Sample Solution from
Engineering Mechanics: Dynamics (14th Edition)
14th Edition
ISBN: 9780133915389
Chapter 12.2
Problem 1PP
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Textbook Problem

a. If s = (2t3) m, where t is in seconds, determine v when t = 2 s.

b. If v = (5s) m/s, where s is in meters, determine a at s = 1 m.

c. If v = (4t + 5) m/s, where t is in seconds, determine a when t = 2 s.

d. If a = 2 m/s2, determine v when t = 2 s if v = 0 when t = 0.

e. If a = 2 m/s2, determine v at s= 4 m if v = 3 m/s at s = 0.

f. If a = (s) m/s2, where s is in meters, determine v when s = 5 m if v = 0 at s = 4 m

g. If a = 4 m/s2, determine s when t = 3 s if v = 2 m/s and s = 2 m when t = 0.

h. It a = (8t2) m/s2, determine v when t = 1 s if v = 0 at t = 0.

i. If s = (3t2 + 2) m, determine v when t = 2 s.

j. When t = 0 the particles is at A. In four seconds it travels to B, then in another six seconds it travels to C. Determine the average velocity and the average speed. The origin of the coordinate is at O.

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Expert Solution

a)

To determine

The velocity when time is t=2s .

Answer

The velocity when time is t=2s is 24m/s .

Explanation of Solution

Given:

The time is t=2s .

The distance equation is s=(2t3)m .

Write the distance equation.

s=(2t3)m

Here, average velocity is υavg , change in distance is Δs and change in time is Δt .

Write the expression velocity.

υ=dsdt (I).

Here, velocity is υ and rate of change of distance s with respect to time (t) is dsdt .

Conclusion:

Substitute (2t3)m for s in Equation (I).

υ=ddt(2t3)=6t2m/s (II).

Substitute 2s for t in Equation (II).

υ=6t2m/s=6(2)2=24m/s

Thus, the velocity when time is t=2s is 24m/s .

Expert Solution

b)

To determine

The acceleration when distance s=1m .

Explanation of Solution

Answer:

The acceleration a when distance s=1m 25m/s2 .

Given:

The distance is s=1m .

The velocity equation is υ=(5s)m/s .

Write the velocity equation.

υ=(5s)m/s

Write the expression acceleration.

a=υdυds (I).

Here, velocity is υ , acceleration is a and rate of change of velocity υ with respect to distance (s) is dυds .

Conclusion:

Substitute (5s)m/s for υ in Equation (I).

a=υdυds=(5s)dds(5s)=5s(5)=25s (II).

Substitute 1m for s in Equation (II).

a=25s=25(1)=25m/s2

Thus, the acceleration a when distance s=1m is 25m/s2 .

Expert Solution

c)

To determine

The acceleration when distance t=2s .

Answer

The acceleration a when distance t=2s is 4m/s2 .

Explanation of Solution

Given:

The distance is s=1m .

The velocity equation is υ=(4t+5)m/s .

Write the velocity equation.

υ=(4t+5)m/s

Write the expression acceleration.

a=dυdt (I).

Here, acceleration is a and rate of change of velocity υ with respect to time (t) is dυdt .

Conclusion:

Substitute (4t+5)m/s for υ in Equation (I).

a=dυdt=ddt(4t+5)=4m/s (II).

Thus, the acceleration a when distance t=2s is 4m/s2 .

Expert Solution

d)

To determine

The velocity when time is t=2s .

Answer

The velocity υ when time is t=2s is 4m/s .

Explanation of Solution

Given:

The time is t=2s .

The acceleration is a=2m/s2 .

The initial velocity is υ0=0

Write the expression for final velocity in y direction.

υ=υ0+act (III).

Here, final velocity is υ , initial velocity is υ0 , acceleration due to gravity is ac and time is t .

Conclusion:

Substitute 2s for t , 0 for υ0 (s0) and 2m/s2 for ac in Equation (I).

υ=υ0+act=0+(2)(2)=4m/s

Thus, the velocity υ when time is t=2s is 4m/s .

Expert Solution

e)

To determine

The velocity when distance is s=4m .

Answer

The velocity υ when distance is s=4m is 5m/s .

Explanation of Solution

Given:

The time is t=3s .

The acceleration is a=2m/s2 .

The initial velocity is υ0=3m/s

The initial distance is (s0)=0 .

The final distance is s=4m .

Write the expression for final velocity in y direction..

υ2=(υ02)+2ac(sy(s0)) (II).

Here, final velocity is υ , initial velocity is (υ0)y , acceleration due to gravity is ac , final distance is s , initial distance is (s0)y .

Conclusion:

Substitute 3m/s for υ0 , 4m for s , 0 for (s0) and 2m/s2 for ay in Equation (I).

υ2=(υ02)+2ac(s(s0))υ2=(3)2+2(2)(40)υ=5m/s

Thus, the velocity υ when distance is s=4m is 5m/s .

Expert Solution

f)

To determine

The velocity

Answer

The velocity υ is 3m/s .

