Week

Conservation of Energy Pt. 1

Objective

Student will: Demonstrate and apply the laws

of conservation of energy in terms ofKinetic And Potential Energy

Cornell Notes (1/4)Energy: Ability to do work or cause change in a system Scalar Measured in Joules (J)

Gravitational Potential Energy (PE): Stored potential energy due to height

Potential Energy (J)= mass (kg) x gravity (9.81 m/s2) x

change in height (m)

Questions

1) Can an object that has zero height have PE?

g is positive because since energy is not a vector, no direction is

needed

No Trig needed!

Cornell Notes (2/4)Kinetic Energy: Energy due to an object’s motion.

KE = ½ mv2

Kinetic Energy (J) = ½ mass (kg) x (velocity)2 (m/s)

2) Can an object have both potential an kinetic energy?

Cornell Notes (3/4)

Mechanical Energy: Sum of kinetic and potential energy.

ME = KE + PE

Cornell Notes (4/4)

The law of conservation of energy: In a closed system, energy cannot be created or destroyed, it can only change forms.

PEi + KEi = PEf + KEf

mghi + ½ mvi2 = mghf +

½ mvf2

If friction is involve, that means energy is

lost somewhere

The Amazing Spider-Man

Cornell Notes (1/5)

Example: Conservation of Energy

Gwen Stacy is dropped from 84.3 meters (273.4 ft) from the top of the Brooklyn Bridge. Calculate her velocity right before she hits the water. Assume her mass is 70 kg.

Cornell Notes (2/5)Given:

vi = 0 m/s

hi = 84.3 m

hf = 0 mm = 70 kg

ay = g = 9.81 m/s2

Unknown: vf = ?

Steps1) Define

Gwen Stacy is dropped from 84.3 meters (273.4 ft) from the top of the Brooklyn Bridge. Calculate her velocity right before she hits the water. Assume her mass is 70 kg.

Cornell Notes (3/5)Choose an equation or situation:

Rearrange the equation to isolate the unknown:

2) Plan

𝑚𝑔 h𝑖+12𝑚𝑣 𝑖

2=𝑚𝑔 h𝑓 +12𝑚𝑣 𝑓

2

√2𝑔h 𝑖=𝑣

Gwen’s potential energy is all converted to kinetic

energy

𝑚𝑔 h𝑖+12𝑚𝑣 𝑖

2=𝑚𝑔 h𝑓 +12𝑚𝑣 𝑓

2

𝑚𝑔 h𝑖+0=0+12𝑚𝑣 𝑓

2

𝑚𝑔 h𝑖=12𝑚𝑣𝑓

2

𝑔 h𝑖=12𝑣 𝑓2

Cornell Notes (4/5)Substitute the values into the equation and solve

3) Calculate

40.669=𝑣

√2𝑔h 𝑖=𝑣

√2 ∙9.8 ∙84.3=𝑣

40.669𝑚 /𝑠=𝑣

Cornell Notes (5/5)

So Gwen Stacy’s final velocity is 40.67 m/s (Approximately 91 mph) before Spider-Man saves her (and kills her)

4) Evaluate