Work, Energy, Power

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# Work, Energy, Power - PowerPoint PPT Presentation

Work, Energy, Power. Jamie, Tommy, Jordan. Work. W= F( cosθ )d OR W= FΔr cosθ. Work Example.

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### Work, Energy, Power

Jamie,Tommy, Jordan

Work

W= F(cosθ)d

OR

W= FΔrcosθ

Work Example
• A person pulls a toboggan for a distance of 35.0 m along the snow with the rope directed 25.0° above the snow. The tension in the rope is 94.0 N. (a) How much work is done on the toboggan by the tension force? (b) How much work with the same tension parallel to the snow?
Formulas

Kinetic Energy is an object with mass and speed.

KE= ½mv2

Potential energy due to gravity:

PE= mgh OR Ug= mgh

Work Energy Theorem

W= ΔKE = KEf – Keo= ½ mvf2 – ½ mvo2

Work-Energy Theorem Example

A 7.3 kg ball starts from rest is whirled around in a circle several times and released. In one throw the ball is given a speed of 29 m/s. A .22 caliber bullet has a mass of 2.6 g and starting from rest exits the barrel at 410 m/s. Determine the work done to launch the motion of (a) the ball and (b) the bullet.

Forces
• Conservative (gravity)

Work is independent of path between initial and final positions.

Does no net work, closed path object starts and finishes at the same spot.

• Non-Conservative (friction, air resistance, tension, normal force)

Work depends on path of motion.

W= Wc+ Wnc

Wnc= ΔKE + ΔPE = (KEf – Keo) + (PEf– PEo) = (1/2 mvf2 – ½ mvo2) + (mghf – mgho)

Non-conservative Work Example

“Rocketman” has a propulsion unit strapped to his back. He starts from rest on the ground, fires the unit, and is propelled straight upward. At a height of 16m his speed is 5.0 m/s. His mass, including the propulsion unit is 136 kg. Find the work done by the force generated by the propulsion unit.

Conservation of Mechanical Energy
• E = KE + PE
• Wnc= (KEf – KEo) + (PEf – PEo) = (KEf + PEf) – (KEo + PEo) = Ef – Eo
• When Wnc= 0J then Ef= Eo (mechanical energy conserved)
• Ef= Eo(KEf + PEf) = (KEo + PEo) ½mvf2 + mghf = ½mvo2 + mgho
• When Wnc ≠ 0 (mechanical energy not conserved)
• Wnc = ( ½mvf2 + mghf) – ( ½mvo2 + mgho) = ½m (Vf2 – Vo2) – mg (ho – hf)
Conservation of Mechanical Energy

When an earthquake strikes a 300 kg hot-dog cart rolls down Nob Hill and reaches point A at a speed of 8.00 m/s. How fast is the hot-dog cart going when it reaches point B?

Power
• P = W/Δt
• W = F (Cos θ) d
• P = F (Cos θ) d/ Δt
• F = ma
• P = ma (Cos θ) d/ Δt
• V = d/ Δt
• P = ma (Cos θ) V
Power

Atlas and Hercules each lift 200 kg barbells 2.00 m off the ground. Atlas lifts his barbells in 1.00s and Hercules lifts his in 3.00 s. Calculate the amount of power each man produces.