OCR M1 2015 June — Question 7 15 marks

Exam BoardOCR
ModuleM1 (Mechanics 1)
Year2015
SessionJune
Marks15
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicPulley systems
TypeMulti-stage motion: changing surface conditions or external intervention
DifficultyStandard +0.3 This is a standard M1 pulley problem with connected particles on an inclined plane. While it has multiple parts and changing conditions (smooth to rough surface), each stage uses routine mechanics techniques: resolving forces, applying F=ma, and using constant acceleration equations. The multi-stage nature adds some complexity, but the problem-solving is methodical rather than requiring novel insight.
Spec3.03l Newton's third law: extend to situations requiring force resolution3.03o Advanced connected particles: and pulleys3.03t Coefficient of friction: F <= mu*R model3.03v Motion on rough surface: including inclined planes

7 \includegraphics[max width=\textwidth, alt={}, center]{8b79facc-e37f-45c3-95c0-9f2a30ca8fe4-4_392_1192_255_424} \(A B\) and \(B C\) are lines of greatest slope on a fixed triangular prism, and \(M\) is the mid-point of \(B C . A B\) and \(B C\) are inclined at \(30 ^ { \circ }\) to the horizontal. The surface of the prism is smooth between \(A\) and \(B\), and between \(B\) and \(M\). Between \(M\) and \(C\) the surface of the prism is rough. A small smooth pulley is fixed to the prism at \(B\). A light inextensible string passes over the pulley. Particle \(P\) of mass 0.3 kg is fixed to one end of the string, and is placed at \(A\). Particle \(Q\) of mass 0.4 kg is fixed to the other end of the string and is placed next to the pulley on \(B C\). The particles are released from rest with the string taut. \(P\) begins to move towards the pulley, and \(Q\) begins to move towards \(M\) (see diagram).
  1. Show that the initial acceleration of the particles is \(0.7 \mathrm {~m} \mathrm {~s} ^ { - 2 }\), and find the tension in the string. The particle \(Q\) reaches \(M 1.8 \mathrm {~s}\) after being released from rest.
  2. Find the speed of the particles when \(Q\) reaches \(M\). After \(Q\) passes through \(M\), the string remains taut and the particles decelerate uniformly. \(Q\) comes to rest between \(M\) and \(C 1.4 \mathrm {~s}\) after passing through \(M\).
  3. Find the deceleration of the particles while \(Q\) is moving from \(M\) towards \(C\).
  4. (a) By considering the motion of \(P\), find the tension in the string while \(Q\) is moving from \(M\) towards \(C\).
    (b) Calculate the magnitude of the frictional force which acts on \(Q\) while it is moving from \(M\) towards \(C\). \section*{END OF QUESTION PAPER} \section*{OCR
    Oxford Cambridge and RSA}

Question 7:
Part (i)
AnswerMarks Guidance
AnswerMarks Guidance
\(T - 0.3g\sin30 = 0.3a\) *OR* \(0.4g\sin30 - T = 0.4a\)B1 Either correct N2L for one particle; Putting \(a=0.7\) into correct equation for a single particle and working out \(T\) correctly gets B1M0A0M1A1
\(0.4g\sin30 - 0.3g\sin30 = 0.7a\)M1 Allow combined approach as "method", must be components of weight, allow \(mg(\cos/\sin)30\); Consult TL if this is done for both particles
\(a = 0.7\) m s\(^{-2}\) AGA1 May use the other equation
\(T = 0.3g\sin30 + 0.3\times0.7\)M1 Allow \(0.3g(\cos/\sin)30\). Accept cv(0.7)
\(T = 1.68\) NA1
Part (ii)
AnswerMarks Guidance
AnswerMarks Guidance
\(V = 1.8 \times 0.7\)M1 Accept cv(0.7)
\(V = 1.26\) m s\(^{-1}\)A1
Part (iii)
AnswerMarks Guidance
AnswerMarks Guidance
Dec \(= 1.26/1.4\)M1 Accept \(1.8\times0.7/1.4\); cv(1.26)
Dec \(= 0.9\) m s\(^{-2}\)A1 Or \(a = +/-0.9\)
Part (iv)(a)
AnswerMarks Guidance
AnswerMarks Guidance
\(T - 0.3g\sin30 = -0.3\times0.9\)M1 N2L, 2 forces including cmpt of weight cv(0.9) but signs must be consistent with the direction of motion; Allow \(mg(\cos/\sin)30\)
A1ft
\(T = 1.2\)A1
Part (iv)(b)
AnswerMarks Guidance
AnswerMarks Guidance
\(-0.4\times0.9 = 0.4g\sin30 - T - F_r\)M1 N2L, 3 forces including cmpt of weight cv(0.9) and cv(1.2) but signs must be consistent with the direction of motion; Allow \(mg(\cos/\sin)30\)
\(-0.4\times0.9 = 0.4g\sin30 - 1.2 - F_r\)A1ft
\(F_r = 1.12\) NA1
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## Question 7:

