Physics questions
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1. when liquid water is cooled to 0 deg cel and freezes, ___
A the water molecules stop moving
B the water molecules vibrate more slowly about their fixed positions
C energy is supplied to the water
D the water molecules lose potential energy
answer given is D. but from my point of view, i thought the molecules should gain energy. this is because as water is cooled , water molecules lose K.E, so in turn they should have gained P.E (according to principle of conservation of energy)
2. A solid has a fixed shape and size because________
A the molecules of a solid are large
B there are large attractive forces between the molecules of a solid
C the molecules of a solid are closely packed together
D the molecules of a solid can only vibrate about their fixed positions.
answer given is D but i think it should be B. basically, if the molecules are held by strong attractive forces, they would thus not be able to move about freely , leading to them vibrating about their fixed positions and also the fixed shape.
3.
let's say X is a baby pram, F is the force exerted by the mother pushing the pram and f is the resultant force
F---------> X <--------------f
F = 50 N, f = 20 N, distance moved = 10m
a) what is the work done by the mother?
For me, my working would be
work done = 50 X 10
=500 N
however, a similar (not the same) question in the TYS (p.s. i merely came up with this question myself) does the following:
work done = (50-20) X 10
= 300 N
I mean how can you minus away the 20 N (that should be the overall work done) since work is ALSO done to overcome the friction so we must take into account the 50 N altogether
b) if they ask for work done against friction, how would you present your answer?
for me,
work done against friction = 20 X 10
=200 N
Am I right? cuz i notice for such questions TYS solutions are ALWAYS different from assessment books solutions (i mean the approach they use to solve the question) so i don't really quite get which is the correct method for doing such questions (eg. when to minus away the frictional force, etc)
Thanks!
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Originally posted by anpanman:
1. when liquid water is cooled to 0 deg cel and freezes, ___
A the water molecules stop moving
B the water molecules vibrate more slowly about their fixed positions
C energy is supplied to the water
D the water molecules lose potential energy
answer given is D. but from my point of view, i thought the molecules should gain energy. this is because as water is cooled , water molecules lose K.E, so in turn they should have gained P.E (according to principle of conservation of energy)
2. A solid has a fixed shape and size because________
A the molecules of a solid are large
B there are large attractive forces between the molecules of a solid
C the molecules of a solid are closely packed together
D the molecules of a solid can only vibrate about their fixed positions.
answer given is D but i think it should be B. basically, if the molecules are held by strong attractive forces, they would thus not be able to move about freely , leading to them vibrating about their fixed positions and also the fixed shape.
3.
let's say X is a baby pram, F is the force exerted by the mother pushing the pram and f is the resultant force
F---------> X <--------------f
F = 50 N, f = 20 N, distance moved = 10m
a) what is the work done by the mother?
For me, my working would be
work done = 50 X 10
=500 N
however, a similar (not the same) question in the TYS (p.s. i merely came up with this question myself) does the following:
work done = (50-20) X 10
= 300 N
I mean how can you minus away the 20 N (that should be the overall work done) since work is ALSO done to overcome the friction so we must take into account the 50 N altogether
b) if they ask for work done against friction, how would you present your answer?
for me,
work done against friction = 20 X 10
=200 N
Am I right? cuz i notice for such questions TYS solutions are ALWAYS different from assessment books solutions (i mean the approach they use to solve the question) so i don't really quite get which is the correct method for doing such questions (eg. when to minus away the frictional force, etc)
Thanks!
Hi anpanman,
For question 1), Ice has lower potential energy and kinetic energy than Liquid Water, thus it is correct that water molecules lose potential energy.
For question 2), D will be a technically more correct answer. The point to note is that there is no specification as to how large is the "large attractive force".
For question 3), The resultant force refers to the force working against the mother now. Hence work done = ( Total force - opposing force ) X Distance
For question 4), I believe your workings should be acceptable.
Cheers.
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i need help too..
5)A ball weighing 20N is thrown upwards untilit reaches a maximum height and then falls back down. What is the resultant force on the ball when it is at its maximum height?
A) 0N
B) 20N
C) 30N
D) 40N
Ans: B Why? I've always thought that the ball stops momentarily when it's at its maximum height which means that there's no resultatnt force right?
