Which of the following is a simple permanent tissue?

The correct answer is B, Gravitational force and Normal force. A block lying quietly on a table is in equilibrium, so the forces on it must add up to zero, but that does not mean there are no forces at all. The Earth pulls the block down with its weight, mg. The table pushes back on it with an equal and opposite normal reaction, acting perpendicular to the surface, and these two balance each other. Friction is a force that appears only when there is motion along the surface or a tendency towards such motion; nothing is pushing the block sideways here, so no friction acts. This is Newton's first law in a familiar everyday form. Option A is wrong because gravity alone would make the block accelerate downward through the table. Option C is wrong because static friction needs an applied horizontal force that the block is resisting. Option D is wrong because sliding friction acts only while a body is actually sliding over a surface. Exam tip: on a body at rest with no horizontal push, only weight and normal reaction act, and friction is zero.

The correct answer is A, 1038 m. An echo is the sound heard when a wave is reflected from a distant obstacle and comes back, so in the time measured the sound covers the distance to the cliff and then the same distance back again. The total path is therefore twice the distance, and twice the distance equals v into t, that is 346 into 6 = 2076 metres. Dividing by two gives a distance of 1038 metres. For an echo to be heard as a separate sound the reflecting surface must be far enough away that the return takes at least 0.1 second, which in air needs a distance of about 17 metres. Option B is wrong because 1083 is only a rearrangement of the digits of the correct figure. Option C is wrong because 2076 m is the full to-and-fro path rather than the distance to the cliff. Option D is wrong because 2706 follows from neither step of the calculation. Exam tip: the distance of the reflector equals v into t divided by 2, and the minimum distance for a distinct echo in air is about 17 m.

The correct answer is A, hammer, anvil and stirrup. The middle ear holds three tiny bones, the smallest in the human body, called the malleus, the incus and the stapes, which in everyday English are the hammer, the anvil and the stirrup. Sound waves collected by the pinna travel down the ear canal and set the eardrum vibrating. The three ossicles pass that vibration along in a chain and amplify it about twenty times before handing it to the oval window of the cochlea, where the fluid inside converts it into nerve signals for the auditory nerve. The stapes is the smallest bone in the body, roughly 3 mm long. Option B is wrong because the cochlea is the spiral fluid-filled part of the inner ear, not a middle-ear bone. Option C is wrong because the pinna is the outer flap of the ear. Option D is wrong because there is no bone called the auditory bone. Exam tip: malleus, incus and stapes are the hammer, anvil and stirrup, and the stapes is the smallest bone in the human body.

The correct answer is B, in the downward direction. The force on a current-carrying conductor placed in a magnetic field is found with Fleming's left-hand rule. Stretch the thumb, the forefinger and the middle finger of the left hand at right angles to one another: the forefinger points along the magnetic field, which is north here, the middle finger along the current, which runs from east to west here, and the thumb then gives the direction of the force, which comes out vertically downward. The size of this force is F = BIL sin of the angle between the current and the field, and it is greatest when the two are at right angles, as they are here. This force is the principle behind the electric motor. Option A is wrong because a sideways force would need the field and the current to lie in a vertical plane. Option C is wrong because reversing either the current or the field would be needed to push the conductor upward. Option D is wrong for the same reason as option A. Exam tip: Fleming's left hand is for the motor and gives force, while Fleming's right hand is for the generator and gives induced current.

The correct answer is D, Solids. Sound is a mechanical wave: it travels only when the particles of a medium push against their neighbours, so the closer and more tightly bound those particles are, the faster the disturbance is passed on. Particles in a solid are packed most closely and held by the strongest intermolecular forces, so sound moves through a solid quickest. Typical speeds at ordinary temperature are about 346 metres per second in air, roughly 1500 metres per second in water and about 5000 metres per second in steel, which is why a rail track carries the sound of an approaching train long before the air does. Option A is wrong because gas particles lie far apart, giving the slowest speed of the three. Option B is wrong because a vacuum has no particles at all, so sound cannot travel through it. Option C is wrong because a liquid carries sound faster than a gas but slower than a solid. Exam tip: the order is solids faster than liquids faster than gases, and sound needs a medium while light does not.