CONSERVATION OF MECHANICAL ENERGYKinetic Energy and Potential Energy

• The land-speed record for a wheel-driven car is nearly 738 km/h. (recorded at the Bonneville Salt Flats on 10/18/01) If the mass of this car was approximately 1.0x103 kg, what was its kinetic energy?

http://www.loqu.com/top-10-most-significant-land-speed-records-of-all-time/blogs/hotday-5390

𝐾=12𝑚𝑣2=

12

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𝐾=2.1 𝑥107 𝐽

Potential Energy (U)• The amount of energy that is stored in a body• A measure of how much work CAN be done• Gravitational Potential Energy: the amount of work

that can be done on a body as a result of its position above a reference point (in Earth’s gravitational field)

Ep = mgh

Where m = mass (kg)

g = acceleration due to gravity (9.81 m·s-2)

And h = height (m) above reference level

The Principle of Energy Conservation

• The total amount of energy a body possesses will remain constant, although the type of energy may be transformed from one form to another• Note: many times the energy transforms into a “useless” form, so it

appears that energy has been lost…when it really hasn’t!

• Conservation of Mechanical Energy:

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Review questions! • How much energy does a 875 g flower pot have if it is on

a shelf 95.0 cm above the ground?

• How fast is the flower pot traveling when it is 35.0 cm above the ground?

𝐸𝑝= h𝑚𝑔 =(0.875𝑘𝑔 )(9.81𝑚𝑠2 ) (0.950𝑚)=8.15 𝐽

𝐾 𝑖+𝑈 𝑖=𝐾 𝑓+𝑈 𝑓12𝑚𝑣❑

2 + h𝑚𝑔 =12𝑚𝑣❑

2 + h𝑚𝑔

0+𝑚 (9.81 ) (0.95 )=12𝑚𝑣2+𝑚(9.81)(0.35)

𝑣=3.43𝑚𝑠

• A car slides down an icy hill that has a height from top to bottom of 14.0 m. What is its speed when it hits the snow bank at the bottom of the hill?

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