Figure 8.7 shows a modified form of Atwood's machine where one of the blocks slides on a table instead of hanging from the pulley. The blocks are released from rest. Find the speed of the blocks after they have moved a distance h in terms of m1, m2, R, and h. Ignore friction.
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Use conservation of energy:
gravitational potential energy(GPE) lost = kinetic energy (KE) gained
GPE lost by m₂ = KE gained by m₁ +KE gained by m₂ + KE gained by pulley.
m₂gh = ½m₁v² + ½m₂v² + ½Iω²
Since ω = vR: m₂gh = ½m₁v² + ½m₂v² + ½I(v²/R²)
2m₂gh = v²(m₁ + m₂ + I/R²)
v = √[2m₂gh/((m₁ + m₂ + I/R²)]
Physics Torque..
First, draw the body mass diagram of each object.
Using Newton's Law
for object 1 : sigma_F = m*a --> T1 = m1 * a (eq 1)
for object 2 : sigma_F = m*a --> T2 = W2 - m2 * a (equation 2)
sigma_torque = moment_of_inertia x (linear_acceleration / R)
(R x T2) - (R x T1) = 2/5 M*R^2 x (a/R) (equation 3)--> T2 = tension of m2, T1 = tension of m1, M = mass of pulley
substituting eq 1 and 2 to eq 3, we got :
R*(T2-T1)=2/5 M*R^2 x (a/R)
R (W-m2*a-m1*a)=2/5 M*R^2 x (a/R)
eliminating R :
(W-m2*a-m1*a)=2/5 M*R x (a)
thus, a equal
a =(2/5 M + m2 - m1) / (m2 * g)
speed after moving h distance :
V_end^2 = V_start^2 + 2 * a * h
V_end^2 = 0+ 2*(2/5 M + m2 - m1) / (m2 * g) *h
V_end^2 = 2*h*(2/5 M + m2 - m1) / (m2 * g)
v_end = sqrt(2*h*(2/5 M + m2 - m1) / (m2 * g))