Explanation of Solution

Given:

The distance is s1=5m .

The distance is s2=4m .

The acceleration equation is a=(s)m/s2 .

Write the acceleration equation.

a=(s)m/s2

Write the expression acceleration.

a=υdυdsads=υdυ (I).

Here, velocity is υ , acceleration is a and rate of change of velocity υ with respect to distance (s) is dυds .

Conclusion:

Substitute (s)m/s2 for a in Equation (I).

Integrate the Equation (I) at the limits 0 to υ for υ and s1=5m to s2=4m for s .

0υdυ=45(s)ds[υ22]0υ=[s22]45

υ22=(5)22(4)22υ2=2516υ=3m/s

Thus, the velocity υ is 3m/s .

Expert Solution

g)

To determine

The distance when time is t=3s .

Answer

The distance s when time is t=3s is 26m .

Explanation of Solution

Given:

The time is t=3s .

The acceleration is a=4m/s2 .

The velocity is υ=2m/s

The distance is (s0)=2m

Write the expression for final distance in y direction.

s=(s0)+(υ)t+12at2 (I).

Here, final distance is s , initial distance is (s0) , initial velocity is (υ) , acceleration due to gravity is a and time is t .

Conclusion:

Substitute 3s for t , 2m/s for υ , 2m for (s0) and 4m/s2 for ay in Equation (I).

s=(s0)+(υ)t+12at2=2+2(3)+12(4)(3)2=26m

Thus, the distance s when time is t=3s is 26m .

Expert Solution

h)

To determine

The velocity when time is t=1s .

Answer

The velocity υ when time is t=1s is 2.67m/s .

Explanation of Solution

Given:

The time is t=1s .

The acceleration equation is a=(8t2)m/s2 .

Write the acceleration equation.

a=(8t2)m/s2

Write the expression acceleration.

a=dυdtadt=dυ (I).

Here, acceleration is a and rate of change of velocity υ with respect to time (t) is dυdt .

Conclusion:

Substitute (8t2)m/s2 for a in Equation (I).

Integrate the Equation (I) at the limits 0 to υ for υ and 0 to t for t .

0υdυ=0t(8t2)dt[υ]0υ=[8t33]0tυ=(2.667t3)m/s (II).

Substitute 2s for t in Equation (II).

υ=(2.667t3)m/s=2.667(1)=2.67m/s

Thus, the velocity when time is t=1s is 2.67m/s .

Expert Solution

i)

To determine

The velocity when time is t=2s .

Answer

The velocity υ when time is t=2s is 12m/s .

Explanation of Solution

Given:

The time is t=2s .

The distance equation is s=(3t2+2)m .

Write the distance equation.

s=(3t2+2)m

Here, average velocity is υavg , change in distance is Δs and change in time is Δt .

Write the expression velocity.

υ=dsdt (I).

Here, velocity is υ and rate of change of distance s with respect to time (t) is dsdt .

Conclusion:

Substitute (3t2+2)m for s in Equation (I).

υ=ddt(3t2+2)=6tm/s (II).

Substitute 2s for t in Equation (II).

υ=6tm/s=6(2)=12m/s

Thus, the velocity υ when time is t=2s is 12m/s .

Expert Solution

j)

To determine

The average velocity and the average speed of the particle.

Answer

The average velocity of particle is 0.7m/s .

The average speed of particle is 2.1m/s .

Explanation of Solution

Given:

The distance traveled by the particle from A to C is shown in Figure (1).

images

The time traveled by the particle from A to B is 4s .

The time traveled by the particle from B to C is 6s .

Write the expression for the average velocity.

υavg=ΔsΔt (I).

Here, average velocity is υavg , change in distance is Δs and change in time is Δt .

Write the expression for the average speed υsp .

υsp=sTotaltTotal (II).

Here, the total distance is sTotal and the total time traveled by the particle is tTotal .

Refer Figure (1) and calculate the total distance traveled by the particle.

sTotal=sAB+sBC (III)

Refer Figure (1) and calculate the total time traveled by the particle.

tTotal=tAB+tBC (IV)

Conclusion:

From the Figure (1) calculate the change in distance.

Δs=(s2s1)=(6(1))=7m

Calculate the change in distance

Δt=(t2t1)=(100)=10s

Substitute 10s for Δt and 7m for Δs in Equation (1).

υavg=ΔsΔt=7m10s=0.7m/s

Thus, the average velocity of particle is 0.7m/s .

The time traveled by the particle from A to B .

tAB=6s

The time traveled by the particle from B to C .

tBC=4s

Substitute 7m for sAB and 14m for sBC in Equation (III).

sTotal=sAB+sBC=7m+14m=21m

Substitute 6s for tAB and 4s for tBC in Equation (IV).

ttotal=6s+4s=10s

Substitute 21m for sTotal and 10s for tTotal in Equation (II).

υsp=sTotaltTotal=21m10s=2.1m/s

Thus, the average speed of particle is 2.1m/s

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