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 Work is energy transferred to or from an object by means of a force
acting on the object. Energy transferred to the object is positive work,
and energy transferred from the object is negative work.

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 In the figure: Two men slide an initially
stationary safe of mass m=225 kg
on a frictionless floor. F_{1} =12.0 N and F_{2} = 10.0 N
 What is the net work done on the safe by F_{1} and F_{2}
 during the displacement
d.
 during the displacement d, what is the work done by the weight of the
safe and the normal force?
 What is the final speed of the safe if d= 8.50 m?

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 In the figure: a box moves on a frictionless
surface through a displacement
 d = 3 i m. The wind is
acting on the box by force F =
(2 i – 6 j) N,
 a) How much work does F do during the displacement?
 b) If the box has a kinetic energy of 10 J
at the beginning of the displacement, what is the kinetic energy at the
end.

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 An initially stationary crate in is pulled, via a cable, a distance L =
5.70 m
 up a frictionless ramp, to a height h of 2.50 m where it stops.
 a) How much work W_{g} is done on the crate by the gravitational
force F_{g} during the lift?
 b) How much work W_{T} is done on the crate by the force T from
the cable during the lift?

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 An elevator cab of mass m=500kg Is descending with speed v_{i} =
4.0 m/s when
 Its supporting cable begins to slip, allowing it to fall with constant
acceleration
 a = g/5
 a) During the fall through a distance d=12 m, what is the work W_{g}
done on the cab by the gravitational force F_{g}?
 b) During the 12 m fall, what is the work W_{T} done on the cab
by upward pull T of the elevator cable?
 c) What is the net work W done on the cab during the fall?
 d) What is the cab kinetic energy at the end of the 12m fall?

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 In the figure the floor is frictionless and the applied
 Force F_{a} = 4.9 N needed to hold the block stationary
 at x_{1} = 12 mm.
 How much work does the spring force do on the block if the block is
pulled rightward from x_{0} = 0 to x_{2} = 17 mm?
 Next the block is moved leftward to x_{3} = 12 mm. How much
work does the spring force do on the block during the displacement?
Explain the sign of this work.

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 A block of mass m = 0.40 kg slides on a horizontal
 frictionless table with a speed v = 0.50 m/s. It then runs
 into and compresses a spring of spring constant
 k = 750 N/m. When the block is momentarily stopped by
 the spring, by what distance d is the spring compressed?

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 P_{ave} = W/Δt (average power)
 P = dW/dt ( instantaneous
power)
 1 watt = 1 W = 1 J/s
 1 kWh (unit of work)
 P = dW/dt
 = F cos
Φ ds /dt
 = F v
cos Φ
 = F
. v

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 F_{1} and F_{2} constant forces acting on the box as the
box slides rightward across a frictionless floor. F_{1 }is
horizontal with magnitude 2.0 N; F_{2} is applied upward by 60^{o}
to the floor and has magnitude 4.0 N. The speed v of the box at certain
instant is 3.0 m/s.
 What is the power due to each force acting on the box at the instant and
what is the net power? Is the net power changing at that instant
 If the magnitude of F_{2} is, instead, 6.0 N, what is the net
power, and is it changing?

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