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 (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.

The Work-Energy Theorem

Kinetic Energy  A form of mechanical energy  Energy due to motion  K = ½ m v 2 –K: Kinetic Energy in Joules. –m: mass in kg –v: speed in m/s

The Work-Energy Theorem

W net =  KE –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.

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.

Power  The rate at which work is done.  P ave = W / t  P = dW/dt  P = F v

Units of Power  Watt = J/s  ft lb / s  horsepower 550 ft lb / s 746 Watts

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 F D = bv 2. By what factor must the power be increased to increase airspeed by 25%?