# 7.4 Power  (Page 4/8)

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Calculate the power output needed for a 950-kg car to climb a $2.00\text{°}$ slope at a constant 30.0 m/s while encountering wind resistance and friction totaling 600 N.

A man of mass 80 kg runs up a flight of stairs 20 m high in 10 s. (a) how much power is used to lift the man? (b) If the man’s body is 25% efficient, how much power does he expend?

a. 1.57 kW; b. 6.28 kW

The man of the preceding problem consumes approximately $1.05\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{7}\phantom{\rule{0.2em}{0ex}}\text{J}$ (2500 food calories) of energy per day in maintaining a constant weight. What is the average power he produces over a day? Compare this with his power production when he runs up the stairs.

An electron in a television tube is accelerated uniformly from rest to a speed of $8.4\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{7}\phantom{\rule{0.2em}{0ex}}\text{m/s}$ over a distance of 2.5 cm. What is the power delivered to the electron at the instant that its displacement is 1.0 cm?

$6.83\text{μW}$

Coal is lifted out of a mine a vertical distance of 50 m by an engine that supplies 500 W to a conveyer belt. How much coal per minute can be brought to the surface? Ignore the effects of friction.

A girl pulls her 15-kg wagon along a flat sidewalk by applying a 10-N force at $37\text{°}$ to the horizontal. Assume that friction is negligible and that the wagon starts from rest. (a) How much work does the girl do on the wagon in the first 2.0 s. (b) How much instantaneous power does she exert at $t=2.0\phantom{\rule{0.2em}{0ex}}\text{s}$ ?

a. 8.51 J; b. 8.51 W

A typical automobile engine has an efficiency of 25%. Suppose that the engine of a 1000-kg automobile has a maximum power output of 140 hp. What is the maximum grade that the automobile can climb at 50 km/h if the frictional retarding force on it is 300 N?

When jogging at 13 km/h on a level surface, a 70-kg man uses energy at a rate of approximately 850 W. Using the facts that the “human engine” is approximately 25% efficient, determine the rate at which this man uses energy when jogging up a $5.0\text{°}$ slope at this same speed. Assume that the frictional retarding force is the same in both cases.

1.7 kW

A cart is pulled a distance D on a flat, horizontal surface by a constant force F that acts at an angle $\theta$ with the horizontal direction. The other forces on the object during this time are gravity ( ${F}_{w}$ ), normal forces ( ${F}_{N1}$ ) and ( ${F}_{N2}$ ), and rolling frictions ${F}_{r1}$ and ${F}_{r2}$ , as shown below. What is the work done by each force?

Consider a particle on which several forces act, one of which is known to be constant in time: ${\stackrel{\to }{F}}_{1}=\left(3\phantom{\rule{0.2em}{0ex}}\text{N}\right)\stackrel{^}{i}+\left(4\phantom{\rule{0.2em}{0ex}}\text{N}\right)\stackrel{^}{j}.$ As a result, the particle moves along the x -axis from $x=0$ to $x=5\phantom{\rule{0.2em}{0ex}}\text{m}$ in some time interval. What is the work done by ${\stackrel{\to }{F}}_{1}$ ?

$15\phantom{\rule{0.2em}{0ex}}\text{N}·\text{m}$

Consider a particle on which several forces act, one of which is known to be constant in time: ${\stackrel{\to }{F}}_{1}=\left(3\phantom{\rule{0.2em}{0ex}}\text{N}\right)\stackrel{^}{i}+\left(4\phantom{\rule{0.2em}{0ex}}\text{N}\right)\stackrel{^}{j}.$ As a result, the particle moves first along the x -axis from $x=0$ to $x=5\phantom{\rule{0.2em}{0ex}}\text{m}$ and then parallel to the y -axis from $y=0$ to $y=6\phantom{\rule{0.2em}{0ex}}\text{m}\text{.}$ What is the work done by ${\stackrel{\to }{F}}_{1}$ ?

