A-level Mechanics OCR G484

Question 1

Define Linear momentum

Answer 1

the product of mass and velocity

Question 2

Define net force on a body

Answer 2

equal to the rate of change of momentum

Question 3

Define impulse of a force

Answer 3

Area under a force/time graph.
Change in momentum

Question 4

Define a perfectly elastic collision

Answer 4

A collision which results in no change/loss of kinetic energy

Question 5

Define an inelastic collision

Answer 5

Total energy is conserved though some loss of kinetic energy (during collision). The magnitude of the impulse on each object is the same.

Question 6

Define the radian

Answer 6

The angle where the arc of a circle equals the radius

Question 7

Define gravitational field strength

Answer 7

Force per unit mass (at a point in a gravitational field)

Question 8

Define the period of an object describing a circle

Answer 8

The time taken for the object to describe a complete circle/orbit

Question 9

Define a geostationary orbit of a satellite

Answer 9

Equatorial orbit
Same period as Earth (fixed point above the Earth’s surface)

Question 10

Define displacement

Answer 10

Is the distance of a body from the equilibrium position and is directed from the start to end point

Question 11

Define amplitude

Answer 11

The maximum displacement

Question 12

Define period

Answer 12

Time taken to compete one oscillation/cycle

Question 13

Define frequency

Answer 13

Number of oscillations/cycles per unit time

Question 14

Define angular frequency

Answer 14

Product of 2π x frequency or 2π/period

Question 15

Define phase difference

Answer 15

The angle, in radians between subsequent wave peaks

Question 16

Define simple harmonic motion

Answer 16

Force/acceleration is (directly) proportional to displacement (from the equilibrium position) and is directed towards the equilibrium position.

Question 17

Define pressure of a gas

Answer 17

Collisions with surface of large numbers of particles travelling randomly exerts a force (or each collision has a change of momentum)
Pressure = Force / Area

Question 18

Define internal energy

Answer 18

The sum of the random distribution of kinetic and potential energies associated with the molecules of a system

Question 19

Define specific heat capacity

Answer 19

The amount of thermal energy required to raise 1kg of substance through one degree Kelvin.

Question 20

Define the newton

Answer 20

The force which gives a mass of 1kg an acceleration of 1 ms-2

Question 21

Define the Kilowatt-hour

Answer 21

1kWh is the energy used/provided by a 1 kW device in 1 hour

Question 22

State the uses of geostationary satellites

Answer 22

Communication
Weather

Question 23

State Newton’s three laws of motion

Answer 23

1st: A body will remain at rest or continue to move with constant velocity unless acted upon by a force
2nd: Force is proportional to rate of change of momentum
3rd: When one body exerts a force upon another, the other body exerts an equal but opposite force on the first body.

Question 24

State the principle of conservation of momentum

Answer 24

(linear momentum) Total momentum is conserved.
For a closed system / no external forces

Question 25

State Newton’s law of gravitation

Answer 25

The attraction of two masses is directly proportional to the product of their mass and inversely proportional to the square of their distance apart/separation

Question 26

State Boyle’s law

Answer 26

Pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature

Question 27

State the effect of temperature on total internal energy

Answer 27

As temperature decreases, so does total internal energy; absolute zero is the temperature at which a substance has minimum internal energy.

Question 28

State the basic assumptions of the kinetic theory of gases;

Answer 28

Volume of particles negligible compared to volume of container OR molecules much smaller than distance between them.
No intermolecular forces (except during collision) OR molecules only have kinetic energy.
Elastic collisions
Particles travel at a constant, rapid velocity (in straight lines) between collisions OR effect of gravity is small
Time of collision is much smaller than time between collision.
Gas consists of a large number of molecules moving randomly

Question 29

State what is meant by a “mole”

Answer 29

One mole of any substance contains 6.02 × 10²³ particles and that 6.02 × 10²³ mol-1 is the Avogadro constant N_A

Question 30

Explain that F = ma is a special case of Newton’s Second Law

Answer 30

When the mass is constant, the rate of change of momentum ([change in mass] x [change in velocity] / time) can be expressed as mass x acceleration.

