AS June 2024 Q4
4.

A particle \(P\) of mass \(m\) and a particle \(Q\) of mass \(4m\) are at rest on a smooth horizontal plane, as shown in Figure 2.
Particle \(P\) is projected with speed \(u\) along the plane towards \(Q\) and the particles collide.
The coefficient of restitution between the particles is \(e\), where \(e \gt \dfrac{1}{4}\)
As a result of the collision, the direction of motion of \(P\) is reversed and \(P\) has speed \(\dfrac{u}{5}(4e-1)\).
After the collision, \(P\) goes on to hit a vertical wall which is fixed at right angles to the direction of motion of \(P\).
The coefficient of restitution between \(P\) and the wall is \(f\), where \(f \gt 0\)
Given that \(e = \dfrac{3}{4}\)
After its impact with the wall, \(P\) goes on to collide with \(Q\) again.
| Scheme | Marks | AO |
|---|---|---|
| Use of CLM OR NEL | M1 | 3.1a |
| \(mu = -m\dfrac{u}{5}(4e-1) + 4mv_Q\) OR \(v_Q + \dfrac{u}{5}(4e-1) = eu\) | A1 | 1.1b |
| \(v_Q = \dfrac{u}{5}(e+1)\) | A1 | 1.1b |
| (3) |
Notes
M1: CLM: Correct no. of terms, condone consistent extra \(g\)’s, sign errors, cancelled \(m\)’s
OR
NEL: \(e\) on the correct side but condone sign errors
A1: Correct equation
A1: Cao. Accept any equivalent two term expression.
| Scheme | Marks | AO |
|---|---|---|
| \(v_P = \pm\dfrac{fu}{5}(4e-1)\) | B1 | 3.3 |
| \(= \pm\dfrac{2fu}{5}\) | B1 | 1.1b |
| KE Loss \(= \dfrac{1}{2}m\left(\dfrac{2u}{5}\right)^2 - \dfrac{1}{2}m\left(\dfrac{2fu}{5}\right)^2\) | M1 | 3.1a |
| \(= \dfrac{2mu^2}{25}(1 - f^2)\) | A1 | 1.1b |
| (4) |
Notes
B1: Seen or implied.
B1: Seen or implied.
M1: Allow negative of this and without \(e\) being substituted.
N.B. Allow anything of the form: \(\pm\left(\dfrac{1}{2}m\left(v_P\right)^2 - \dfrac{1}{2}m\left(fv_P\right)^2\right)\), provided that \(v_P\) has come from an attempt to put \(e = \tfrac{3}{4}\) in the given expression
A1: Cao. Accept any equivalent two term expression, isw
| Scheme | Marks | AO |
|---|---|---|
| \(v_Q = \dfrac{7u}{20}\) | M1 | 1.1b |
| \(\dfrac{7u}{20} \lt \dfrac{2fu}{5}\) | M1 | 2.1 |
| \(\dfrac{7}{8} \lt f \leqslant 1\) | A1 B1 | 1.1b 1.2 |
| (4) | ||
| (11 marks) |
Notes
M1: For attempt to put \(e = \dfrac{3}{4}\) in their \(v_Q\) expression to give a multiple of \(u\), seen or implied at some stage.
M1: Correct inequality for their speeds (which could involve \(e\)), provided it’s dimensionally correct
Not available if \(Q\) is moving towards the wall.
A1: \(\dfrac{7}{8} \lt f\) oe
B1: For \(f \leqslant 1\)
N.B. All the marks are available if they go straight to \(\dfrac{7}{20} \lt \dfrac{2f}{5}\)