### Part (i)
| Answer | Marks | Guidance |
|--------|-------|----------|
| $T - 0.3g\sin30 = 0.3a$ *OR* $0.4g\sin30 - T = 0.4a$ | B1 | Either correct N2L for one particle; Putting $a=0.7$ into correct equation for a single particle and working out $T$ correctly gets B1M0A0M1A1 |
| $0.4g\sin30 - 0.3g\sin30 = 0.7a$ | M1 | Allow combined approach as "method", must be components of weight, allow $mg(\cos/\sin)30$; Consult TL if this is done for both particles |
| $a = 0.7$ m s$^{-2}$ AG | A1 | May use the other equation |
| $T = 0.3g\sin30 + 0.3\times0.7$ | M1 | Allow $0.3g(\cos/\sin)30$. Accept cv(0.7) |
| $T = 1.68$ N | A1 | |

### Part (ii)
| Answer | Marks | Guidance |
|--------|-------|----------|
| $V = 1.8 \times 0.7$ | M1 | Accept cv(0.7) |
| $V = 1.26$ m s$^{-1}$ | A1 | |

### Part (iii)
| Answer | Marks | Guidance |
|--------|-------|----------|
| Dec $= 1.26/1.4$ | M1 | Accept $1.8\times0.7/1.4$; cv(1.26) |
| Dec $= 0.9$ m s$^{-2}$ | A1 | Or $a = +/-0.9$ |

### Part (iv)(a)
| Answer | Marks | Guidance |
|--------|-------|----------|
| $T - 0.3g\sin30 = -0.3\times0.9$ | M1 | N2L, 2 forces including cmpt of weight cv(0.9) but signs must be consistent with the direction of motion; Allow $mg(\cos/\sin)30$ |
| | A1ft | |
| $T = 1.2$ | A1 | |

### Part (iv)(b)
| Answer | Marks | Guidance |
|--------|-------|----------|
| $-0.4\times0.9 = 0.4g\sin30 - T - F_r$ | M1 | N2L, 3 forces including cmpt of weight cv(0.9) and cv(1.2) but signs must be consistent with the direction of motion; Allow $mg(\cos/\sin)30$ |
| $-0.4\times0.9 = 0.4g\sin30 - 1.2 - F_r$ | A1ft | |
| $F_r = 1.12$ N | A1 | |

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7\\
\includegraphics[max width=\textwidth, alt={}, center]{8b79facc-e37f-45c3-95c0-9f2a30ca8fe4-4_392_1192_255_424}\\
$A B$ and $B C$ are lines of greatest slope on a fixed triangular prism, and $M$ is the mid-point of $B C . A B$ and $B C$ are inclined at $30 ^ { \circ }$ to the horizontal. The surface of the prism is smooth between $A$ and $B$, and between $B$ and $M$. Between $M$ and $C$ the surface of the prism is rough. A small smooth pulley is fixed to the prism at $B$. A light inextensible string passes over the pulley. Particle $P$ of mass 0.3 kg is fixed to one end of the string, and is placed at $A$. Particle $Q$ of mass 0.4 kg is fixed to the other end of the string and is placed next to the pulley on $B C$. The particles are released from rest with the string taut. $P$ begins to move towards the pulley, and $Q$ begins to move towards $M$ (see diagram).
\begin{enumerate}[label=(\roman*)]
\item Show that the initial acceleration of the particles is $0.7 \mathrm {~m} \mathrm {~s} ^ { - 2 }$, and find the tension in the string.

The particle $Q$ reaches $M 1.8 \mathrm {~s}$ after being released from rest.
\item Find the speed of the particles when $Q$ reaches $M$.

After $Q$ passes through $M$, the string remains taut and the particles decelerate uniformly. $Q$ comes to rest between $M$ and $C 1.4 \mathrm {~s}$ after passing through $M$.
\item Find the deceleration of the particles while $Q$ is moving from $M$ towards $C$.
\item (a) By considering the motion of $P$, find the tension in the string while $Q$ is moving from $M$ towards $C$.\\
(b) Calculate the magnitude of the frictional force which acts on $Q$ while it is moving from $M$ towards $C$.

\section*{END OF QUESTION PAPER}
\section*{OCR \\
 Oxford Cambridge and RSA}
\end{enumerate}

\hfill \mbox{\textit{OCR M1 2015 Q7 [15]}}