Question 31

Explain, using the kinetic model, how pressure is exerted by gases

Answer 31

Collisions with surface of large numbers of particles travelling randomly exerts a force (or each collision has a change of momentum)

Each collision exerts a mean force of 2mv

Total force = 2mv/t where t is the average time between collisions.

Pressure = Force / Area

Question 32

Explain that whilst the momentum of a system is always conserved in the interaction between bodies, some change in kinetic energy usually occurs.

Answer 32

Changes in kinetic energy occur because not all collisions are perfectly elastic, some energy is lost in deformation, thermal changes etc

Question 33

Explain that a force perpendicular to the velocity of an object will make the object describe a circular path;

Answer 33

(Resultant) force acts perpendicular to velocity (towards the centre)

The net effect of this force is to change the direction of the velocity vector without affecting its magnitude.

The change in direction is such that the object travels in a circular path with a radius that increases as the velocity increases.

Question 34

Explain what is meant by centripetal acceleration and centripetal force;

Answer 34

Velocity or direction is always changing
Acceleration is in the direction of the force OR towards the centre/perpendicular to velocity

Question 35

Explain that close to the Earth’s surface the gravitational field strength is uniform and approximately equal to the acceleration of free fall;

Answer 35

Close to Earth, field lines are effectively parallel and therefore uniform.

Question 36

Explain that the period of an object with simple harmonic motion is independent of its amplitude;

Answer 36

T = 2 π √m/k

Question 37

Explain that the rise in temperature of a body leads to an increase in its internal energy;

Answer 37

The total internal energy of a substance is the kinetic energy and the potential energy. Only kinetic energy contributes to temperature.

Question 38

Explain that a change of state for a substance leads to changes in its internal energy but not its temperature;

Answer 38

When undergoing a state change heat energy is used to increase the potential energy of a substance (to “break/weaken bonds”). The kinetic energy of the particles does not change; so the temperature does not change. Internal energy is the sum of kinetic and potential energies so as the P.E. increases so does the I.E.

Question 39

Explain that thermal energy is transferred from a region of higher temperature to a region of lower temperature;

Answer 39

Whenever there is a temperature gradient (a difference in temperature) heat will ‘flow’ from the warmer body to the colder body. This is due to collisions between more energetic (hotter) particles with less energetic (colder)particles. This results in a mean increase in the velocities of the colder particles, and a mean decrease in the velocities of the hotter particles i.e. the two become the same temperature

Question 40

Explain that regions of equal temperature are in thermal equilibrium;

Answer 40

No net heat flow between objects

Question 41

Explain that the mean translational kinetic energy of an atom of an ideal gas is directly proportional to the temperature of the gas in kelvin;

Answer 41

E = 3/2kT KE = 1/2mv2
3/2kT = 1/2mv² (3/2k is a constant)
T is proportional to KE

Question 42

What is the area under a force-time graph equal to?

Answer 42

that the area under a force against time graph is equal to impulse;

impulse = change in momentum.

Question 43

Use gravitational field lines to represent a gravitational field;

Answer 43

Question 44

Derive the equation the equation
T²={4π²/GM} r³
from first principles;

Answer 44

1) F = GMm/r² = mv²/r
2) Rearrange to get v² = GM/r
3) T = 2πr/v hence T² = 4π²r²/v²
4) Substitute for v²: T² = 4π²r²r/GM and rearrange

T²={4π²/GM} r³

Question 45

Describe how a mass creates a gravitational field in the space around it;

Answer 45

F = GM/r²

Question 46

Describe simple examples of free oscillations;

Answer 46

Pendulum, mass on a spring

Question 47

Describe, with graphical illustrations, the changes in displacement, velocity and acceleration during simple harmonic motion;

Answer 47

Question 48

Describe the interchange between kinetic and potential energy during simple harmonic motion;

Answer 48

As one increases the other decreases, total energy remains constant.

