Mechanics Flashcards
(33 cards)
why circular motion won’t change speed even though there is net force
F is perpendicular to electrons displacement
no work is done so KE remain unchanged
why people feel weightless in space
astronauts and spacecraft both moving with the same acceleration towards the earth
when will momentum not conserved
when there is external force
eg the object is fixed in position
or there is mgsin veta on a slope
or friction
why beam balance fails to work while free fall
weightless
when answering energy conservation questions
it is a must to state that
By conservation of energy
How to determine the centre of gravity in an experiment
- Use a hole punch to punch a small hole in the cardboard
- Hang the cardboard and the string, which acts as the vertical, on the hook. Wait for equilibrium
- Use the pencil and ruler to draw a line on the cardboard along the string.
- Punch another hole on the cardboard and repeat the procedures. Draw another line along the strings.
- 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
why the airship comes to a constant velocity after some time
due to air resistance
the airship reaches its terminal velocity.
experiment question:
include physical quantities to be measured and the result expected
state that we should plot a graph y against x
then (a straight line passing through the origin is obtained)
describe the movement of bungee jumper
accelerate at g before the elastic cord stretches
accelerate decreases as the cord stretches
decelerate until momentarily at rest.
why the full body harness is better than ankle attachment only for a bungee jumper
contact area is larger
hence pressure is smaller during the fall and the structure is less likely to break
maximum error 表達形式
eg +-1g
X +1g/-1g
X 100 +- 1g
can a beam balance (秤） still be used when freefalling
The beam balance will fail to work
as the apparent weight is zero or weightless, the counterweight can take any position
what is the name of newtons law
newton’s (first/second/third) law of motion
how to describe a block motion when its accelerating
The block decelerate/ accelerate UNIFORMLY
what is vector
its a physical quantity that require both magnitude and direction for a complete description
what is scalar
its a physical quantity that require magnitude only for description
what is uniform motion
at a constant velocity
what is inertia
inertia is the tendency of an object to maintain its state of rest or of constant velocity
Newton’s first law of motion states that
every object remains a state of rest or uniform motion unless acted on by a net force or an unbalanced force
Newton’s second law of motion states that
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
what are fluids
gases or liquid
Newton’s third law of motion states that
to every action there is an equal and opposite reaction. The action and reaction forces act on different interacting objects simultaneously.
unit of moment
Nm
what is couple in moment
A couple forms when two equal and opposite parallel forces apply simultaneously to the same body and do not act in the same line
