When the jaws are closed, it means there is nothing to measure. So the reading given by the scales should be zero.

If the reading is not zero, then that reading is the zero error. So the answer is just the reading in the picture:

To take the reading:

1. check the position of the zero mark of the vernier scale. This is between 1 and 2 mm on the main scale. So write down 1 mm on a piece of paper.

2. Find the mark on the vernier scale that coincides with a mark on the main scale. This happens at mark number 5. So write down 0.5 mm.

3. Finally, add them up: 1 + 0.5 = 1.5 mm.

This is the reading, and also the zero error.

A vector has direction and magnitude.

The quantities in the choices that have directions are: force, velocity, displacement and acceleration.

So the answer is (d).

When Mary pulls quickly, the strip moves a larger distance in between her dots. So the dots are further apart.

When she pulls slowly, the strip moves a shorter distance, so the dots are closer together.

Slowly-quickly-slowly means that the dots are closer-further-closer.

When the ball falls, it increases in speed uniformly. So the graph should be a straight line going up initially.

Possible answers are (c) and (d).

Just after it bounces, its speed decreases uniformly as it goes up. So the graph changes to a stright line going down.

So the answer is (c).

If the plane stays horizontal, it means there is no resultant upward or downward force. So the lift and weight must cancel exactly.

So lift is equal to weight.

If it accelerates, there must be a resultant force in the forward direction.

So thrust must be larger than air resistance.

Something causes her to decide to stop. Maybe a cat runs across the road.

The distance the bicycle moves between this situation and her applying the brakes is the thinking distance.

The distance between applying the brakes and coming to a stop is the braking distance.

Definition: stopping distance = thinking distance + braking distance

Pulling the brake harder affects the braking distance, which in turn affects the stopping distance.

It is not related to the thinking distance. So this stays the same.

W = mg, or weight = mass x gravitational field strength

So mass is m = W/g = 300 / 10 = 30 kg.

Mass depends on the amount of atoms in the body. So at the top of a mountain, the mass remains the same - at 30 kg.

The possible answers are (a) and (b).

The weight is given by the same formula W = mg.

g has changed. So the weight also changes.

(a) is the only possible option, because the weight in (b) has not changed.

The volume of wood = volume of box - volume of air = 1200 - 720 = 480 cm³.

So the mass of wood = density x volume = 8 x 480 = ___ g.

Stability is about what angle you can tilt the object before it falls. This happens when the centre of mass tilts over the edge of the base.

This is a geometrical thing. So the weight is not relevant. So the answer is not (c).

For the same base area, lower centre of mass is more stable. For the same height of centre of mass, large base area is more stable.

So the answer is (b).

The weight of the rod acts at the centre of mass. Since the rod is uniform, the centre of mass is at the mid point.

Since the rod is 1 m long, the weight acts 50 cm from one end of the rod. This is 30 cm from the pivot.

Principle of moments:

clockwise moment = anticlockwise moment

W x 30 cm = 1.5 N x 20 cm

W = 1.5 N x 20 cm / 30 cm = 1 N

P = F/A, or pressure = force / area

F = 96 N. A = 8 cm x 20 cm = 0.08 m x 0.20 m = 0.016 m²

P = 96 N / 0.016 m² = 6000 Pa = 6 kPa

Water pressure depends only on depth.

It may seem that if the tank is longer, there is more water, and the larger weight would give a larger pressure.

However, this weight is spread out over the length of the bottom. So per unit area (e.g. for every 1 cm²), the force at the bottom is the same. So pressure is the same,

Potential energy depends only on the vertical height.

So the ball at the lowest position has the minimum potential energy.

The formula is E = mc², or energy = mass x (speed of light)²

So E = 17.8 x 10^-30 kg x (3 x 10⁸ m/s)² = 15.8 x 10^-13 J

efficiency = (output energy)/(input energy)

In option (b), (output energy)/(input energy) = 6 / 30 = 0.2.

This agrees with the efficiency given in (b). So this is the correct answer.

Molecules in water move randomly. Some are faster, some slower. The average energy is related to the temperature.

At the water surface, molecules that happen to have high enough energy can overcome the attraction by other molecules and escape into the air.

This is evaporation.

Since molecules with high energies left, those that remain have lower average energy. So temperature falls.

A black object absorbs heat faster than objects of other colours.

If you wear a black dress under the sun, you will feel a lot hotter than if you wear a white one.

The answer is (b).

Consider two thermometers, one with a wider bore, one with a narrower bore.

Suppose they have the same volume of mercury. When the temperature increases by 1 °C, the mercury expands by the same volume.

This extra volume moves along the bore. The same volume in the narrower bore will move further.

This means it is more sensitive.

