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Principles of Force and Motion

Unit Plan

Principles of Force and Motion

Objectives

Through demonstrations and worksheets, students will understand:

  • that motion is the product of unbalanced force.

  • the difference between kinetic and potential energy.

  • the advantages offered by simple machines.

Essential Questions

  • What causes objects to move?

Related Unit and Lesson Plans

Related Materials & Resources

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Formative Assessment

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    Multiple Choice Items:

    1. What kind of force is a car experiencing if it is proceeding down a highway at a constant speed, in a straight line?

    1. unbalanced
    2. balanced
    3. inertial
    4. kinetic
    1. If car Alpha is going twice as fast as car Beta, how much braking distance will Alpha require as compared to Beta?

    1. the same amount
    2. twice as much
    3. three times more
    4. four times more
    1. What statement best describes kinetic energy?

    1. same as potential energy
    2. energy of something in motion
    3. energy of stored water
    4. same as electricity
    1. Which is approximately equal to one joule?

    1. one horsepower
    2. sunlight hitting one square meter
    3. one AA battery
    4. an apple falling one meter
    1. What kind of energy is represented by water stored in the reservoir of a hydroelectric dam?

    1. potential
    2. kinetic
    3. mechanical
    4. aquatic
    1. Lifting a 100-pound weight onto a loading dock requires 100 pounds of force. Sliding it up particular loading ramp requires 20 pounds of force. What is the mechanical advantage of the loading ramp?

    1. 20
    2. 100
    3. 4
    4. 5
    1. Which simple machine best represents a crowbar?

    1. lever
    2. inclined plane
    3. pulley
    4. wheel-and-axle
    1. Which simple machine is a variation of the screw and the wedge?

    1. pulley
    2. lever
    3. inclined plane
    4. wheel-and-axle
    1. What is the formula for work?

    1. work = force × distance
    2. work = distance × speed
    3. work = force ÷ distance
    4. work = distance ÷ speed

    Multiple Choice Answer Key:

    1. B

    2. D

    3. B

    4. D

    5. A

    6. D

    7. A

    8. C

    9. A

     

    Short Answer Items:

    1. You are a passenger in a car driven by a friend on a dark snowy night when the roads are iced. You feel he is following the car ahead too closely. He replies that he normally allows one car-length spacing of following distance for every ten miles per hour. Based on what you have learned about coefficients of friction, what change do you advise?

    1. You remember that a horsepower is nearly 750 watts, and that the sunlight hitting the surface of the Earth amounts to 1,366 joules per square meter. A friend insists that his cousin’s boss has built a solar-powered car that uses a special photovoltaic body point on its roof (measuring 5 square meters) to generate the equivalent of 30 horsepower, thanks to its revolutionary 100 percent conversion efficiency. Do you believe him or not, and why?

    Short-Answer Key and Scoring Rubrics:

    1. You are a passenger in a car driven by a friend on a dark snowy night when the roads are iced. You feel he is following the car ahead too closely. He replies that he normally allows one car-length spacing of following distance for every ten miles per hour. Based on what you have learned about coefficients of friction, what change do you advise?

     

    Points

    Description

    2

    • Student’s answer allows five times the usual following distance, since a tire’s coefficient of friction in icy conditions is a fifth of what it is in good conditions.

    1

    • Student’s answer allows more following distance, but without specifying a factor of five; or increases it by a factor of five without saying why.

    0

    • Student’s answer does not allow for more following distance, or the student gives no answer or does not address the issue.
    1. You remember that a horsepower is nearly 750 watts, and that the sunlight hitting the surface of the Earth amounts to 1,366 joules per square meter. A friend insists that his cousin’s boss has built a solar-powered car that uses a special photovoltaic body point on its roof (measuring 5 square meters) to generate the equivalent of 30 horsepower, thanks to its revolutionary 100 percent conversion efficiency. Do you believe him or not, and why?

    Points

    Description

    2

    • Student answers no. Explanation includes: Since a joule is a watt-second, sunlight amounts to less than two horsepower per square meter. So with a collector of 5 square meters, he could not develop 30 horsepower, even assuming an unlikely 100 percent conversion efficiency.

