G484 - Standard Answers

Question 1

State in words Newton’s first law of motion

Answer 1

A body will remain at rest or continue to move with constant velocity unless acted upon by a force.

Question 2

State in words Newton’s second law of motion

Answer 2

The resultant force acting on an object is directly proportional to the rate of change of momentum;
and occurs in the same direction as the change in momentum.

Question 3

State in words Newton’s third law of motion.

Answer 3

When one body exerts a force on a second body, the second body exerts a force which is equal in magnitude and opposite in direction on the first body

Question 4

Two objects A and B collide. Explain, using Newton’s third law of motion, the relationship between the impulse experienced by A and the impulse experienced by B during the collision.

Answer 4

By Newton’s 3rd Law, the force exerted on B due to A is equal in magnitude and opposite in direction to the force exerted on A due to B;  the time of contact during the collision is the same for both objects and impulse = Ft;  Hence, the impulse on A is equal in magnitude and opposite in direction to the impulse on B.

Question 5

Define the impulse of a force

Answer 5

Force x time for which the force acts.

Question 6

Define linear momentum.

Answer 6

mass x velocity.

Question 7

Explain why linear momentum is a vector quantity.

Answer 7

velocity is a vector;  the product of a scalar (mass) and a vector (velocity) is a vector (momentum).

Question 8

State the principle of conservation of linear momentum

Answer 8

Total momentum is conserved;  for a closed system / provided no external forces are applied.

Question 9

State what is meant by a perfectly elastic collision.

Answer 9

A collision in which no kinetic energy is lost (kinetic energy is conserved).

Question 10

State what is meant by an inelastic collision.

Answer 10

A collision in which there is some loss of kinetic energy

Question 11

Explain how the speed of an object undergoing circular motion remains constant even though there is a resultant force acting on it.

Answer 11

The resultant force acts at a right angle to the motion of the object;
and so no work is done by the force;
hence, the kinetic energy (and speed) of the object does not change.

Question 12

State, in terms of force, the conditions necessary for an object to move in a circular path at constant speed.

Answer 12

The resultant force acts on the object in a direction which is perpendicular to the direction of motion (velocity).

Question 13

State what is meant by a geostationary orbit

Answer 13

The spaceship/satellite is always vertically above the same point on the surface of the Earth/planet.

Question 14

State some of the properties of a geostationary orbit.

Answer 14

The orbit is equatorial (that is it is above the equator);  The velocity of the satellite is parallel to the velocity of a point on the surface of the planet at all times;  The satellite orbits in the same direction as the rotation of the planet.

Question 15

Describe the pattern of gravitational field lines in a uniform field.

Answer 15

The field lines are parallel to each other;  The field lines are equally spaced.

Question 16

State, in words, Newton’s law of gravitation

Answer 16

The gravitational force exerted on one object due to another object is proportional to the product of their masses;  and inversely proportional to the square of the distance between their centres of mass.

Question 17

Define gravitational field strength.

Answer 17

Force per unit mass at a point in a gravitational field.

Question 18

State Kepler’s Third Law

Answer 18

The cube of the planets distance from the Sun divided by the square of the orbital period is the same (for all planets).

Question 19

Define, in words, simple harmonic motion.

Answer 19

Acceleration is proportional to displacement from the equilibrium position;  and is always directed towards the equilibrium position.

Question 20

For simple harmonic motion, describe the difference between displacement and amplitude

Answer 20

Displacement is the distance of the body from the equilibrium position;  whereas amplitude is the maximum displacement.

Question 21

For simple harmonic motion, describe the difference between frequency and angular frequency

Answer 21

Frequency is the number of oscillations per unit time;  whereas angular frequency is the product of 2π x frequency

Question 22

Explain what is meant by a resonance of a mechanical system.

Answer 22

Resonance occurs when the driving frequency applied to the system matches the natural frequency of the system;  When this happens, the amplitude of vibrations is then a maximum.

Question 23

State and explain an everyday example of resonance.

Answer 23

A child on a swing that is being pushed is an example of resonance:  In this example, the person pushing provides the driving frequency, and the child on the swing is the part of the system being driven;  When the driving frequency of the person pushing the swing matches the natural frequency of the swing, the amplitude of oscillations will increase.

Question 24

Give an example where resonance is a problem

Answer 24

Earthquakes; the ground vibrating causes buildings to vibrate and possibly collapse.

Question 25

State the effects of damping on resonance.

Answer 25

Smaller amplitude of vibrations;  The system will have a lower natural frequency (so driving frequency must change in order to produce resonance).

Question 26

Describe the motion of atoms in a solid at a temperature well below its melting point

Answer 26

The atoms vibrate about their fixed positions

Question 27

Describe the effect that a small increase in temperature would have on the motion of atoms in a solid at a temperature well below its melting point.

Answer 27

The atoms would vibrate with a greater amplitude / greater frequency, still about their fixed positions.

