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Forces

Forces. Scott Sharp Rockwall-Heath Physics. Force. Mass (kg) x Acceleration (m/s 2 ) F = m x a Units of force are (kgm / s 2 ). TAKS Objective 4. The students will demonstrate an understanding of motion, forces and energy. TEKS 8.7.

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Forces

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  1. Forces Scott Sharp Rockwall-Heath Physics

  2. Force • Mass (kg) x Acceleration (m/s2) • F = m x a • Units of force are (kgm / s2)

  3. TAKS Objective 4 • The students will demonstrate an understanding of motion, forces and energy.

  4. TEKS 8.7 • The student knows that there is a relationship between force and motion. The student is expected to: • A. Demonstrate how unbalanced forces cause changes in the speed or direction of an object’s motion.

  5. Learning Objective • The learner will interpret free-body force diagrams

  6. Newton’s 1st Law An object in motion stays in motion in a straight line, unless acted upon by unbalanced force. A push or pull will cause an object to (accelerate ) speed up, slow down, or change direction.

  7. Forces that are balanced have an acceleration of 0. An object @ rest V=Zero m/s A=0 m/s …. It will stay @ rest unless acted upon by an outside force!

  8. Unbalanced Forces Basically, objects just keep doing whatever it is they are doing unless acted upon by an UNBALANCED FORCE. So, one side has to have more force than another side, causing the object to accelerate in the direction where there is more force.

  9. Unbalanced Forces = Net Force Example…. Pretend that you make 1,000,000 $ as your salary. Pretend, also, that your monthly expenses are: 2.000 a month for the house 1,000 a month for the car And 4,000 a month for other bills What is the NET income?

  10. Net Force • Net forces are exerted when balances are not completely balanced, or cancelled by other individual forces. • An unbalanced force exists when the vertical or horizontal forces do not cancel each other out.

  11. Example 1 of Net Force • Notice the upward force of 1200 N is more than gravity (800N). The net force is 400 N up.

  12. Example #2 of Net Force • Notice that while the normal force and gravitation forces are balanced (50 N each), the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left.

  13. Free-Body Diagrams • Free-body diagrams show the relative magnitude (#) and direction of all forces acting upon an object.

  14. Free-Body Diagrams • This diagram shows four forces acting upon an object. However, there isn’t always four forces. There could be three, two, or even just one force acting upon the object. • Notice, these forces are BALANCED.

  15. Example Problem #1 • Draw a free-body diagram of a book that is @ rest on a table.

  16. Answer to example problem #1 • In this problem, there are normal and gravitational forces acting upon the book.

  17. Practice Problem #2 • An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

  18. Answer to practice problem #2 • Gravity is the only force acting on the egg as it falls.

  19. Practice Problem #3 • A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Draw a free-body diagram of this scenario.

  20. Answer to practice problem #3 • Gravity pulls the squirrel downward, while air resistance keeps the squirrel in the air for a while. They have to be equal forces because the a=0 (constant velocity).

  21. Practice Problem #4 • A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free body diagram.

  22. Answer to practice problem #4 • Note that the applied force arrow is pointing to the right. Notice how friction force points in the opposite direction. Finally, there is still gravity and normal forces involved.

  23. Practice problem #5 • A skydiver is descending with a constant velocity. Consider air resistance. Draw a FBD.

  24. Answer to example problem #5 • Gravity pulls down the skydiver, while air resistance pushes up as she falls.

  25. Practice Problem #6 • A man drags a sled across loosely packed snow with a rightward acceleration. Draw a FBD.

  26. Answer to Practice Problem #6 • The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Since there is not information that we are in the blizzard, normal forces still apply as does gravitational force since we are on planet Earth.

  27. Practice Problem #7 • A football is moving upwards toward its peak after having been booted by the punter. Draw a FBD.

  28. Answer to Practice Problem #7 • The force of gravity is the only force described. It is not a windy day (no air resistance).

  29. Practice Problem #8 • A car runs out of gas and is coasting down a hill.

  30. Answer to Practice Problem #8 • Even though the car is coasting down the hill, there is still the dragging friction of the road (left point arrow) as well as gravity and normal forces.

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