    1

    • Student answers no, but without giving details.

    0

    • Student answers yes, for any reason, or the student gives no answer or does not address the issue.

    Performance Assessment:

    Twenty-five years from now you are on a jury hearing a lawsuit arising from a traffic accident between two cars that collided right after sunset near a curve on a country road. Both cars received damage so severe that their travel recorders (mandatory in the future) were destroyed. But thanks to modern energy-absorbing construction, neither driver was hurt.

    The jury hears from the responding police officer, who says that the remains of both cars were found on the shoulder of Car A’s side of the road. Because of the sharp curve, the posted speed limit was 25 miles per hour. But the disturbances to the scene caused by later rains and traffic made it impossible to tell what happened.

    Driver A then took the stand and said that he had just gone around the curve when he found Car B coming at him on the wrong side of the road at a tremendous rate of speed. He veered to the right onto the shoulder but Car B must have veered in the same direction and they collided there.

    Driver A’s lawyer presents expert testimony that both cars weighed about the same, or 1,500 kilograms. The damage to both cars indicated a release of kinetic energy from a combined impact speed of 85 miles per hour. Since his client says he was going only 20 miles per hour, the other car must have been going the excessive speed of 65 miles per hour, and is at fault.

    On cross-examination by the opposing lawyer, the expert admitted that other combinations of speed between the two cars could have accounted for the impact energy, especially as they weighed the same, as long as the total was 85 miles per hour.

    Driver B then took the stand and said he was not moving at all when Driver A hit him. He said the streets were slick from recent rain. As he approached the curve from the opposite direction as Car A, he lost control when he veered to avoid a jumping deer. He ended up in the ditch on the other side of the road, with his car pointed in the direction of the oncoming traffic. There, his tires spun in the mud without traction.

    A truck then stopped and its driver lifted the front of the car so that Driver B was able to shovel gravel under the front wheels, which were the ones driven by the engine. That truck and its driver then left, and Driver B could only say that the person had been “some kind of plumber.”

    Meanwhile it got dark and mist began blowing in from the woods. With the traction of the gravel, Driver B was able to drive his car onto the shoulder of the road. Then Car A came around the turn at such a great rate of speed that it could not quite make the turn, ran onto the shoulder, and hit his car head-on.

    On cross-examination, the opposing attorney mocks the idea of a mysterious passer-by lifting the car, and who somehow was not around when the accident took place.

    Driver B replies that the truck driver who stopped took a piece of steel pipe about 12 feet long out of his truck and placed one end under the front of the Car B, and placed a rock about a foot in front of the car as a fulcrum. The man easily weighed 100 kilograms, and was able to apply his full weight to the other end of the pipe and lift one end of the car. After shoveling in some gravel, Driver A was able to move his car by backing up. The truck driver than reloaded the pipe and left. There was a delay before Driver B drove onto the shoulder because he had to remove the rock in front of the car that had served as the fulcrum.

    Driver A’s lawyer insists that Driver B’s story has all the marks of a fantasy, that one man cannot lift a car weighing 1,500 kilograms, and therefore the jury must believe Driver A. He sums up by saying that the impact obviously would have been much less if Car B really were standing still.

    Who do you believe, Driver A or Driver B? Give at least two reasons, based on physics.

     

    Performance Assessment Scoring Rubric:

    Points

    Description

    4

    Student answers Driver B. Explanation includes: The lever would have provided a mechanical advantage of 11 to 1, enough for 100 kilograms of force to lift half the weight of the car, or 750 kilograms. Since kinetic energy depends on the object’s frame of reference, the same damage would have resulted if Car B was standing still as long as Car A was going 85 miles per hour.

    3

    Student answers Driver B, but with only one correct reason.

    2

    Student answers Driver B, but with no correct reason.

    1

    Student answers Driver A, because if Driver A were going 20 miles per hour, then Driver B must have been going 65 miles per hour, and the mechanical advantage of the lever was not enough to lift the car’s full weight of 1,500 kilograms.

    0

    Student answers Driver A for any other incorrect reasons, or the student gives no answer or does not address the issues.

DRAFT 11/19/2010
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