Presentation on theme: "Kinetic energy – work theorem"— Presentation transcript:
1Kinetic energy – work theorem Positive net work?Kinetic energy increases.negative net work?Kinetic energy decreases.Zero net work?Constant speed.
2Average driving force of the biker? A 90.0-kg biker (bike and person) approaches a 10-% hill at a speed of 10.0 m/s. After 60.0 m uphill, his speed is 4.00 m/s. Neglect friction.Work of gravity?Work of biker?Average driving force of the biker?L17sp09
3A 50. 0-kg child drops off a ladder onto a thick spring mattress A 50.0-kg child drops off a ladder onto a thick spring mattress. The top of the ladder is 2.50 m above the surface of the mattress. If the mattress springs have an effective force constant of 50 kN/m, what is the maximum depression of the mattress?
4A particle starts from rest at x = 0 and moves to x = L under the action of a variable force F(x), which is shown in the figure. What is the particle's kinetic energy at x=L/2 and at x=L?a) Fmax L/2, FmaxL b) Fmax L/4, 0 c) Fmax L, 0 d) Fmax L/4, Fmax L/2 e) Fmax L/2, Fmax L/4
5Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. Gravity is doing work on them. Just before hitting the ground, the heavier marble hasas much kinetic energy as the lighter one.twice as much kinetic energy as the lighter one.half as much kinetic energy as the lighter one.four times as much kinetic energy as the lighter one.impossible to determine
7Example for Power calculations A car is accelerating from 0 to 100 km/h in the time of 10 seconds. The mass of the car is 1000 kg.Assuming constant acceleration, determinethe average power of the car enginethe power the car engine has to expand at the end of the time interval.
8A sports car accelerates from zero to 30 mph in 1.5 s. How long does it take for it to accelerate from zero to 60 mph, assuming the power of the engine to be independent of velocity and neglecting friction?2 sec3 sec4.5 sec6 sec9 sec12 sec
9Work of gravity Final position Final position Final position Initial positionFinal positionInitial positionFinal positionInitial positionFinal position
10Work of gravity Work is not dependent on path Initial positionFinal positionInitial positionFinal positionInitial positionFinal positionWork is not dependent on pathWork is only dependent on initial and final position
11Conservative Force The force of gravity is a conservative force. The work of a conservative force isnot dependent on pathonly dependent on initial and final positionThe force of gravity is a conservative force.
12Examples for non-conservative forces Initial positionFinal positionInitial positionFinal positionFriction forceApplied force of pushing or pullingInitial positionFinal positionInitial positionFinal position
13Which of the following are conservative forces? Justify answer. FrictionGravity close to the surface of the earthForce of the engine of a snow mobileForce exerted by a horizontal spring on a massDrag force on a parachute at terminal velocityGravity far awayNormal force