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Net Work

Net Work. Net work (W net ) is the sum of the work done on an object by all forces acting upon the object. The Work-Energy Theorem. Consider a force applied to an object ( Σ F ≠ 0). Newton’s second law tells us that this net force will produce an acceleration.

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Net Work

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  1. Net Work Net work (Wnet) is the sum of the work done on an object by all forces acting upon the object.

  2. The Work-Energy Theorem • Consider a force applied to an object (ΣF ≠ 0). • Newton’s second law tells us that this net force will produce an acceleration. • Since the object is accelerating, its displacement will change, hence the net force does work.

  3. The Work-Energy Theorem

  4. Kinetic Energy • A form of mechanical energy • Energy due to motion • K = ½ m v2 • K: Kinetic Energy in Joules. • m: mass in kg • v: speed in m/s

  5. The Work-Energy Theorem

  6. The Work-Energy Theorem Wnet = DKE • When net work due to all forces acting upon an object is positive, the kinetic energy of the object will increase. • When net work due to all forces acting upon an object is negative, the kinetic energy of the object will decrease. • When there is no net work acting upon an object, the kinetic energy of the object will be unchanged.

  7. Power • Power is the rate of which work is done. • No matter how fast we get up the stairs, our work is the same. • When we run upstairs, power demands on our body are high. • When we walk upstairs, power demands on our body are lower.

  8. Power • The rate at which work is done. • Pave = W / t • P = dW/dt • P = F • v

  9. Units of Power • Watt = J/s • ft lb / s • horsepower • 550 ft lb / s • 746 Watts

  10. Power Problem Develop an expression for the power output of an airplane cruising at constant speed v in level flight. Assume that the aerodynamic drag force is given by FD = bv2. By what factor must the power be increased to increase airspeed by 25%?

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