Work and Energy

Work is force x displacement

Only the force parallel to the displacement applies to work

Guy is pulling a sled witha force of 50 N at an angleof 40°, what work does hedo over 20 m?

W = Fd(cos)

W = (50 N)(20 m)(cos 40°)

W = 766 = 766 JJoules

50 N

40°

Nm

I push a red ball up an 8 m slide, with a force of 20 N.The slide is frictionless andmakes a 48° angle with theground. What work do I do?

20 N

48°

What am I working against?Gravity!

W = Fd(sin) = (20 N)(8 m)(sin 48°) = 119 J

Kinetic Energy is energy of motion

2

2

1mvKE

units: kg m2/s2 = J

A girl throws a .8 kg softball with aspeed of 20 m/s. What is the ball’skinetic energy?

KE = ½mv2

KE = ½(.8 kg)(20 m/s)2

KE = 160 J

Potential Energy is energy of position

Gravitational Potential Energy PE = mgh

units: kg (m/s2) (m) = J

Elastic Potential Energy PE = 1/2kx2

units: (N/m)m2 = J

A 15 kg white ball is raised 12 meters. How muchpotential energy is it given?

12 m

PE = mgh = (15 kg)(9.81 m/s2)(12 m) = 1766 J

A spring stretches 5.0 cm from its restposition. The spring has a spring constantof 25 N/m. How much potential energyhas it been given?

rest5 cm

PE = ½kx2

PE = ½(25 N/m)(.05 m)2

PE = .031 J