When x=A all P.E no K.E

When x = 0 All K.E no P.E

Question 49

Describe the effects of damping on an oscillatory system;

Answer 49

Damping an effect that reduces the amplitude of oscillations

Question 50

Describe practical examples of forced oscillations and resonance;

Answer 50

Where a force is applied with a frequency matching the natural frequency of the system, resulting in resonance. For example Barton’s pendulums or a wine glass.

Question 51

Describe graphically how the amplitude of a forced oscillation changes with frequency near to the natural frequency of the system;

Answer 51

Question 52

Give examples where resonance is useful and other examples where resonance should be avoided.

Answer 52

Useful: microwaves cause water molecules to vibrate. Woodwind reed/lips cause air column to resonate. MRI radio waves cause nuclei to vibrate

Problem: walking in step on a bridge. Engine vibrations causing car to shake. Earthquake ground vibrations causing buildings to collapse.

Question 53

Describe solids, liquids and gases in terms of the spacing, ordering and motion of atoms or molecules;

Answer 53

Question 54

Describe a simple kinetic model for solids, liquids and gases;

Answer 54

Solids: Particles closely packed together, particles have vibrational energy and vibrate about an equilibrium position. These particles have no translational kinetic energy. Low potential energy

Liquids: particles marginally further apart than in solids, some translational kinetic energy, high vibrational kinetic energy. Moderate amount of potential energy

Gas: Large inter-molecular distance, particles have very high translational kinetic energy. Particles have large potential energy.

Question 55

Describe an experiment that demonstrates Brownian motion and discuss the evidence for the movement of molecules provided by such an experiment;

Answer 55

Movement of smoke particles caused by being hit by randomly moving, different speed, air molecules
Smoke particles are constantly moving because the air particles are continuously moving
Smoke particles are visible but air molecules aren’t hence air molecules must be very small
Small movement of smoke particles is due to the large numbers of air molecules hitting from all sides

Question 56

Describe, using a simple kinetic model for matter, the terms melting, boiling and evaporation.

Answer 56

Melting: When a substance changes from a solid to a liquid. During melting: KE of particles stays the same, PE increases, temperature remains constant. Inter-molecular distance increases as energy is used to break/weaken bonds.

Boiling: When a substance changes from a liquid to a gas. During boiling; KE of particles stays the same, PE increases, temperature remains constant. Inter-molecular distance increases as energy is used to break/weaken bonds.

Question 57

Describe how there is an absolute scale of temperature that does not depend on the property of any particular substance (ie the thermodynamic scale and the concept of absolute zero);

Answer 57

Temperature arises from the random submicroscopic vibrations of the particle constituents of matter. These motions comprise the kinetic energy in a substance. More specifically, the thermodynamic temperature of any bulk quantity of matter is the measure of the average kinetic energy of a certain kind of vibrational motion of its constituent particles.

Thermodynamic temperature’s null point, absolute zero, is the temperature at which the particle constituents of matter are as close as possible to complete rest; that is, they have minimal motion or zero kinetic energy

Thermodynamic temperature is scaled so that the triple point of (Vienna standard mean ocean) water is 273.16 degrees kelvin.

Question 58

Descibe an electrical experiment to determine the specific heat capacity of a solid or a liquid;

Answer 58

Must show liquid in vessel with electrical heater with thermometer, ammeter and voltmeter
Measure mass of liquid, temperature change, values of I, V & t.
Rearrange E=mcΔθ
Identify uncertainties
Note ‘specific’ means ‘per unit mass’.

Question 59

Describe what is meant by the terms latent
heat of fusion and latent heat of vaporisation.

Answer 59

Latent Heat of Fusion: Thermal energy required to change a solid into a liquid at constant temperature.
Latent Heat of vaporisation: Thermal energy required to change a liquid into a gas at constant temperature