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Force: The Cause of Motion

Driving Questions. What is the natural state of objects in regard to their motion?Under what conditions will an object move with constant velocity?Under what conditions will an object accelerate?How do the forces that objects exert on one another compare?How is acceleration related to force and

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Force: The Cause of Motion

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    1. Force: The Cause of Motion SCI 201

    2. Driving Questions What is the natural state of objects in regard to their motion? Under what conditions will an object move with constant velocity? Under what conditions will an object accelerate? How do the forces that objects exert on one another compare? How is acceleration related to force and mass?

    3. Inquiry#1 Pushing and pulling different masses What is the relationship between the mass of an object and the force needed to slide it across a surface?

    4. Inquiry#2 Push against push

    5. Inquiry #3 Forces on cars What did you learn about forces from inquiries #2 and #3?

    6. Can we now address any of our Driving Questions? What is the natural state of objects in regard to their motion? Under what conditions will an object move with constant velocity? Under what conditions will an object accelerate? How do the forces that objects exert on one another compare?

    7. Newton’s Laws Aristotle vs. Galileo/Newton The 1st Law Inertia & Mass The 3rd Law The bug and the truck

    8. Newton’s 1st Law of Motion An object in motion (or at rest) will remain in motion (or at rest) with constant velocity (i.e. no change in speed or direction a.k.a. no acceleration) unless acted upon by an unbalanced force. Inertia - the tendency of an object to resist a change in motion Mass - the measurement of inertia SI units - kilogram Other metric units - gram, milligram British units - the slug

    9. Newton’s 3rd Law of Motion For every force there is an equal and opposite force.

    10. Consider the following set-up:

    11. How would the force required to lift the load with no additional mass on the sled compare to the force required with 1,2,3, or 4 masses on the sled?

    12. How would the force required to lift the load with with 1,2,3, or 4 masses on the sled compare to the previous situation if rollers were placed under the sled?

    13. How can you account for (i.e. explain) the results you obtained?

    14. Friction A force that opposes motion (or “desired” motion) Electromagnetic in nature Depends on the nature of the two surfaces in contact and how hard they are pressed together The amount of friction that must be overcome to get something moving (static friction) is greater than the amount of friction that must be overcome to keep something moving (kinetic friction)

    15. Driving Questions What is the natural state of objects in regard to their motion? Under what conditions will an object move with constant velocity? Under what conditions will an object accelerate? How do the forces that objects exert on one another compare? How is acceleration related to force and mass?

    16. Newton’s 2nd Law Acceleration is directly proportional to the unbalanced force on an object, inversely proportional to its mass, and in the direction of the unbalanced force on an object. a = FUB/m where “a” is the acceleration, “FUB” is the unbalanced force, and “m” is the mass It is often written: FUB = m X a

    17. Newton’s 2nd Law (cont’d) There are 3 statements: a ? FUB a ? 1/m a is in the direction of FUB Statement #3 merits additional consideration

    18. a is in the direction of FUB Under what conditions will an object speed up? “ “ slow down? Change direction? Move along a circlular path?

    19. Centripetal Force Any force that is directed consistently at a right angle to the motion of an object will cause the object to follow a circular path path. This kind of force ends up being directed toward the center of the circle (i.e. it is “center-seeking” and is called centripetal).

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