CHAPTER 5
WORK AND ENERGY
I NTERNET A SSIGNED P ROBLEMS
1 – 30
James Prescott Joule (1818 – 1889)
WORK AND CONSTANT FORCE
1. How much work is done by the gravitational force when a Hint: The force and the displacement are both 265-kg object falls
2.80 m? downwards, so the angle between them is 0o .
2. A 65.0-kg physics student climbs a flight of stairs 20.0 m high. Hint: The minimum force required to lift the
How much work is required?
六和塔firefighter is equal to his weight. The force and the displacement are both upwards so the angle between them is 0o .
3. A 1300-N box rests on the floor. How much work is required to Hint: The work done to move it across the floor move it at constant speed
4.0 m along the floor against a is the work done by the pulling force. The friction force of 230 N? angle between the pulling force and the direction of motion is 0o .
G G Δx
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4. How much work is done pushing a 160-kg crate 10.3 m across Hint: Since it is not accelerating horizontally or a rough floor without acceleration, if the coefficient of friction vertically, the work done to move it across was 0.50? the floor is done by the pushing force.
. A
5.0-kg bo across a floor at a rate of Hint: Since the acceleration of the box is constant
G F G Δx
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5x is accelerated from rest 2.0 m/s 2 for 7.0 s. Find the net work done on the box.
find the distance moved. Assume the box starts from rest.
6. A 330-kg piano slides 3.6 m down a 28o incline and I kept from Hint: The piano is moving with a constant velocity accelerating by a man who is pushing back on it parallel to the
down the plane. P F G
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is the force of the man
incline. The coefficient of friction is 0.40. Calculate the force pushing on the pia o. n
exerted by the man.
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长江七号观后感7. Using the information from problem 6, calculate the work done Hint: The work done by the man is the work done by the man on the piano. by P F G . The angle between P F G
and the
direction of the motion is 180.
o
t is the minimum work
. Wha 8 needed to push a 950-kg car 810 m Hint: Ignore the frictional force. INETIC ENERGY; WORK-ENERGY PRINCIPLE
. At room temperature, an oxygen molecule, with mass of Hint: Find the velocity from the kinetic energy. up along a 9.0o incline?
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K
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9 5.31 x 10-26Kg, typically has a KE of about 6.21 x 10-21J. How fast is the molecule moving?
10. How much work is needed for a 68-kg runner to accelerate Hint: Solve by using the KE formula. 1. How much work from rest to 7.5 m/s?
1 must be done to stop a 1250-kg car traveling Hint: The work done on the car is equal to the 2. An 88-g arrow is fired from a bow who string exerts an Hint : The force exerted by the bow on the arrow t at 105 km/h? change in its kinetic energy.
1 average force of 110 N on the arrow over a distance of is in the same direction as the displacemen 78 cm. What is the speed of the arrow as it leaves the bow of the arrow. ? 3. At an accident scene on a level road, investigators measure a Hint: The work needed to stop the car is equal
1 car’s skid mark to be 88 m long. The accident occurred on a to the change in the car’s kinetic energy. rainy day, and the coefficient of kinetic friction was estimated That work comes from the force of friction
to be 0.42. What was the speed of the car when the driver
on the car, which is assumed to be static slammed on the brakes? friction since the driver locked the brakes.
d = stopping distanc
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POTENTIAL ENERGY
4. A spring has a spring constant of 440 N/m. How much must this Hint: U the elastic potential equation and solve
1 spring be stretched to store 25 J of potential energy?
for distance.
15. A 4.1-kg block is attached to a spring with a force constant of Hint: Sum the work done by the spring for each 550 N/m, as shown below.
gment of path one.
Find the work done by the spring on the block as the block 6. A 7.0-kg monkey swings from one branch to another 1.2 m Hint: Subtract the initial gravitational PE from
Moves from A to B along paths 1 and 2?
1 higher. What is the change in potential energy?
鲁班技能
the final gravitational PE.
