Show the step by step of equations of 2.68, 2.69 ,2.71 and 2.72

College Physics
1st Edition
ISBN:9781938168000
Author:Paul Peter Urone, Roger Hinrichs
Publisher:Paul Peter Urone, Roger Hinrichs
Chapter4: Dynamics: Force And Newton's Laws Of Motion
Section: Chapter Questions
Problem 2CQ: What properties do forces have that allow us to classify them as vectors?
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Show the step by step of equations of 2.68, 2.69 ,2.71 and 2.72
EXAMPLE 2.9
Atwood's machine consists of a smooth pulley with two masses suspended from
a light string at each end (Figure 2-11). Find the acceleration of the masses and
the tension of the string (a) when the pulley center is at rest and (b) when the
pulley is descending in an elevator with constant acceleration a.
Fixed
Elevator
T
(a)
(b)
FIGURE 2-11 Example 2.9; Atwood's machine.
CASE I
Solution. We neglect the mass of the string and assume that the pulley is
smooth-that is, no friction on the string. The tension Tmust be the same
throughout the string. The equations of motion become, for each mass, for
case (a),
m,k, = mg - T
m2ä = mag – T
(2.66)
(2.67)
Notice again the advantage of the force concept: We need only identify the
forces acting on each mass. The tension T is the same in both equations. If
the string is inextensible, then * = - , and Equations 2.66 and 2.67 may be
combined
m,ä = mig – (mog – mgž2)
= mig - (mog + m2ä)
Rearranging,
g(m, - m2)
*, =
= -*,
(2.68)
т + т,
If m, > mg, then ä, > 0, and #, < 0. The tension can be obtained from
Equations 2.68 and 2.66:
T = mig - m
(m, - mg)
T= mg - mig
m, + m2
2m, mog
T =
m, + m2
(2.69)
---
---------
Transcribed Image Text:EXAMPLE 2.9 Atwood's machine consists of a smooth pulley with two masses suspended from a light string at each end (Figure 2-11). Find the acceleration of the masses and the tension of the string (a) when the pulley center is at rest and (b) when the pulley is descending in an elevator with constant acceleration a. Fixed Elevator T (a) (b) FIGURE 2-11 Example 2.9; Atwood's machine. CASE I Solution. We neglect the mass of the string and assume that the pulley is smooth-that is, no friction on the string. The tension Tmust be the same throughout the string. The equations of motion become, for each mass, for case (a), m,k, = mg - T m2ä = mag – T (2.66) (2.67) Notice again the advantage of the force concept: We need only identify the forces acting on each mass. The tension T is the same in both equations. If the string is inextensible, then * = - , and Equations 2.66 and 2.67 may be combined m,ä = mig – (mog – mgž2) = mig - (mog + m2ä) Rearranging, g(m, - m2) *, = = -*, (2.68) т + т, If m, > mg, then ä, > 0, and #, < 0. The tension can be obtained from Equations 2.68 and 2.66: T = mig - m (m, - mg) T= mg - mig m, + m2 2m, mog T = m, + m2 (2.69) --- ---------
CASE I.
For case (b), in which the pulley is in an elevator, the coordinate system
with origins at the pulley center is no longer an inertial system. We need an in-
ertial system with the origin at the top of the elevator shaft (Figure 2-11b). The
equations of motion in the inertial system (x = xi + x, xg = x + x2) are
mä¡ = m, (* + #1)
%3D
mig - T
M2g - T
so
mig - T- mží = m1(g – a) – T
ma = mag – T- mgäž = m2(g – æ) – TJ
(2.70)
where * = = a. We have ä,
ing T:
- #1, so we solve for ä, as before by eliminat-
(m,
* = - * = (g – a)-
m2)
(2.71)
m, + mg
and
2m, my(g – a)
T =
(2.72)
m1 + m2
Transcribed Image Text:CASE I. For case (b), in which the pulley is in an elevator, the coordinate system with origins at the pulley center is no longer an inertial system. We need an in- ertial system with the origin at the top of the elevator shaft (Figure 2-11b). The equations of motion in the inertial system (x = xi + x, xg = x + x2) are mä¡ = m, (* + #1) %3D mig - T M2g - T so mig - T- mží = m1(g – a) – T ma = mag – T- mgäž = m2(g – æ) – TJ (2.70) where * = = a. We have ä, ing T: - #1, so we solve for ä, as before by eliminat- (m, * = - * = (g – a)- m2) (2.71) m, + mg and 2m, my(g – a) T = (2.72) m1 + m2
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