Nuclear forces

Nuclear forces act between neutrons and

protons within the nuclei of atoms (see chapter 33). They are the forces that hold the protons and neutrons of the atoms together within the nucleus (figure 6.19). These are immensely strong forces and they act only over short distances within the nucleus. You will learn more about this force in your later study of physics. This is perhaps the only type of force that we do not come

across in our everyday activities, since it is found only in the nuclei of atoms.

ITQ7

A bar magnet is supported in a vertical position by a short length of thread, AB, an elastic band, BC, with its lower end resting on a horizontal table as shown in figure 6.18.

Name and label each type of force acting on the magnet, indicating by an arrow the direction of each force.

nuclear force ❯

CHAPTER 33

proton

neutron

Figure 6.19 Nucleus of an atom.

Chapter summary

• A force is always either a push or a pull. • Forces act in pairs.

• Forces acting between two bodies always act in opposite directions on these bodies. • A force produces one or more of the following effects on a body on which it acts:

– change of position; – change of dimensions; – change of shape;

Answers to ITQs

ITQ1 The separation of the strings is the same all along their length.

ITQ2 That there is now a resultant force acting on the sphere when there was none before. Because there is now a resultant force (the weight) causing the sphere to fall.

ITQ3 Weight = m × g. So on Earth, astronaut’s weight = 72 kg × 10 N kg–1 ₌

720 N and on the Moon, weight = 1₆ of 720 N = 120 N

ITQ4 mg = W, so m = W/g = 6 N/(g/6) N kg–1 _{= 3.6 kg.}

ITQ5 (i) Thrust; (ii) normal reaction

ITQ6 A charged body brought near to the magnet is not attracted or repelled, so force is not electrostatic. A non-magnetic body brought near to the magnet is not attracted, so force is not gravitational. Further, gravitational forces are not repulsive, but attractive.

ITQ7 Your answer should show: (i) an arrow along AB directed away from the magnet (a tension); (ii) an arrow along BC directed away from the magnet (a tension); (iii) an arrow from the centre of the magnet directed vertically downwards (the weight of the magnet); (iv) an arrow from the point of contact of the magnet with the table directed vertically upwards (the normal reaction of the table to the magnet).

ITQ8 (i)(a) By using the pull of the elastic cord, which would have been the same throughout the length of the cord. (b) By using a smooth bench top where the friction would be negligible.

(ii) No, since there would have been some friction between the masses and the bench-top.

(iii) Since the friction between the kilogram mass and the bench top would

have been the smaller of the two, the net force on the kilogram mass is likely to have been larger.

– change of speed in a moving body; – change of direction in a moving body. • There are six main types of force:

– Gravitational force: the attraction between two masses, whether or not they are separated.

– Mechanical force: a force that acts between two bodies which are in contact; such a force can be produced in a number of different ways.

– Electrostatic force: acts between bodies that are electrically charged. – Magnetic force: acts between magnetised bodies and current-carrying

conductors.

– Elastic force: occurs in bodies which can be stretched, compressed, or squashed. – Nuclear force: binds protons and neutrons together in an atomic nucleus.

• The weight of a body acts from its centre of mass.

• The ‘centre of mass’ of a single body is the point at which the entire mass of the body can be considered to act. The centre of mass may also be called ‘centre of gravity’.

Examination-style questions

1 The roughly circular path followed by the Moon around the Earth is due to the gravitational force between them.

(i) Copy the diagram and draw arrows showing the force acting:

(a) on the Earth; (b) on the Moon.

(ii) What is the relationship between these forces? Imagine a satellite half-way between the centres of the Moon and the Earth.

(iii) Draw arrows showing the forces acting on the satellite due to the mass of the Moon and that of the Earth.

(iv) Which of the two masses will exert the greater force on the satellite? Give a reason for your answer.

2 Two identical bar magnets are suspended in stirrups from two strings so that they are initially in line, with an N-pole and an S-pole facing each other. When released, they remain at rest as shown below.

Copy the diagram and add arrows showing the forces acting on each of the magnets. State clearly which of the forces is/are mechanical, magnetic and/or gravitational.

3 (i) Explain the relationship between the mass of a body and its weight.

(ii) What do you understand by the statement: ‘the value of g on the Moon is 1/6 of that on the Earth’?

(iii) If g on the Earth is 10 N kg–1_{, what mass (in kg) will weigh 10 N on the Moon?}

Earth

Moon

















(or the centre of mass) of a body in the shape of a lamina





whether or not that body will be stable





in equilibrium