Particle A of mass $9 m$ and particle $B$ of mass $m$ travel towards each other along a smooth horizontal surface in a straight line and collide head-on. The initial speed of particle A before the collision is $u$. In Fig. 1.1, the variation with time $t$ of momentum $p$ is shown from $t=0$ to $t=3 T$ for particle $A$ and from $t=0$ to $t=T$ for particle $B. Fig. 1.1

A uniform square box with sides 0.80 m and mass 2.0 kg is at rest on the ground. One end of a light rope is attached to the box and the other end is attached to the wheel of a motor. The motor applies a constant clockwise torque of 5.0 N m on the wheel of radius 0.20 m . At the instant shown in Fig. 2.1, the rope is taut and it makes an angle of $\theta=20^{\circ}$ with the vertical side of the box. The system remains in equilibrium. Fig. 2.1

(a) Explain why gravitational potential is always negative whereas electric potential can have positive and negative values, given that both potentials are zero at infinity.

(b) Fig. 4.1 shows how the gravitational potential $\phi$ from the surface of a planet varies with distance $r$ from the centre of the planet. Fig. 4.1

A transverse wave on a rope is travelling to the right. Fig. 5.1 shows the waveform at a particular time. Particles Q, R, S, T, U and V are labelled. Fig. 5.1

Fig 6.1 shows a miniature E10 filament light bulb with a rating of $6.0 \mathrm{~V}, 3.0 \mathrm{~W}$. Fig. 6.1

A medical treatment makes use of a sample of americium-240 that emits alpha particles to kill cancer cells. In one such treatment, a total energy of 1140 J is applied to a tumour of mass 0.500 kg . At the start of the treatment, the mass of the americium-240 sample is $2.00 \times 10^{-9} \mathrm{~kg}$. Americium-240 has a half-life of 50.2 hours, and it decays by emitting an alpha particle of kinetic energy 5.71 MeV .

Read the passage below and answer the questions that follow. Despite the increasing popularity of laser printers, inkjet printers remain a common choice for many people who are looking to print documents and photos from their computers due to their relatively low cost, smaller size and ability to print photos with better quality than laser printers. In inkjet printers, tiny droplets of ink are ejected and directly applied onto the paper, which then form the text or image. Traditionally, there are two main technologies used for droplet ejection in inkjet printers: continuous inkjet (CIJ) and drop-on-demand (DOD). In CIJ printers, a vibrating nozzle ejects a stream of regularly spaced ink droplets at a high velocity towards a pair of charging electrodes that deposits electrons on the droplets, giving the appropriate amount of charge to each droplet. The charged droplets then enter a region of uniform electric field between two deflection plates, which causes the droplets to deflect by different amounts corresponding to the amount of charge deposited on them. These droplets finally land on the piece of paper to form the desired image. This process in a CIJ printer is shown in Fig. 8.1. Fig. 8.1 Not every ink droplet that passes through the charging electrodes will get charged - instead, charged droplets will usually be separated by one or more uncharged "guard droplets". These uncharged droplets, which will not be deflected by the electric field, will enter a gutter that will divert these unused droplets back into the ink reservoir to be reused. Typical data for a commercial CIJ printer are given in Table 8.1. Table 8.1 | velocity of ink droplets $/ \mathrm{m} \mathrm{~s}^{-1}$ | 20 | | :-- | :-- | | frequency of droplet ejection $/ \mathrm{kHz}$ | 110 | | average diameter of ink droplet $/ \mu \mathrm{m}$ | 80 | | density of ink $/ \mathrm{g} \mathrm{cm}^{-3}$ | 0.84 | | p.d. across deflection plates $/ \mathrm{V}$ | 10000 | In DOD inkjet printers, ink droplets are ejected from a nozzle one drop at a time. While there are a number of different methods by which the droplets are ejected out individually, a common method used is called the thermal inkjet, or sometimes known as the "bubble jet". In thermal inkjet printers, the printhead consists of an ink chamber connected to a nozzle from which the ink will be ejected. There is a square-shaped thin-film resistor of sides length $20 \mu \mathrm{m}$ that acts as a heating element on top of the ink chamber. A small pulse of current is passed through the thin-film resistor, which quickly heats up and vapourises a thin layer of ink just below the thin-film resistor. This creates a bubble which expands rapidly. The pressure within the ink chamber increases and pushes a tiny drop of ink out of the nozzle onto the paper underneath to form the desired image. This process in a thermal inkjet printer is shown in Fig. 8.2. Fig. 8.2 Typical data for a commercial thermal inkjet printer are given in Table 8.2. Table 8.2 | velocity of ink droplets $/ \mathrm{m} \mathrm{~s}^{-1}$ | 4.5 | | :-- | :-- | | frequency of droplet ejection $/ \mathrm{kHz}$ | 18 | | average diameter of ink droplet $/ \mu \mathrm{m}$ | 10 | | density of ink $/ \mathrm{g} \mathrm{cm}^{-3}$ | 1.17 | | specific heat capacity of ink $/ \mathrm{J} \mathrm{kg}^{-1} \mathrm{~K}^{-1}$ | 2090 | | specific latent heat of vaporisation of ink $/ \mathrm{kJ} \mathrm{kg}^{-1}$ | 444 | | boiling point of ink $/{ }^{\circ} \mathrm{C}$ | 80 | | resistance of thin-film resistor $/ \Omega$ | 30 |