(More sensitive means that for the same temperature change, the mercury thread moves further so it can be read more accurately.)

Temperature stays the same during melting or boiling. This shows up as the flat part of the graph.

In this case, since we start with a solid, it would be melting.

Even though it continues to take in heat, the temperature does not change. This is because the energy is used to overcome the potential energy between atoms, and not to increase the kinetic energies.

As it melts, the part that becomes liquid can also evaporate.

The answer is (d).

Hot air rises because air expands when heated. It becomes less dense than the surrounding air, and floats upwards.

Reflection of sound from the flat wall is like reflection of light from a mirror. It is as if there is an image of the sound wave in the wall. The reflected wave is like this image wave coming out through the wall.

So the reflected wave is exactly the same as the incident wave, except that the direction has changed.

The refraction formula, called Snell's law, is

n = sin i / sin r

where i is the angle of incidence, r the angle of refraction and n is the refractive index.

Since we want to find r, we make sin r the subject first:

sin r = sin i / n.

Then we substitute i and n:

sin r = sin 30° / 1.33 = 0.3759

Then we find

r = sin^-1 0.3759 = 22.08°

Only a convex lens can produce a real image. A real image is one where light is made to meet. As a result, you can put a screen there an see the picture on the screen. So it cannot be (a) or (b).

A concave lens can only give virtual image. The image is always smaller than the object, and always upright.

So (d) is the only option.

X ray, microwave and ultra violet wave are all electromagnetic waves, and all travel at the speed of light.

Alpha particle has mass. Particle with mass can never reach the speed of light. So it cannot be (a) or (d).

Sound is much slower than light. So it cannot be (c).

So the only option is (b).

The actual time that the thunder takes to reach the boy is equal to the time that the flash reaches the boy plus 5 seconds.

Light (the flash) takes a much shorter time than sound (the thunder) to reach the boy. So the time for the thunder is very close to 5 seconds

Taking it to be approximately 5 seconds, the distance travelled by the thunder is velocity x time = 340 x 5 = 1700 m.

Since thunder is caused by lightning, this is also the distannce of the lightning from the boy.

If a metal is nonmagnetic, magnetic field goes right through it as if it is not there. So the box cannot be copper.

Iron is magnetic. (This means it can be attracted by a magnet, and not that it must be a magnet.) It guides magnetic field through the length of its body.

If the instrument is placed in an iron box, magnetic field through the iron would follow the length of the iron, but not go through it. In this way, the field cannot reach the instrument.

When P is close to Q, charges are induced on Q. Electrons on Q are attracted and move towards P.

So Q is negative on the side closer to P, and positive on the other side.

When the girl's finger touches Q, electrons in her body is attracted to Q.

These electrons are attracted by P via Q, and not by the positive charges in Q. So even if the finger touches the side of Q with negative charge, the electrons would still be attracted to Q.

These electrons neutralises the positive charge on Q. So Q is left with the negative charge.

In ths way, Q become negatively charged.

The formula for electric power is P = VI, where V is the voltage and I is the current.

We know P and the question is to find V. We can use the formula if we know I.

We are give the time (t) and the charge (Q). These are related to current by I = Q/t.

So we can find the current: I = Q/t = 60 / 30 = 2 A.

To find V, we rearrange P = VI. We make V the subject (divide both sides of P = VI by I): V = P/I.

Now substitute P and I : V = 24 / 2 = 12 V.

Electrons are negative.

In a circuit, current flows from positive pole of the battery to negative pole. Electrons flow in the opposite direction: from negative pole to positive pole.

We can think of the combined resistance as a single resistor that would give the same current in the circuit, if it replaces the two original resistors.

For two resistors R₁ and R₂ in series, the combined resistance R_c is give by

R_c = R₁ + R₂.

Let the resistance for each resistor be R. We are given that the combimed resistance is 4 Ω.

Substituting into the formula, we have

4 = R + R.

So R = 2 Ω

For two resistors in parallel, the combined resistance is

1/R_c = 1/R₁ + 1/R₂.

1/R_c = 1/2 + 1/2 = 1,

R_c = 1 Ω.

There is a heating element in the kettle. When current passes through it, it gets hot and heats the water.

The heating element is a coil of resistance wire. The two ends of the element is connected to two wires in the cable, called live and neutral. These go to the mains socket on the wall, where they are connected to the alternating (ac) voltage.

The neutral wire is connected to a 0 V point, and live wire is at 240 V ac.

In the cable, there is a third wire, called the earth wire. At the kettle, this is connected to the body of the kettle. At the wall socket, it is connected to earth.

It can happen that if the insulation between live wire and the kettle body is poor, the body can be at 240 V, or become "live". This is very dangerous if someone touches it.

