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translation - 2.35

Show that a force applied to one side of a massless spring is the reaction force at the other side.

ans.

Figure 2.37 Drill problem: Prove that the force on both sides is equal

2.4 OTHER TOPICS

Designing a system in terms of energy content can allow insights not easily obtained by the methods already discussed. Consider the equations in Figure 2.38. These equations show that the total energy in the system is the sum of kinetic and potential energy. Kinetic energy is half the product of mass times velocity squared. Potential energy in translating systems is a force magnitude multiplied by a distance (that force was applied over). In addition, the power, or energy transfer rate is the force applied multiplied by the velocity.

translation - 2.36

E = EP + EK

(7)

 

E

 

Mv2

(8)

K

= ----------

 

 

2

 

EP = Fd = Mgd

(9)

 

 

 

d

(10)

P = Fv = ----E

dt

Figure 2.38 Energy and power equations for translating masses

2.5SUMMARY

FBDs are useful for reducing complex systems to simpler parts.

Equations for translation and rotation can be written for FBDs.

The equations can be integrated for dynamic cases, or solved algebraically for static cases.

2.6PRACTICE PROBLEMS

1.If a spring has a deflection of 6 cm when exposed to a static load of 200N, what is the spring constant?

translation - 2.37

2. Derive the effective damping coefficients for the pairs below from basic principles,

a) Kd1

Kd2

b)

 

 

Kd1

 

 

Kd2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3. Write a differential equation for the mass pictured below.

x

M

F

 

 

B

4. Write the differential equations for the translating system below.

Ks

F

M1

 

 

 

M2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

translation - 2.38

 

 

 

 

 

5. Write the differential equations for the system below.

 

 

 

 

 

 

 

 

Kd1

 

x1

 

 

x2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

F

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ks1

 

 

 

 

 

 

M1

Ks2

M2

 

 

 

 

 

 

 

 

 

 

B

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6.

Write the differential equations for the system given below.

 

 

 

 

 

 

 

Kd

 

Ks

 

 

 

 

F

 

 

 

 

 

 

 

 

 

 

M1

 

M2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

B1

 

 

B2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7.

Write the differential equations for the system below.

 

 

 

 

 

 

 

 

 

 

x1

x2

 

 

 

 

 

Kd1

 

 

Ks1

M1

Ks2

F

M2

 

 

 

B

8. Write the differential equations for the system below.

 

x1

x2

Kd1

 

 

Kd2

 

Kd3

Ks1

M1

Ks2

M2

F

Ks3

 

translation - 2.39

9. Write the differential equations for the system below.

Ks2

 

x2

 

Kd1

M2

F

 

B

 

 

x1

Ks1

M1

 

10. Write the differential equations for the system below.

Ks2

 

x2

 

M2

F

Kd1

 

s, k

Ks1

M1

x1

11. Write the differential equations for the system below.

F

 

 

 

 

 

 

 

 

 

 

 

 

 

M1

 

 

x1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ks1

 

 

 

 

Kd1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

M2

 

 

x2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ks2

 

 

 

 

Kd2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

translation - 2.40

12. Write the differential equations for the system below. Assume that the pulley is massless and frictionless and that the system begins undeflected.

 

 

x

Ks1

M

R

 

 

 

 

B

Ks2

13. Write the differential equations for the system below. In this system the upper mass, M1, is between a spring and a cable and there is viscous damping between the mass and the floor. The suspended mass, M2, is between the cable and a damper. The cable runs over a massless, frictionless pulley.

Ks

x1

R

 

M1

 

B

 

 

 

 

 

 

 

 

 

M2

 

 

x2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Kd