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Clicker Question

Clicker Question. Room Frequency BA. A mass m is sliding on a frictionless plane tilted at an angle q as shown. The mass is moving up the plane because it was pushed up the plane by a spring a short time before (note that the spring is no longer in contact). .

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Clicker Question

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  1. Clicker Question Room Frequency BA A mass m is sliding on a frictionless plane tilted at an angle q as shown. The mass is moving up the plane because it was pushed up the plane by a spring a short time before (note that the spring is no longer in contact). Which of the following free-body diagrams most accurately represents the forces on the mass m at the moment shown in the diagram (after it was given an uphill push)? 59% 32% 1% 8%

  2. Announcements • CAPA Set #8 due tonight at 10 pm • CAPA Set #9 now available, due next Friday • Next week in Section: Lab #3 – Momentum • Exam #2 – scores and solutions posted on CULearn • detailed information on “exam info” web link Average = 67.4 out of 100 Stan. Dev. = 17.6 A range (85-100) B range (70-85)C range (55-70) D range (40-55) F range (0-40)

  3. Clicker Question Room Frequency BA A mass m is pulled to the right along a frictionless surface by a force of magnitude F at an angle q, as shown. N Fsinq Fcosq Fnet,y=N+Fsinq-mg=0 mg N=mg-Fsinq What is the magnitude of the normal force N exerted on the mass by the table? 31% 3% 15% 3% 48%

  4. This physics material is hard. It is particularly difficult to overcome preconceived notions that are backed upby years of observations. “Do not try to bend the spoon — that's impossible. Instead, only try to realize the truth: there is no spoon.” No force is required in the direction of the velocity. Keep working on these concepts and we are here to help.

  5. Clicker Question Room Frequency BA Roller Coaster Problem A B Assume an isolated system (no external work). Assume no dissipation from friction (i.e. thermal energy generation) Which of the following is true: Speed at position A = Speed at position B Speed at position A > Speed at position B Speed at position A < Speed at position B

  6. Clicker Question Room Frequency BA Roller Coaster Problem N Fnet,y = N-mg = may = 0 mg If the car is just sitting at the top of this hill: The net force on the car is upward. The net force on the car is downward The net force on the car is zero

  7. Clicker Question Room Frequency BA Roller Coaster Problem N Fnet,y = N-mg = may = mv2/r v mg Approximately circular arc If the car moving rightward at the top of this hill: The net force on the car is upward. The net force on the car is downward The net force on the car is zero

  8. General Statement of Conservation of Energy ΔEmechanical + ΔEthermal = Wexternal Emechanical = KE + PE ΔEthermal = thermal energy generated = -Wfriction Wfrict = -FfrictionΔx = -μk N Δx Wexternal = external work done on the system System Wexternal Energy can move from KE to PE to Ethermal Can change energy of the system

  9. General Statement of Conservation of Energy ΔEmechanical+ ΔEthermal = Wexternal KEi + PEi + Wfrict + Wexternal= KEf + PEf or Wexternal > 0 if external work is done on the system Wexternal < 0 if external work is done by the system Wexternal = 0 if no external work is done on or by the system Wfrict < 0 if there is friction Wfrict = 0 if there is no friction

  10. Clicker Question Room Frequency BA KEi + PEi + Wfrict + Wexternal= KEf + PEf • Under what condition will the final mechanical energy be greater than the initial mechanical energy? • Wexternal > |Wfriction| • Wexternal < |Wfriction| • Wexternal = |Wfriction| • It will never be greater than the initial mechanical energy • It will always be greater than the initial mechanical energy

  11. Clicker Question Room Frequency BA KEi + PEi + Wfrict + Wexternal= KEf + PEf Under what condition will Wexternal be negative? If the external work is done on the system. If the external work is done by the system. It will never be negative. It will always be negative. If there is work done by the system, the system loses energy to the surroundings that it does work on.

  12. One has to be careful about the definition of the “system” System = everything in the box Friction between hippo and rug converts mechanical energy to thermal energy. However, no external work. All energy remains within the system (isolated). System = everything in the box Friction between hippo and rug converts mechanical energy to thermal energy. Friction force of rug on the hippo is now an external force. Thus energy leaves the system.

  13. Clicker Question Room Frequency BA A mass m slides down a roughramp of height h. Its initial speed is zero. Its final speed at the bottom of the ramp is v. m h Which of the following expressions gives the speed squared (v2) of the block when it reaches the bottom of the ramp? KEi + PEi + Wfrict + Wexternal= KEf + PEf 0 + mgh+ Wfrict + 0 = ½ mv2+ 0

  14. Another Look A mass m slides down a roughramp of height h. Its initial speed is zero. Its final speed at the bottom of the ramp is v. h Find the work done by friction… KEi + PEi + Wfrict + Wexternal= KEf + PEf 0 + mgh+ Wfrict + 0 = ½ mv2+ 0 Wfrict = ½ mv2 - mgh <0 D(Mechanical Energy) = -Wfric = mgh- ½mv2

  15. Another Look A mass m slides down a roughramp of height h. Its initial speed is zero. Its final speed at the bottom of the ramp is v. h q Find the work done by friction using another method… Wfric = Forcefric x displacement Wfric = mk N x displacement Wfric = mk (mgcosq) x (h/sinq) Wfric = mkmgh cot(q) =DME = ½ mv2 - mgh

  16. The effect of some forces is expressed as a Potential Energy: e.g., gravity, elastic forces produced by springs (later) These forces are said to be Conservative. They have the property that the work done depends only on the starting and ending points, not the path taken while the force is being applied. The work done by some other forces depends on the path taken: e.g., friction. Under the action of these forces, mechanical energy is lost. The friction force is Non-Conservative. Its effect cannot be expressed as a Potential Energy.

  17. Clicker Question Room Frequency BA A blue car coasts in neutral (no stepping on the gas or brakes) down the blue path. A red car coasts in neutral (no stepping on the gas or brakes) down the red path. If only gravity is acting, then The red car ends up with a larger velocity The blue car ends up with a larger velocity The two velocities are equal at the end

  18. Springs Spring in “relaxed” position exerts no force Force Compressed spring pushes back Stretched spring pulls back Force Hooke’s “Law” k = “spring constant” units = N/m x = displacement of spring Force = -kx Large k = stiff spring Small k = floppy spring

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