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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. F1 =12.0 N and F2 = 10.0 N
- What is the net work done on the safe by F1 and F2
- 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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10
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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 Wg is done on the crate by the gravitational
force Fg during the lift?
- b) How much work WT 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 vi =
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 Wg
done on the cab by the gravitational force Fg?
- b) During the 12 m fall, what is the work WT 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 Fa = 4.9 N needed to hold the block stationary
- at x1 = 12 mm.
- How much work does the spring force do on the block if the block is
pulled rightward from x0 = 0 to x2 = 17 mm?
- Next the block is moved leftward to x3 = -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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- Pave = W/Δt (average power)
- P = dW/dt ( instantaneous
power)
- 1 watt = 1 W = 1 J/s
- 1 kW-h (unit of work)
- P = dW/dt
- = F cos
Φ ds /dt
- = F v
cos Φ
- = F
. v
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- F1 and F2 constant forces acting on the box as the
box slides rightward across a frictionless floor. F1 is
horizontal with magnitude 2.0 N; F2 is applied upward by 60o
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 F2 is, instead, 6.0 N, what is the net
power, and is it changing?
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