Question 28

Describe the effect on the internal energy and temperature of a solid when it melts

Answer 28

The internal energy increases (that is the potential energy of the solid’s molecules increases whilst the kinetic energy remains constant);  and the temperature remains constant.

Question 29

Explain the term thermal equilibrium.

Answer 29

There is no net heat flow between two objects (they are at the same temperature)

Question 30

Define specific heat capacity.

Answer 30

The energy required to raise the temperature of a unit mass of a substance by a unit rise in temperature.

Question 31

Define the term latent heat of fusion

Answer 31

The energy required to change a substance from solid to liquid.

Question 32

Define the term latent heat of vaporisation.

Answer 32

The energy required to change a substance from liquid to gas.

Question 33

Define the internal energy of a system

Answer 33

The sum of the randomly distributed kinetic energy and potential energy of the atoms/molecules in the system.

Question 34

There is a change in internal energy when a mass of water at 100 oC becomes an equal mass of vapour at 100 oC. Explain why.

Answer 34

The potential energy of the molecules increases as work is done breaking the bonds between the molecules;
the kinetic energy of the molecules is the same in the liquid and gaseous states as the temperature is the same;
as internal energy is the sum of the potential energy and kinetic energy, internal energy increases.

Question 35

State some conclusions that may be deduced about the motion of the molecules of a liquid/gas by observing the motion of pollen grains/smoke particles (that are suspended in the liquid/gas) under a microscope.

Answer 35

The movement of the pollen grains/smoke particles is caused by the molecules of the liquid/gas moving in a random/haphazard manner;  Pollen grains/smoke particles are visible but the liquid/gas molecules are not, hence the molecules of the liquid/gas are much smaller than the pollen grains/smoke particles;  The pollen grains/smoke particles are continuously moving, which means the liquid/gas molecules are continuously moving.

Question 36

State the assumptions made in the development of the kinetic model of an ideal gas.

Answer 36

The collisions between the gas molecules and the walls of the container are perfectly elastic;  The force between molecules is negligible except during collisions;  The volume of the molecules is negligible compared to the volume of the container;  The time during a collision is negligible compared to the time between collisions.

Question 37

Use the kinetic model of a gas and Newton’s laws of motion to explain how a gas exerts a pressure on the walls of its containers.

Answer 37

When a molecule collides with a wall, its momentum is changed;  in order to change its momentum, the wall must have provided a force which acted on the molecule as the rate of change of momentum is proportional to the force exerted (by Newton’s 2nd Law);  By Newton’s 3rd Law, the force exerted on the wall by the molecule is equal in magnitude and opposite in direction to the force exerted on the molecule by the wall;  The total pressure experienced by the wall is = (sum of all forces exerted by the molecules) / (area of the wall).

Question 38

A gas molecule of mass m travelling perpendicular to the wall of a container hits the wall with speed v. Explain why the molecule rebounds with speed v and undergoes a change of momentum 2mv.

Answer 38

As the collision between the molecule and the wall is elastic, kinetic energy is conserved and so the molecule rebounds with the same speed it hit the wall with;  the velocity with which it rebounds is -v;  the change in momentum is mv - (-mv) = 2mv.

Question 39

State the conclusions about the movement of gas molecules provided by the observations of Brownian motion.

Answer 39

Gas molecules moving in random motion.

Question 40

A constant mass of gas occupies a container of constant volume. Use the kinetic theory of gases to explain the increase in the force exerted on the wall of the container by the gas when the temperature is increased.

Answer 40

As the temperature increases, the speed of the gas molecules increases;  As a result, collisions between the molecules and the walls of the container are more frequent;  and as the molecules are travelling (and rebounding) with larger speeds, the change in momentum of the molecules increases;  and so the total force exerted on the walls of the container increases.

Question 41

A constant mass of gas occupies a container whose volume can change. Use the kinetic theory of gases to explain why the volume of the gas must increase if the pressure is to remain constant as the gas is heated

Answer 41

As the temperature increases, the speed of the gas molecules increases;  As a result, collisions between the molecules and the walls of the container are more frequent;  and as the molecules are travelling (and rebounding) with larger speeds, the change in momentum of the molecules increases;  For a constant pressure fewer collisions per unit time are required;  this can be achieved by increasing the volume as the molecules will have to travel a greater distance between collisions, decreasing the number of collisions per unit time they undergo

Question 42

State, in words, Boyle’s law

Answer 42

For a fixed mass of gas at constant temperature;  the pressure of the gas is inversely proportional to the volume of the gas

Question 43

The escape velocity is the minimum vertical velocity a particle must have in order to escape from the Earth’s gravitational field. Explain why atoms such as helium, which have a mean speed lower than the escape velocity, still escape from the Earth’s atmosphere. 

Answer 43

The helium atoms have a range of speeds and kinetic energies;  Some of the atoms will have a velocity greater than the escape velocity and will therefore be able to leave the atmosphere.