Consider a particle on which several forces act, one of which is known to be constant in time: ${\stackrel{\to }{F}}_{1}=\left(3\phantom{\rule{0.2em}{0ex}}\text{N}\right)\stackrel{^}{i}+\left(4\phantom{\rule{0.2em}{0ex}}\text{N}\right)\stackrel{^}{j}.$ As a result, the particle moves along a straight path from a Cartesian coordinate of (0 m, 0 m) to (5 m, 6 m). What is the work done by ${\stackrel{\to }{F}}_{1}$ ?

$39\phantom{\rule{0.2em}{0ex}}\text{N}·\text{m}$

what is electromagnetism
It is the study of the electromagnetic force, one of the four fundamental forces of nature. ... It includes the electric force, which pushes all charged particles, and the magnetic force, which only pushes moving charges.
Energy
what is units?
units as in how
praise
What is th formular for force
F = m x a
Santos
State newton's second law of motion
can u tell me I cant remember
Indigo
force is equal to mass times acceleration
Santos
The acceleration of a system is directly proportional to the and in the same direction as the external force acting on the system and inversely proportional to its mass that is f=ma
David
The uniform seesaw shown below is balanced on a fulcrum located 3.0 m from the left end. The smaller boy on the right has a mass of 40 kg and the bigger boy on the left has a mass 80 kg. What is the mass of the board?
Consider a wave produced on a stretched spring by holding one end and shaking it up and down. Does the wavelength depend on the distance you move your hand up and down?
how can one calculate the value of a given quantity
means?
Manorama
To determine the exact value of a percent of a given quantity we need to express the given percent as fraction and multiply it by the given number.
AMIT
meaning
Winford
briefly discuss rocket in physics
ok let's discuss
Jay
What is physics
physics is the study of natural phenomena with concern with matter and energy and relationships between them
Ibrahim
a potential difference of 10.0v is connected across a 1.0AuF in an LC circuit. calculate the inductance of the inductor that should be connected to the capacitor for the circuit to oscillate at 1125Hza potential difference of 10.0v is connected across a 1.0AuF in an LC circuit. calculate the inducta
L= 0.002H
NNAEMEKA
how did you get it?
Favour
is the magnetic field of earth changing
what is thought to be the energy density of multiverse and is the space between universes really space
tibebeab
can you explain it
Guhan
Energy can not either created nor destroyed .therefore who created? and how did it come to existence?
this greatly depend on the kind of energy. for gravitational energy, it is result of the shattering effect violent collision of two black holes on the space-time which caused space time to be disturbed. this is according to recent study on gravitons and gravitational ripple. and many other studies
tibebeab
and not every thing have to pop into existence. and it could have always been there . and some scientists think that energy might have been the only entity in the euclidean(imaginary time T=it) which is time undergone wick rotation.
tibebeab
What is projectile?
An object that is launched from a device
Grant
2 dimensional motion under constant acceleration due to gravity
Awais
Not always 2D Awais
Grant
Awais
why not? a bullet is a projectile, so is a rock I throw
Grant
bullet travel in x and y comment same as rock which is 2 dimensional
Awais
components
Awais
no all pf you are wrong. projectile is any object propelled through space by excretion of a force which cease after launch
tibebeab
for awais, there is no such thing as constant acceleration due to gravity, because gravity change from place to place and from different height
tibebeab
it is the object not the motion or its components
tibebeab
where are body center of mass on present.
on the mid point
Suzana
is the magnetic field of the earth changing?
tibebeab
does shock waves come to effect when in earth's inner atmosphere or can it have an effect on the thermosphere or ionosphere?
tibebeab
and for the question from bal want do you mean human body or just any object in space
tibebeab
A stone is dropped into a well of 19.6m deep and the impact of sound heared after 2.056 second ,find the velocity of sound in air.
9.53 m/s ?
Kyla
In this case, the velocity of sound is 350 m/s.
Zahangir
why?
Kyla
some calculations is need. then you will get exact result.
Zahangir
i mean how? isn't it just a d over t?
Kyla
calculate the time it takes the stone to hit the ground then minus the stone's time to the total time... then divide the total distance by the difference of the time
Snuggly
awit lenard. Hahahah ari ga to!
Kyla