|
|
Because the magnitude of the torque due to F about O is defined as rF sin 3 by Equation 10.19, we can write the work done for the infinitesimal rotation as (10.22) The rate at which work is being done by F as the object rotates about the fixed axis through the angle d! in a time interval dt is Because dW/dt is the instantaneous power ! (see Section 7.8) delivered by the force (10.23) This expression is analogous to ! " Fv in the case of linear motion, and the expres- x dx. In studying linear motion, we found the energy approach extremely useful in de- scribing the motion of a system. From what we learned of linear motion, we expect that To show that this is in fact the case, let us begin with 2 " I(. Using the chain rule from calculus, we can express the resultant torque as Rearranging this expression and noting that 2 d! " dW, we obtain Integrating this expression, we obtain for the total work done by the net external force (10.24) where the angular speed changes from & i to & f . That is, the work–kinetic energy theorem for rotational motion states that In general, then, combining this with the translational form of the work–kinetic en- In addition to the work–kinetic energy theorem, other energy principles can also be applied to rotational situations. For example, if a system involving rotating objects is Table 10.3 lists the various equations we have discussed pertaining to rotational mo- tion, together with the analogous expressions for linear motion. The last two equations the net work done by external forces in rotating a symmetric rigid object about a
% W " & & f & i I& d& " 1 2 I& f
2 % 1 2 I& i
2 % 2 d! " dW " I& d& % % 2 " I( " I d& dt " I d& d!
d! dt " I d& d! & % ! " dW dt " 2&
dW dt " 2
d! dt dW " 2 d! SECTION 10.8 • Work, Power, and Energy in Rotational Motion 313 Power delivered to a rotating rigid object Work–kinetic energy theorem for rotational motion |