Mechanics

Question 1

why circular motion won’t change speed even though there is net force

Answer 1

F is perpendicular to electrons displacement

no work is done so KE remain unchanged

Question 2

why people feel weightless in space

Answer 2

astronauts and spacecraft both moving with the same acceleration towards the earth

Question 3

when will momentum not conserved

Answer 3

when there is external force
eg the object is fixed in position
or there is mgsin veta on a slope
or friction

Question 4

why beam balance fails to work while free fall

Answer 4

weightless

Question 5

when answering energy conservation questions

it is a must to state that

Answer 5

By conservation of energy

Question 6

How to determine the centre of gravity in an experiment

Answer 6

1. Use a hole punch to punch a small hole in the cardboard
2. Hang the cardboard and the string, which acts as the vertical, on the hook. Wait for equilibrium
3. Use the pencil and ruler to draw a line on the cardboard along the string.
4. Punch another hole on the cardboard and repeat the procedures. Draw another line along the strings.
5. The intersection of the two lines is the center of mass of the cardboard.

Source of error: The centre of mass may alter when punching holes

Question 7

why the airship comes to a constant velocity after some time

Answer 7

due to air resistance

the airship reaches its terminal velocity.

Question 8

experiment question:

include physical quantities to be measured and the result expected

Answer 8

state that we should plot a graph y against x

then (a straight line passing through the origin is obtained)

Question 9

describe the movement of bungee jumper

Answer 9

accelerate at g before the elastic cord stretches
accelerate decreases as the cord stretches
decelerate until momentarily at rest.

Question 10

why the full body harness is better than ankle attachment only for a bungee jumper

Answer 10

contact area is larger

hence pressure is smaller during the fall and the structure is less likely to break

Question 11

maximum error 表達形式

Answer 11

eg +-1g

X +1g/-1g
X 100 +- 1g

Question 12

can a beam balance (秤） still be used when freefalling

Answer 12

The beam balance will fail to work

as the apparent weight is zero or weightless, the counterweight can take any position

Question 13

what is the name of newtons law

Answer 13

newton’s (first/second/third) law of motion

Question 14

how to describe a block motion when its accelerating

Answer 14

The block decelerate/ accelerate UNIFORMLY

Question 15

what is vector

Answer 15

its a physical quantity that require both magnitude and direction for a complete description

Question 16

what is scalar

Answer 16

its a physical quantity that require magnitude only for description

Question 17

what is uniform motion

Answer 17

at a constant velocity

Question 18

what is inertia

Answer 18

inertia is the tendency of an object to maintain its state of rest or of constant velocity

Question 19

Newton’s first law of motion states that

Answer 19

every object remains a state of rest or uniform motion unless acted on by a net force or an unbalanced force

Question 20

Newton’s second law of motion states that

Answer 20

the acceleration of an object is directly proportional to and in the same direction as the net force acting on it and inversely proportional to the mass of the object
ie F=ma

Question 21

what are fluids

Answer 21

gases or liquid

Question 22

Newton’s third law of motion states that

Answer 22

to every action there is an equal and opposite reaction. The action and reaction forces act on different interacting objects simultaneously.

Question 23

unit of moment

Answer 23

Nm

Question 24

what is couple in moment

Answer 24

A couple forms when two equal and opposite parallel forces apply simultaneously to the same body and do not act in the same line

Question 25

what are the conditions for a body to be in equilibrium

Answer 25

no net force and no net moment

Question 26

what is work

Answer 26

energy transferred due to a force

Question 27

why aircraft cant fly in space

Answer 27

it needs air to burn fuel

the wing need air to provide the lifting force

Question 28

no satellites in geostationary orbit can pass vertically above Hong Kong
T/F?

Answer 28

True

There is only one geostationary orbit on the equator

Question 29

what is the name of force N

Answer 29

normal force

?/X Normal reaction(fc ok for 2015dse)

Question 30

describe the energy change when a boy climbs up the slide

Answer 30

from chemical energy to gravitational potential energy

Question 31

how to give the astronaut an artificial weight

Answer 31

give the satellite a rotational motion/spinning

Question 32

one sphere X of mass m travels with speed 4ms-1
it collide with another sphere Y of mass 2m
is it possible that the speed of Y after collision be 3ms-1?

Answer 32

NOOOOOOOOOOOOOO
KE increase
dont just consider momentum

Question 33

why it takes a longer time for the car to accelerate up an inclined road than along a horizontal road

Answer 33

as both KE and PE increases
as power remains unchanged
so it takes more time to accelerate