Two masses m, and m, are situated in a system with ideal pulleys as shown in the figure. This system is in the vertical plane. A spring (constant k, rest length &) connects mass m: with the ground. Data: m= 1.5 kg, m2 = 2 kg, my= 1.5 kg, k = 50 N/m, & = 0.4 m, h= 2 m.

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3. Two masses m, and m, are situated in a system with ideal pulleys as shown in the
figure. This system is in the vertical plane. A spring (constant k, rest length &)
connects mass m: with the ground.
Data: m;= 1.5 kg, m2 = 2 kg, m3= 1.5 kg, k = 50 N/m, 6 = 0.4 m, h= 2 m.
a) The system is in rest at t = 0s. Determine at that instance the tension in all
the ropes and the compression/extension of the spring.
b) Then, an extra mass ms is dropped from a height h, hits mass m2 and sticks to
that mass. During this very short impact, the spring does not exert an impulse.
Determine the velocities of all the masses in the system right after the impact
and also the impulses exerted by the ropes.
c) How much energy was lost during the impact?
d) Determine the accelerations of all the masses right after the impact.
Solution:
1. Make FBD's for m, and for m2 and use the
equations of motion (Newton's laws) for each
mass. Projected on a vertical axis, the accelerations
must be zero.
2. Analyse the pulleys: you will find that there is
a relation between the forces in the ropes. This
gives you enough equations to calculate the tension
(forces) in the ropes and the spring force. From the
latter, you can calculate the spring extension.
3. Use conservation of energy on mass mą to
calculate, using the height h, the velocity of this
mass right before it hits the mass m2.
Use twice the principle of impulse and
momentum: once for the system of masses m2 +
m3 together and once for mass m, each time
projected on the vertical axis (there is no
conservation of momentum, as the ropes apply an
impulse on the masses)
a. The relation between the tension in the
different ropes, obtained in 2, will also determine
the relation between the different impulses of the
m
4.
m.
m,
k, l
ropes.
b. The analysis of the pulleys will also give a relation between the
accelerations and velocities of the masses m: and (m2 + m3).
c. From those four equations, you can calculate the impulses of the ropes
and the velocities of the masses immediately after the impact.
5. Use the work-energy principle (not conservation!) for the entire system
(hence all three masses together) to calculate the difference between the
kinetic energy before and after the impact, to calculate the loss of energy.
6. Make a FBD for the masses m, and (m2 + m3) and write down the
equations of motion (Newton's laws) in the vertical direction. The
spring is still as much extended as immediately before the impact (the
impact was very short and hence the spring could not change length in
that time period). The relations between forces and accelerations of the
different masses have been obtained previously. Hence you have four
equations and four unknowns (Sı, S2+3, a1, az+3), so you can calculate
these values.
Transcribed Image Text:3. Two masses m, and m, are situated in a system with ideal pulleys as shown in the figure. This system is in the vertical plane. A spring (constant k, rest length &) connects mass m: with the ground. Data: m;= 1.5 kg, m2 = 2 kg, m3= 1.5 kg, k = 50 N/m, 6 = 0.4 m, h= 2 m. a) The system is in rest at t = 0s. Determine at that instance the tension in all the ropes and the compression/extension of the spring. b) Then, an extra mass ms is dropped from a height h, hits mass m2 and sticks to that mass. During this very short impact, the spring does not exert an impulse. Determine the velocities of all the masses in the system right after the impact and also the impulses exerted by the ropes. c) How much energy was lost during the impact? d) Determine the accelerations of all the masses right after the impact. Solution: 1. Make FBD's for m, and for m2 and use the equations of motion (Newton's laws) for each mass. Projected on a vertical axis, the accelerations must be zero. 2. Analyse the pulleys: you will find that there is a relation between the forces in the ropes. This gives you enough equations to calculate the tension (forces) in the ropes and the spring force. From the latter, you can calculate the spring extension. 3. Use conservation of energy on mass mą to calculate, using the height h, the velocity of this mass right before it hits the mass m2. Use twice the principle of impulse and momentum: once for the system of masses m2 + m3 together and once for mass m, each time projected on the vertical axis (there is no conservation of momentum, as the ropes apply an impulse on the masses) a. The relation between the tension in the different ropes, obtained in 2, will also determine the relation between the different impulses of the m 4. m. m, k, l ropes. b. The analysis of the pulleys will also give a relation between the accelerations and velocities of the masses m: and (m2 + m3). c. From those four equations, you can calculate the impulses of the ropes and the velocities of the masses immediately after the impact. 5. Use the work-energy principle (not conservation!) for the entire system (hence all three masses together) to calculate the difference between the kinetic energy before and after the impact, to calculate the loss of energy. 6. Make a FBD for the masses m, and (m2 + m3) and write down the equations of motion (Newton's laws) in the vertical direction. The spring is still as much extended as immediately before the impact (the impact was very short and hence the spring could not change length in that time period). The relations between forces and accelerations of the different masses have been obtained previously. Hence you have four equations and four unknowns (Sı, S2+3, a1, az+3), so you can calculate these values.
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