Because the body is connected to the earth wire, a large current then flows to earth. This current comes from the live wire, which has a fuse. The large current heats up the fuse, which then melts and break - or "blow". This breaks the circuit, so the live and kettle body are not at 240 V anymore.

In parallel wires, currents in the same direction attract.

Currents in opposite directions repel.

In the motor, there is a coil of wire in a magnetic field. When current goes round the coil, the field produces a turning effect (moment) on the coil and causes it to rotate.

If we look at a small segment on the coil, there is current in the segment going through the magnetic field. The field produces a force on the current. It is this force that gives rise to the moment.

This force is bigger if there is more current or a stronger field. If there is more turns in the coil, the moment on each turn adds up to give a larger moment.

If the current changes direction, the force changes direction, but the magnitude of the force is the same. So changing the current direction does not change the moment. The answer is (a).

The resistor can be a light bulb, for example.

(a) The voltage is induced in each side of the rotating coil. When one side of the coil rotates up through the magnetic flux, the voltage is in one direction. When it then rotates down, the voltage is in the opposite direction. The voltage through the resistor does charge direction.

(b) Each turn in the coil has a voltage induced. The different turns are connected in series, so the voltages add up. So more turns mean more voltage.

(c) When one side of the coil moves through the magnetic flux, voltage is induced. When the coil is horizontal, the side moves vertically and cuts the flux most quickly. When the coil is at an angle, the side of the coil cuts the flux less quickly, so voltage decreases. So voltage does not stay the same.

(d) An iron cylinder in the coil is also in the magnetic field. So the iron cylinder becomes an induced magnet. It then produces its own field, which adds to the original field. This gives an even stronger field to the coil, and therefore a bigger voltage. So the voltage does not stay the same.

A transformer is usually made up of an iron core, with two separate coils of wires wound on the same core. An ac (alternating current) voltage is connected to one coil, called the primary coil. This produces a changing magnetic field in the iron core. When this changing field goes through the other coil, called secondary coil, a voltage is induced. This is the output voltage.

We can see that in order to produce this output voltage, the input voltage on the primary coil must be ac. Otherwise, if it is dc (direct current), then it is a fixed voltage that does not change. The magnetic field it produces is also fixed. A magnetic field that does not change cannot induce a voltage in the secondary coil.

If there are more turns on the secondary coil, the output voltage would be bigger. By changing the number of turns on the secondary coil, the transformer can be used to produce a voltage that is different from the original voltage at the primary coil.

So the answer is (b) or (d).

So a transformer can be used to change a low voltage to a high voltage. When this happens, the current in the secondary coil will be smaller than the current in the primary coil. The reason is that the power going into the transformer must be the same as the power coming out.

Power = voltage x current, or P = VI. When voltage goes up, current must come down. Then power remains the same. If current is smaller, less energy is wasted. This is because the energy of current going through the resistance R in the cables over large distance is converted to heat. This is wasted energy. The rate of heat loss is given by I²R. So if I is smaller, less energy is wasted as heat.

So the answer is (d).

When a current goes through a magnetic field, the field produces force on the current. The direction of this force is give by Fleming's left hand rule:

To use this, hold out your left hand. Point your first finger in the direction of the field, your second finger in the direction of the current, and your thumb in the direction of the force. Stretch these out so that they are all perpendicular to one another.

If the moving charges in the current are positive, then the direction of the current is in the same as the direction of the moving charges. If the moving charges are negative, the current is defined to be in the opposite direction to the moving charges.

In this question, electrons move to the right. Since these are negative, current flows to the left.

Apply the left hand rule: First finger is upwards and second finger is to the left. So thumb - the force - is into the page. Therefore (a) is the answer.

So answer is (a).

First, there is background radation, because there is reading even when no radioactive sources is around. This means that there can never be zero reading. So (c) and (d) cannot be right.

Next, we need to know that alpha particles can be stopped by a piece of paper. This means that a thick cardboard would block all alpha particles. So (b) cannot be right.

So (a) must be the answer.

¹⁶₈O means that there are 8 protons. Each proton has a positive charge that is equal and opposite to the charge of an electron.

To have a neutral atom, there must be exactly 8 electrons to cancel the positive charges.

In the beginning, there are 32000 atoms of P.
After 1 day, this is halved, so there are 16000 nuclides.
After 2 days, this is halved again to 8000 nuclides.
After 3 days, it becomes 4000 nuclides.
After 4 days, it is 2000 nuclides.
After 5 days, it is 1000 nuclides.
After 6 days, it is 500 nuclides.

Likewise for Q, are are 4000 atoms at first.
After 2 days, this is halved to give 2000 nuclides.
After 4 days, it is 1000.
After 6 days, it is 500.

Using this way, we see that they are both 500 after 6 days. So answer is(c).