Mechanics as a whole

Question 1

Which one of the following has the same unit as the rate of change of momentum?

Answer 1

Weight

Question 2

2.The nucleus of a radioactive isotope X is at rest and decays by emitting an α particle so
that a new nuclide Y is formed.
Which one of the following statements about the decay is correct?
A The momentum of Y is equal and opposite to the momentum of the α particle.
B The momentum of Y is equal to the momentum of X.
C The kinetic energy of Y is equal to the kinetic energy of the α particle.
D The total kinetic energy is the same before and after the decay.

Answer 2

A

Question 3

Elastic Collision:

Answer 3

One in which there is no loss of kinetic energy in the collision.

Question 4

Define the moment of a force about a point (2 marks)

Answer 4

• Moment = Force x Perpendicular distance

- BETWEEN LINE OF ACTION and PIVOT POINT

Question 5

Tensile stress:

Answer 5

Force exerted per CROSS SECTIONAL AREA

Question 6

Tensile Strain:

Answer 6

Extension over ORIGINAL LENGTH

Question 7

State which feature of a graph confirms that a material is elastic:

Answer 7

Returns to original shape

NO PERMANENT EXTENSION

Question 8

With reference to a spring, explain what is meant by limit of proportionality: (1 mark)

Answer 8

Point beyond which
Graph is no longer linear
OR
Force no longer proportional to extension

Question 9

What happens when an object in free fall is no longer accelerating?

Answer 9

Resulting forces are equal, e.g in a bungee jump f in rope F=K/_\L and W=mg
When acceleration is zero
kdeltaL=mg and can solve

Question 10

Should you use only part of the information in a question?

Answer 10

NO, use ALL FIGURES given in the question otherwise won’t be right

Question 11

The bungee jump operator intends to use a bungee rope of the same unextended
length but with a much greater stiffness. The rope is to be attached in the same way
as before.
Explain, with reference to the kinetic energy of the jumper, any safety concerns that
may arise as the jumper is slowed down by the new rope. (3 marks)

Answer 11

-Same kinetic energy when rope begins to stretch

-More work done per unit extension / stops in shorter distance
“Shorter time” gets no credit

-Increases force on jumper (increasing the risk of injury)

Question 12

The car does not reach the height calculated in part (b).

i) Explain the main reason why the car does not reach this height. (2

Answer 12

• (work done) by friction

- (energy converted) to thermal energy

Question 13

State and explain what happens to the gravitational potential energy lost between D
and E. (Bottom of slope of snowboarder, 3 marks)

Answer 13

• Idea that GPE is ultimately transferred to: thermal (energy)
• Due to WORK BEING DONE
• By FRICTION
• ALL GPE BECOMES HEAT

Question 14

Ball moving on frictionless track in absence of air.
Galileo thought that, under these circumstances, the ball would reach position C if
released from rest at position A. Position C is the same height above the ground as
A.
Using ideas about energy, explain why Galileo was correct. (3 marks)

Answer 14

• GPE > KE > KE
• No energy lost from system as no energy is lost by work being done by resistive forces eg friction
• Therefore initial gpe= final gpe

Question 15

Between A and B, the work done by the electric motor is 3700 J, and the work done
by the cyclist pedalling is 5300 J.
(i) Calculate the wasted energy as the cyclist travels from A to B.

State two causes of this wasted energy. (2)

Answer 15

5300+3700=9000
9000-Ke in bike - Gpe in bike

second part:

• Friction IN THE TYRE losing energy as thermal energy (1)
• Air restistance (1)

Question 16

Is the force acting on an object equal to its change of momentum?

Answer 16

NO. but, the force acting on an object is equal to its RATE of change of momentum

The force acting on an object is also equivalent to the impulse it receives per second.

Question 17

State the principle of moments (3 marks)

Answer 17

• Sum of clockwise moments= sum of anticlockwise moments (2)

- For a system in equilibrium

Question 18

State and explain one characteristic of the aircraft that limits its maximum
horizontal velocity. (2 marks)

Answer 18

• Aerodynamics of the plane

- As air resistance due to aerodynamics will limit the maximum velocity it can travel at

Question 19

State how a velocity-time graph could be used to find the maximum acceleration.

Answer 19

use MAXIMUM gradient

Question 20

Question with a hockey ball and golf ball hanging down.
Show that momentum is conserved in the collision and that the collision is
perfectly elastic. (4 marks)

Answer 20

-Momentum before = momentum after (1)
(m1v1)+(m2v2)=(m1v1)+(m2v2)
(0.15x0)+(0.05x1.3)=(0.15x0.65)+(0.05x-0.65) (- due to opposite direction)
0.065=0.065
Therefore momentum is conserved (2)
-Initial kinetic energy=final kinetic energy (3)
(0.5x0.05x1.3^2)= (0.5x0.15x0.65^2) + (0.5x0.05x0.65^2)
0.04225=0.04225
Therefore ke =ke after so collision is perfectly elastic

Question 21

Explain how this demonstration relates to the moderation process in a reactor
and state one way in which the collisions in a reactor differ from the collision in
the demonstration. (2 marks)

Answer 21

• Loss of ke of the golf ball due to hockey ball is like neutrons being slowed down
• Collisions in the reactor are rarely head on/not always elastic

Question 22

is acceleration acting upwards positive or negative

Answer 22

negative(-) as acting in the opposite plane to which gravity acts

Question 23

steel ball falling on lens question.

Calculate the change in momentum of the ball due to the impact.

