CHAPTER 8. WORK, ENERGY AND POWER 8.3
- A student with a mass of 60 kg runs up three flights of stairs in 15s, covering a
vertical distance of 10 m. Determine the amountof work done by the student to
elevate her body to thisheight. Assume that herspeed is constant.
More practice video solutions or help at http://www.everythingscience.co.za(1.) 01tk (2.) 01tm (3.) 01tn (4.) 01tp (5.) 01tq (6.) 01tr
(7.) 01ts (8.) 01tt (9.) 01tu (10.) 01tv8.3 Energy
External and Internal Forces ESCEE
In Grade 10, you saw that mechanical energy was conserved in the absence of external
forces. It is important toknow whether a force isan internal force or an external force
in the system, because this is related to whetherthe force can change anobject’s total
mechanical energy when it does work on an object.
When an external force(for example friction, airresistance, applied force) does work on
an object, the total mechanical energy (KE + PE)of that object changes. If positive work
is done, then the objectwill gain energy. If negative work is done, thenthe object will
lose energy. The gain or loss in energy can bein the form of potentialenergy, kinetic
energy, or both. However, the work which is done is equal to the changein mechanical
energy of the object.
Activity: External Forces
We can investigate the effect of external forces on an object’s total mechan-
ical energy by rolling aball along the floor frompoint A to point B.direction of motion of theball� �
A B
Find a nice smooth surface (e.g. a highly polished floor), mark off two posi-
tions, A and B, and rollthe ball between them.
The total mechanical energy of the ball, at eachpoint, is the sum of its ki-
netic energy (KE) and gravitational potential energy (PE):