? Draw these in your lab.
Analysis II – the Photogate Timer
• Calculate the instantaneous velocity of the car using the time and width of flag.
• Compare, by means of % difference, the velocities from both methods. Percent difference is found by taking one value minus the other value, divided by the average of the two values, and multiplied by 100.
In your conclusion, discuss the relative accuracy of the two methods and give methods for improving the lab.
Homework: Read this lab, and come to class with a hypothesis:
How will the speeds compare, ticker tape timer method vs. photogate (LED) timer method?
Here's the lab:
Formal Lab – The Measurement of Velocity
In this experiment, we will determine the velocity of a cart by 2 methods:
• Photogate timer
• Ticker tape timer
Each method can be quite accurate, though what is actually being measured by each is worth some discussion.
Recall that velocity is calculated by knowing the displacement and the amount of time required to traverse it:
v = d / t
Strictly speaking, this is average velocity. In theory, the average velocity is a mathematical average of all (if that were possible) instantaneous velocity points throughout the trip.
Instantaneous velocity is the type of velocity you receive from a speedometer – it is the velocity at that instant. In the case where the object moves at a constant rate, the instantaneous velocity (at all points) is equal to the average velocity. That should be the case (approximately) for this lab. We will determine the extent to which this idea is true in this lab. In this lab, you may work in cm OR m – be consistent.
Procedure
1. Set up a path for your car to travel – 1 meter should be long enough. Place your motorized car on it.
2. Attach a piece of timer tape to the card and ready the cart for motion.
3. Place a photogate timer at some point along the cart’s path. Place a flag on the cart – it must break the light gate fully. Ready the photogate for timing. Measure the width of the flag for future reference.
4. Set the tape timer and note the frequency of operation. Turn on the tape timer.
5. Turn on the car and allow it to run the length of the path.
6. Remove the tape and write the time value from the photogate on the tape for future reference.
7. Repeat for 2 different cart trials, using new tapes each time.
Analysis I – the Ticker Tape Timer
• Examine the ticker tape. If the car is traveling at a uniform velocity, how should the dots appear? Verify that this does occur.
• Starting with the first clear dot, measure the distance that each consecutive dot is from the first dot. Recall the frequency of the timer – this determines the time intervals. For example, if it is set at 10 Hz, the time between each dot is 1/10 of a second. With this in mind, write down the first 30 or so total displacements from the first point. The corresponding times (for 10 Hz) are 1/10, 2/10, 3/10, and so on.
• Plot total displacement versus time on a graph. What type of relationship is it? Does this seem correct?
• Find the slope of the graph. What does this represent?
• What would a (displacement vs. time) graph of an accelerating car look like? How about a decelerating car? How about a car moving backwards with constant velocity? Draw these in your lab.
Analysis II – the Photogate Timer
• Calculate the instantaneous velocity of the car using the time and width of flag.
• Compare, by means of % difference, the velocities from both methods. Percent difference is found by taking one value minus the other value, divided by the average of the two values, and multiplied by 100.
In your conclusion, discuss the relative accuracy of the two methods and give methods for improving the lab.
Friday, April 13, 2012
fun.
http://autos.yahoo.com/blogs/motoramic/physicist-claims-victory-over-traffic-ticket-physics-paper-151808710.html
Monday, April 9, 2012
Hw
By next class, I will assign a large homework assignment, akin to a take home test. It wil be due two classes later. It will be due in class.
Prepare for this. You may only use your notes - no online resources, books or outside help.
There will be 3-4 questions focusing on all things electrical:
Electrostatics
Circuits
Magnetism
electromagnetic induction
Prepare for this. You may only use your notes - no online resources, books or outside help.
There will be 3-4 questions focusing on all things electrical:
Electrostatics
Circuits
Magnetism
electromagnetic induction
Monday, April 2, 2012
HW, E - sorry for delay
1. What is magnetic flux?
2. What is the unit of magnetic field strength?
3. What voltage is induced in a coil of wire (100 turns) if a magnetic field goes from 0.1 T to 15 T in 2 seconds? The coil has a radius of 0.1-m.
4. Now imagine that the coil above has a 15 T magnetic field hitting it constantly, but the coil rotates around it once every 0.5 seconds. What is the maximum voltage tha will be induced?
5. What is Lenz's law and how does it apply to the falling magnet demo?
2. What is the unit of magnetic field strength?
3. What voltage is induced in a coil of wire (100 turns) if a magnetic field goes from 0.1 T to 15 T in 2 seconds? The coil has a radius of 0.1-m.
4. Now imagine that the coil above has a 15 T magnetic field hitting it constantly, but the coil rotates around it once every 0.5 seconds. What is the maximum voltage tha will be induced?
5. What is Lenz's law and how does it apply to the falling magnet demo?
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