Answer 23

going down and bouncing back up therefore change Isn’t mv-mu IT IS 2P (MV+MU)

Question 24

Explain, with reference to momentum, why the test should also specify the material
of the plinth the lens sits on (2 marks)

Answer 24

-A softer plinth would decrease the change in momentum of
the ball (or reduce the height of rebound)
-Increased impact time would reduce the force of the impact.

Question 25

If no acceleration is mentioned in relation to horizontal components what do you NOT Use?

Answer 25

SUVAT (use s=d/t) (Only use suvat for horizontal when you are given a horizontal acceleration, or have to work out a horizontal acceleration)

Question 26

State and explain how the horizontal forward force on the vehicle has to change for
constant acceleration to be maintained from 0 to 27.8 m s–1
( 3marks)

Answer 26

-Horizontal forward force would have to increase
-As when velocity increases the resistive force (air resistance) increases
-So the forward force must be greater than the resistive force *
(so that resultant force stayed the same)

Question 27

How to calculate the angle in degrees of an object turns in circular motion over a time period:

Answer 27

ANGLE : THETA=ANGULAR SPEED(W)xt (Wt)

This calculates the angle in rads. To get to degrees , times by 180/pi

Question 28

How to work out the tension in a string that is undergoing circular motion:
WHEN YOU ASSUME IT IS HORIZONTAL.

Answer 28

USE CENTRIPETAL FORCE

F=mv^2/r

Question 29

How to work out the number of revolutions per second an object will make demonstrating circular motion

Answer 29

w=2pif rearrange for f to work out the revs-1

Question 30

Discuss the motion of the ball in terms of the forces that act on it. In your answer
you should:
• explain how Newton’s three laws of motion apply to its motion in a circle
• explain why, in practice, the string will not be horizontal.
( 6marks)

Answer 30

-First law: ball does not travel in a straight line, so a force must be
acting on it
-although the ball has a constant speed its velocity is not constant
because its direction changes constantly
-because its velocity is changing it is accelerating
-Second law: the force on the ball causes the ball to accelerate (or
changes the momentum of it) in the direction of the force
-the acceleration (or change in momentum) is in the same direction
as the force
-the force is centripetal: it acts towards the centre of the circle
-there is equilibrium (or no resultant force) in the vertical direction
-the weight of the ball, mg, is supported by the vertical component
of the tension, F cos θ, where θ is the angle between the rope and
the vertical and F is the tension
-the horizontal component of the tension, F sin θ, provides the
centripetal force m ω
2
r

Question 31

Explain why the time to fall vertically between the plates is independent of the mass
of a particle. (2 marks)

Answer 31

-Force on a particle = mg and
acceleration = F / m so always = g
-Time to fall (given distance) depends (only) on the distance and acceleration

Question 32

State and explain two reasons, why the horizontal acceleration of a particle is different for each particle. (4 marks)

Answer 32

-Mass is not constant since particle mass will vary
Acceleration = Eq / m
-Charge on a particle is not constant
Acceleration = Eq / m

Question 33

Rifle B is fired and the bullet emerges with a smaller horizontal velocity than the
bullet from rifle A.
Explain why the horizontal distance travelled by bullet B will be less than bullet A. (3 marks)

Answer 33

• their vertical motion is independent of their horizontal motion /vertical speed is same for both
• (bullets A and B will be in the air) for the same time
• BUT distance is proportional to horizontal speed (d=sxt)

Question 34

Describe how a beam of fast moving electrons is produced in the cathode ray tube of
an oscilloscope. (3 marks)

Answer 34

-cathode heated
-accelerated by electric field between
anode and cathode (using anode gets 3rd mark)

Question 35

OHMS LAW

Answer 35

ratio of voltage / current is constant

Question 36

What happens if the beam wasnt negligible (water q, 3 marks)

Answer 36

-Weight of beam acts to right of the pivot/centre of mass is to
the right of the pivot
-provides an additional clockwise moment
-Therefore the value calculated in Q6.2 would be less to have
equilibrium

Question 37

Is impulse a scalar or a vector?

Answer 37

Vector

Question 38

A firework is fired vertically up into the air and subsequently falls to the ground.
Which quantity relating to the motion of the rocket is never zero before it hits the
ground? Assume that air resistance is negligible.

A acceleration
B velocity
C momentum
D kinetic energy

Answer 38

A

Question 39

A pd V is applied across a resistor. Another identical resistor is then connected in
series with it and the same pd V is applied across the combination.
Which statement is incorrect?

[1 mark]

A The total resistance is doubled.
B The pd across one resistor is 2
V .
C The current in the resistors is halved.
D The power dissipated in one resistor is halved.

Answer 39

D

Question 40

Which graph shows how the velocity v of a body moving with simple harmonic
motion varies with its displacement x?

Answer 40

Circe/oval graph

Question 41

A mechanical oscillator is set into motion by a periodic driving force whose
frequency is steadily increased from a low value.
What is correct for this system?

[1 mark]

A Forced vibrations occur only at particular frequencies.
B The oscillator is subject to damping only at the resonant
frequency.
C When resonance occurs the damping force is a
minimum.
D The oscillator will not continue to resonate when the
periodic driving force is removed.

Answer 41

D

Question 42

A small quantity of fine sand is placed onto the surface of the plate. Initially the
sand grains stay in contact with the plate as it vibrates. The amplitude of the
vibrating surface remains constant at 3.5 × 10-4

m over the full frequency range of
the signal generator. Above a particular frequency the sand grains lose contact
with the surface.
Explain how and why this happens.

Answer 42

when the vibrating surface accelerates down with an acceleration
less than the acceleration of free fall the sand stays in contact. 
above a particular frequency, the acceleration is greater than g 
there is no contact force on the sand OR
sand no longer in contact when downwards acceleration of plate
is greater than acceleration of sand due to gravity 