1. Work and Energy

2. Work is force x displacement
Only the force parallel to the displacement
applies to work
Guy is pulling a sled with
a force of 50 N at an angle
of 40°, what work does he
do over 20 m?
50 N
40°
W = Fd(cosq)
W = (50 N)(20 m)(cos 40°)
W = 766 Nm
= 766 J
Joules

3. The slide is frictionless and makes a 48° angle with the
I push a red ball up an 8 m
slide, with a force of 20 N.
The slide is frictionless and
makes a 48° angle with the
ground. What work do I do?
20 N
48°
What am I working against?
Gravity!
W = Fd(sinq)
= (20 N)(8 m)(sin 48°)
= 119 J

4. Kinetic Energy is energy of motion
units: kg m2/s2
= J

5. A girl throws a .8 kg softball with a
speed of 20 m/s. What is the ball’s
kinetic energy?
KE = ½mv2
KE = ½(.8 kg)(20 m/s)2
KE = 160 J

6. 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

7. A 15 kg white ball is raised 12 meters. How much
potential energy is it given?
PE = mgh
= (15 kg)(9.81 m/s2)(12 m)
= 1766 J
12 m

8. A spring stretches 5.0 cm from its rest
position. The spring has a spring constant
of 25 N/m. How much potential energy
has it been given?
5 cm
rest
PE = ½kx2
PE = ½(25 N/m)(.05 m)2
PE = .031 J