6)Two forces, P and Q, are acting on an object of mass 1.8kg. What are the possible values of P and Q if the object is moving at a constant acceleration of 2.5m/s^2?
A) P = 2N , Q = 2.5N
B) P = 4.5N, Q = 4.5N
C) P = 4.5N, Q = 9.0N
D) P = 18N, Q = 0N
Ans: C Why can't it be choice A?
Thanks a lot!
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Originally posted by qdtimes2:
i need help too..
5)A ball weighing 20N is thrown upwards untilit reaches a maximum height and then falls back down. What is the resultant force on the ball when it is at its maximum height?
A) 0N
B) 20N
C) 30N
D) 40N
Ans: B Why? I've always thought that the ball stops momentarily when it's at its maximum height which means that there's no resultatnt force right?
6)Two forces, P and Q, are acting on an object of mass 1.8kg. What are the possible values of P and Q if the object is moving at a constant acceleration of 2.5m/s^2?
A) P = 2N , Q = 2.5N
B) P = 4.5N, Q = 4.5N
C) P = 4.5N, Q = 9.0N
D) P = 18N, Q = 0N
Ans: C Why can't it be choice A?
Thanks a lot!
Hi qdtimes2,
For your 1st question, when the ball is at its maximum height, it is about to start moving downwards, thus there is a gravitational force of 20N working on the ball, hence the resultant force is 20N.
For your 2nd question, I will have to say that it is a bit ambiguous. Perhaps the question is assuming that you cannot have two forces applied at the same spot at the same exact angle at the same time.
Cheers.
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@anpanman's qns 1,
is it also explainable by using sign convention? potential energy arises due to intermolecular forces of attraction, and since this is an attractive force(negative sign convention), when distance between water molecules decreases, the PE also decreases(becomes more negative).
@anpanman qns 3
ur smaller letter f refers to friction? cannot be 20N resultant force opp to the direction of F exerted by the lady...
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Originally posted by qdtimes2:
6)Two forces, P and Q, are acting on an object of mass 1.8kg. What are the possible values of P and Q if the object is moving at a constant acceleration of 2.5m/s^2?
A) P = 2N , Q = 2.5N
B) P = 4.5N, Q = 4.5N
C) P = 4.5N, Q = 9.0N
D) P = 18N, Q = 0N
Ans: C Why can't it be choice A?
Thanks a lot!
Yes, question is not set well.
resultant force = ma
= 1.8 X2.5
= 4.5N
Both A and C can give you 4.5 N resultant force
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fyi, tys answer done by grad students. ~.~
qn5) why 20n ? because thats the weight of the ball ma. it stops at the top because the force u supply= gravity. hence the ball drop =)
or .
f= m x a
f = 2x 10
=20 .
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i think TS got a misconception here... for the first question in the first post..
we always do those physic question that has a slope then one rollercoast, ask u about energy these all...
this type of question is always asuming that there is NO LOSS of energy through friction or any resistance... in reality there is usually a sound or heat energy produced from friction..
now back to the question.. when water change to ice... the water MUST be cooled in order for freezing to take place... thus a overall energy drop... since temperature has no change, molecule move same speed... so no change in kinetic energy... but we only learn 2 type of energy in substance, kinetic and potential... so no change in kinetic, drop in overall, can only result cuz of drop in potential energy...
and potential energy in substance is caused by the intermolecular forces.. not the speed...
so if u compare the type of question that has slope and all that, there is NO overall energy change... so u can always equate them or manipulate it... -
ah i need some help now... what affect the brightness of bulb?
if there is a circuit, 2 identical bulb put together in parallel...
then now i add one more bulb in parallel, how will be the brightness affected? -
Originally posted by ItchyArmpit:
ah i need some help now... what affect the brightness of bulb?
if there is a circuit, 2 identical bulb put together in parallel...
then now i add one more bulb in parallel, how will be the brightness affected?Hi ItchyArmpit,
The brightness of the light bulbs will not be affected.
http://www.rkm.com.au/ANIMATIONS/animation-light-bulbs-parallel.html
Cheers.