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Figure 4 shows two particles P and Q of mass 3 kg and 2 kg respectively, connected by a light inextensible string - Edexcel - A-Level Maths Mechanics - Question 7 - 2007 - Paper 1

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Figure 4 shows two particles P and Q of mass 3 kg and 2 kg respectively, connected by a light inextensible string. Initially P is held at rest on a fixed smooth plan... show full transcript

Worked Solution & Example Answer:Figure 4 shows two particles P and Q of mass 3 kg and 2 kg respectively, connected by a light inextensible string - Edexcel - A-Level Maths Mechanics - Question 7 - 2007 - Paper 1

Step 1

Write down an equation of motion for P and an equation of motion for Q.

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Answer

For particle Q (2 kg):

2gT=2a2g - T = 2a

For particle P (3 kg):

T3gextsin(30exto)=3aT - 3g ext{sin}(30^ ext{o}) = 3a

Step 2

Hence show that the acceleration of Q is 0.98 m s⁻².

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Answer

From the equations derived, we can set them equal to find the acceleration:

Substituting the known values:

2gT=2a extandT3gextsin(30exto)=3a2g - T = 2a \ ext{and } T - 3g ext{sin}(30^ ext{o}) = 3a

We find that the acceleration of Q is 0.98 m s⁻².

Step 3

Find the tension in the string.

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Answer

To find the tension, we can rewrite one of the equations:

Using the first equation:
T=2g2aT = 2g - 2a
Substituting the values gives us:

Textapproximately18NT ext{ approximately } 18 N

Step 4

State where in your calculations you have used the information that the string is inextensible.

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Answer

I used the fact that the string is inextensible to equate the magnitudes of the accelerations of both P and Q. This means both particles must experience the same magnitude of acceleration when the system is released.

Step 5

the speed of Q as it reaches the ground.

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Answer

Using the equation of motion:

v2=u2+2asv^2 = u^2 + 2as
With initial speed u=0u = 0, the total distance s=0.8extms = 0.8 ext{ m} and a=0.98extms2a = 0.98 ext{ m s}^{-2}, we find:

v=extapproximately1.568extms1v = ext{approximately } 1.568 ext{ m s}^{-1}

Step 6

the time between the instant when Q reaches the ground and the instant when the string becomes taut again.

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Answer

Using the equation:

s = ut + rac{1}{2}at^2
Where u=0u = 0, s=0.8s = 0.8 m, and a=0.98a = 0.98 m/s².
Solving for tt gives:
textapproximately0.5extst ext{ approximately } 0.5 ext{ s}

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