Download HW6.2: Work, Energy and Power

HW6.2: Work, Energy and Power
To determine the work done by particular forces, make FBDs with the displacement vector shown clearly so
that you can evaluate
- the orientation between the force and displacement
- the net force (which causes K via Wnet)
1. A parent pushes a baby stroller from home to the park along a level road with a force of 34 N directed at
an angle of 300 below the horizontal. If the park is 0.83 km from home, how much work is done by the
parent?
2. Charlie pulls horizontally to the right on a 14 kg wagon with a force of 37.2 N. Sara pulls horizontally to
the left with a force of 22.4 N. How much work is done on the wagon after it has moved 2.50m to the
right? If the wagon started at rest, how fast is it moving after traveling 2.50m?
3. A proton is moving at 425 m/s
a) How much work must be done on it to stop it? (A proton has a mass of 1.67 x 10-27 kg)
b) Assume the net braking force acting on it has magnitude 8.01 x 10-16 N and it is directed opposite to
its initial velocity. Over what distance must the force be applied? Watch your negative signs in this
problem.
4. A hockey stick applies an average force over a distance of 0.121 m to an initially stationary puck of mass
152 g. The puck then moves with a speed of 51.0 m/s. With what average force did the hockey stick
strike the puck? Neglect friction between the puck and ice.
5. Stuntman’s Freefall, a ride at Six Flags Great Adventure in New Jersey, stands 39.6 m high. Ignoring the
force of friction, what is the minimum power rating of the motor that raises the 1.2 x 105 kg ride from the
ground to the top in 10.0 s at constant velocity?
6. Cary pushes a 15 kg lawn mower across a lawn at a constant speed by pushing with a force of 115 N
along the direction of the handle which makes a 22.50 angle with the horizontal.
a) If Carey develops 64.6 W for 90.0 s, what distance is the lawn mower pushed?
b) What is the work done by friction?
c) What is the coefficient of friction between the lawn mower and the lawn?
7. Lori, who loves to ski, has rigged up a rope tow to pull herself up a local hill that is inclined at an angle of
30.00 from the horizontal. The motor works against a retarding frictional force of 100 N (as well as
gravity). If Lori has a mass of 60.5 kg, and the power of the motor is 1350 W, at what speed can the
motor pull her up the hill? (draw a FBD)
8. A block, of mass 4 kg, is pulled up a smooth (frictionless) inclined plane of angle 30o with horizontal. If
the block moves up with an acceleration of 0.2 m/s2, find the power delivered by the pulling force at a
time 4 seconds after the motion starts from rest. (Draw a free body diagram and use Newton’s Laws to
find the pulling force.)
9. The Porsche 911 GT3 has a 380 hp engine (there are 746 W/hp) and a mass of 1400 kg. The car can
accelerate from 0 to 100 km/hr in 4.3 s. What percentage of the energy supplied by the engine goes into
making the car move (making the K)? Assume that the car’s acceleration is constant.
Answers
1) 24,440 J 2) 37 J and 2.30 m/s
3) -1.51 x 10-22 J b) 1.89 x 10-7 m = 189 nm 4) 1630 N
5) 4.66 x 106 W = 4.66 MW
6) a) 54.7 m b) -5814 J c) 0.56
7) 3.40 m/s
8) 8.16W 9) 44%