17. As a cliff diver drops to the water from a height of 46 m, his Hint: The cliff diver plunges straight downward gravitational potential energy decreas by 25,000 J. What due to the force of gravity. is the diver’s weight in Newtons?
18. A 1200-kg car rolling on a horizontal surface has speed of Hint: Assume that all of the kinetic energy of d 65 km/h when it strikes a horizontal coiled spring and is of the car becomes PE of the compres brought to rest in a distance of 2.2 m. What is the spring spring. constant of the spring?
9. A dart of mass 0.100 kg is presd against the spring of a toy Hint: Initially the dart is at rest so KE = 0. Ignore 1 dart gun as shown. The spring (with spring constant k = 250N) friction and the only PE is elastic. is compresd 6.0 cm and relead. If the dart detaches from the spring when the spring reaches its natural length (x = 0), what speed does the dart acquire?
CONSERVATION OF MECHANICAL ENERGY
0. Jane, looking for Tarzan, is running at top speed 5.3 m/s and Hint: The only forces acting on Jane are gravity模糊近义词
2 grabs a vine hanging vertically from a tall tree in the jungle. and the vine tension. The tension pulls in
How high can she swing upward?
centripetal direction, and so can do no work. 1. A novice skier, starting from rest, slides down a frictionless 35.0o Hint: The forces on the skier are gravity and 11 , v y 22 , v y
2 incline who vertical height is 185 m. How fast is she going the normal force. The normal force is when she reaches the bottom? perpendicular to the direction of motion,
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2. At an amument park, a swimmer ud a water slide to enter Hint: As the swimmer descends the slide her
and so does no work.
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2 the main pool. If the swimmer starts at rest, slides without gravitational potential energy is converted friction, and descends through a vertical height of 2.61 m, what into kinetic energy. Set the loss in
is her speed at the bottom of the slide? gravitational potential energy equal to the 3. A sled is initially given a shove up a frictionless 28.0o incline. It Hint: The forces on the sled are gravity and the gain in kinetic energy by tting her change in mechanical energy equal to zero.
2 reaches a maximum vertical height 1.35 m higher than where normal force. The normal force is
it started. What was its initial speed?
perpendicular to the direction of motion, 4. In the high jump, Tom’s kinetic energy is transformed into Hint: All the forces on the jumper are conrvative. and so does no work.
2 gravitational potential energy without the aid of a pole. With what minimum speed must Tom leave the ground in order to lift her center of mass 2.10 m and cross the bar with a speed of 0.70 m/s?
F G
N
LAWS OF CONSERVATION OF ENERGY
5. A ski starts from rest and slides down a 22o incline 75 m Hint: U Newton’s cond law for forces n,
2 long. If the coefficient of friction is 0.090, what is the ski’s perpendicular to the direction of motio speed at the ba of the incline?
Noting that there is no acceleration perpendicular to the plane. OWER
6. How long
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2 will it take a 1750-W motor to lift a 315-kg piano to Hint: The work necessary to lift the piano is the 7. If a car generates 18 hp when traveling at a steady 88 km/h, Hint: If the car has a constant velocity, then the a sixth-story window 16.0 m above? work done by an upward force, equal to in magnitude to the weight of the piano.
2 what must be the average force exerted on the car due to total resistive force must be of the same friction and air resistance? magnitude as the engine force, so that the 8. A 1400-kg sports car accelerates from rest to 95 km/h in 7.4 s. Hint: The energy transformed is the change
net force is zero.
2 What is the average power delivered by the engine?
in kinetic energy of the car.
29. You rai a bucket of water from the bottom of a deep well. If Hint: The power required is the force times the
your power output is 108 W, and the mass of the bucket and velocity, where the force is just the weight the water in it is 5.00-kg, with what speed can you rai the
of the bucket. bucket?
30. The old record for running the stairs of the Empire State Hint: The power required is the work required Building was t on February 3, 2003. The 86 flight, with a to change the elevation divided by the total of 1576 steps, was run in 9 minutes and 33 conds. If time. the height gain of each step was 0.20 m, and the mass of the runner was 70.0 kg, what was his average power output during the climb? Give your answer in watts.
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