Conservation of Mechanical Energy - PBworksphysicsskills.pbworks.com/f/Conservation+of+energy.pdf · Conservation of Mechanical Energy ... Armstrong jumped up on the moon with an

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Conservation of Mechanical EnergyIn a closed system, total mechanical energy of the

system is conserved

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ho

h1

h2

h3

What is the total mechanical energy of the system (assume no energy loss due to friction) at each point?

PEKE

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Example: A Daredevil Motorcyclist

A motorcyclist is trying to leap across the canyon by driving horizontally off a cliff 38.0 m/s.  Ignoring air resistance, find the speed with which the cycle strikes the ground on the other side.

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General Procedure for Solving Energy Problems

1. Identify the energy added(*) or initially present(**) in the system.

(*) Work is done on the system(**) PEg, PEs, KE of the system

ask yourself:1) Does the object have height?2) Is the object moving?3) Is there a compressed/stretched spring?

2. Identify the energy system has at the end. Where did the energy go? ask the same three questions again...3. Write the equations (Ebefore=Eafter) 4. Substitute5. Solve for unknowns

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In a wild shot, Bo flings a pool ball of mass m off a 0.68 m high pool table, and the ball hits the floor with a speed of 6.0 m/s.  How fast was the ball moving when it left the pool table? 

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Frank, a San Francisco hot dog vendor, has fallen asleep on the job.  When an earthquake strikes, his 300 kg hot­dog cart rolls up and down a street.  It passes the top of hill A, which is 50.0 m high, at a speed of 8.00 m/s.  How fast is the hot dog cart going to pass the top of the next hill ﴾B﴿, which is 30.0 m high when Frank finally wakes up and starts to run after it?

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While on the moon, the Apollo astronauts enjoyed the effects of gravity much smaller than that on Earth.  If Neil Armstrong jumped up on the moon with an initial speed of 1.51 m/s to a height of 0.700 m, what amount of gravitational acceleration did he experience?

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A spring with a spring constant of 400 N/m is compressed 5 cm and a 300 g ball is placed in front of it. How fast will the ball be moving when the spring is released?

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A spring with a spring constant of 400 N/m is compressed 5 cm and a 300 g ball is placed in front of it. How fast will the ball be moving when the spring is released?

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An elevator which is moving down with a speed of 20 m/s when it is 15 m off the ground. The cable holding the elevator snaps and it falls down. At the bottom there is a spring with the spring constant of 4000 N/m. What is the compression of the spring if the mass of the elevator is 500 kg?

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