end of section review questions 13
what is the kinetic energy of a 0.02 kg bullet that is traveling 300 m/s? express answer in Joules
KE = (1/2)mv^2 = (1/2)(.02 kg)(300 m/s)^2 = 900 J
a bus driver applies a force of 55.0 N to the steering wheel, which in turn applies 132 N of force to the steering column. what is the mechanical advantage of the steering wheel?
MA = 2.40
an outboard engine on a boat can do 1.0 x 10^6 J of work in 50.0 s. calculate its power in watts and convert your answer to horsepower.
P = 2.0 x 10^4 W (27 hp)
calculate the gravitational potential energy of a 93 kg sky diver who is 550 m above the ground
PE = mgh = (93.0 kg)((.8 m/s^2)(550) = 5 x 10^5 J
list 3 cases in which potential energy becomes kinetic energy and 3 cases in which kinetic energy become potential energy
PE to KE: a falling ball, anything rolling downhill, a pendulum on the downswing KE to PE: a rising ball, anything rolling uphill, a pendulum on the upswing
how much work is done by a person who uses a horizontal force of 25 N to move a desk 3.0 m?
W = 75 J
explain why it is easier to open a door by pushing near the know than to open it by pushing near the hinges. to which class of levers does a door belong?
a door is normally a second-class lever. pushing near the knob is easier because the input distance is longer. if you push near the hinges, the input arm is shorter than the output arm, and the door becomes a third-class lever, with a MA less than 1
think of a compound machine that you use every day and identify the simple machines that make it up
a pencil sharpener is a compound machine that consists of a couple of screws, wedges, and a wheel and axle
describe how a ramp can make lifting a box easy without changing the amount of work being done
a ramp allows the use of a smaller input force exerted over a longer distance, so that work is unchanged
calculate kinetic or potential energy in joules a. a 2.5 kg book is held 2 m above the ground b. a 15 g snowball is moving through the air at 3.5 m/s c. a 35 kg child is sitting at the top of a slide that is 3.5 m above the ground d. an 8500 kg airplane is flying at 220 km/h
a. 49 J b. 0.092 J c. 1200 J d. 1.6 x 10^7 J
a 400 N student climbs up a 3.0 ladder in 4.0 s. a. how much work does the student do? b. what is the student's power output?
a. W = 1200 J b. P = 300 W
John is using a pulley to lift the sail on his sailboat. the sail weighs 150 N, and he must lift it 4 m a. how much work must be done on the sail? b. if the pulley is 50% efficient, how much work must john do on the rope to lift the sail?
a. W = Fd = 6 x 10^2 J b. work input = useful work output/efficiency = 1200 J
determine what form or forms or energy apply to each of the following, and specify whether each is mechanical or non mechanical a. a flying disk flying through the air b. a hot cup of soup c. a wound clock spring d. sunlight e. a boulder sitting at the top of a cliff
a. gravitational PE and KE (both mechanical) b. kinetic energy of the molecules and chemical energy of the molecules (non mechanical c. elastic potential energy, kinetic energy as the spring unwinds (both mechanical) d. light energy (non mechanical) e. gravitational PE (mechanical)
identify the kind of simple machine represented a. a drill bit b. a skateboard ramp c. a boat oar
a. screw b. inclined plane c. lever
a river does 6500 J of work on a water wheel every second. the wheel's efficiency is 12% a. how much work in joules can the axle of the wheel do? b. what is the power output of the wheel in 1 s?
a. useful work output = (efficiency)(work input) 780 J b. P = W/t 780 W
determine if work is being done a. lifting a spoonful of soup to your mouth b. holding a large stack of books motionless over your head c. letting a pencil fall to the ground
a. yes b. no c. yes (by gravity)
when you do 100 J of work on the handle of a bicycle pump, the pump does 40 J of work pushing the air into the tire. what is the efficiency of the pump?
efficiency = useful work output/work input .4 or 40%
state the law of conservation of energy in you r own words and give and example of a situation that demonstrates this law
energy can neither be created nor destroyed. in a swinging pendulum, energy is constantly transformed from potential to kinetic energy and back again. in all these transformations, the total mechanical energy remains the same
explain how energy differs from work
energy is the ability to do work. when work is done, energy is transferred from one object to another
explain why machines are never 100% efficient
friction!
can an inclined plane have a mechanical advantage of less than one? explain
if the MA were 1, the plane would be as long as it is tall. because you cannot travel a distance shorter than the actual height you need to lift an object, you cannot build an inclined plane with a MA less than 1
list 3 forms of energy
kinetic, potential, mechanical, etc
list the six types of simple machines
lever, pulley, wheel and axle, inclined plane, wedge, and screw
explain the difference between potential energy and kinetic energy
potential energy is energy due to position. kinetic energy is the energy of motion
water storage tanks are usually built on towers on placed on hilltops. why?
storing the water up high gives the water gravitational potential energy, so the water will naturally flow out of the tank if needed
use the concepts of kinetic energy and potential energy to describe the motion of a child on a swing. why does the child need a push from time to time?
the child undergoes energy transformations from maximum PE to maximum KE at the bottom and back to max PE. they need a push to make up for energy loss due to friction between the rope and the support as well as energy lost to air resistance
both a short ramp and a long ramp reach a height of 1 m. which ramp has a greater mechanical advantage?
the long ramp
explain why pulleys are in the lever family
the middle of the pulley is the fulcrum; the wheel of the pulley is like a lever-arm extended into a circle. pulleys are different from ordinary levers because you can change the location of the fulcrum of a lever
describe how a lever can increase the force applied without changing the amount of work being done
the output force will be entered through a smaller distance
describe the rise and fall of a thrown basketball by using the concepts of kinetic energy and potential energy
the player throws the ball, giving it KE. the ball begins to rise and slow down as KE is transformed into PE due to gravity. at its peak, the ball has maximum PE, then begins to fall, transforming PE into KE
name one situation in which gravitational potential energy might be useful, and name one situation in which it might be dangerous
the water tank in the water tank question is a case where gravitational PE is useful. it is dangerous to people hanging from the side of a building
explain why the driver of a car has to continuously apply pressure to the gas pedal in order to keep the car cruising at a steady speed, even on a flat road. does this situation violate the law of conservation of energy
to make up for the losses due to friction within the car's mechanisms, between the tires and the road, and due to wind resistance. this does not violate the law of conservation of energy because mechanical energy is transformed into non mechanical forms
define work and power. how are they related?
work is the quantity of energy transferred by a force applied to an object that moves in the direction of the force. power is a quantity that measures the rate at which work is done. power equals work divided by time
