H2 Physics (questions overload)
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Dear all, here's some questions I need help in, kindly help me. :D I don't need full answers, just guidance, thanks. I know there's a lot of questions, but they are actually MCQ questions and won't take too much of your time. (:
1) A crane has maximum safe working load of 1.2 x 104 N and is used to lift a concrete block of mass 1000 kg. What is the maximum safe upward acceleration of the block while being lifted?
2) The acceleration of free fall is determined by timing the fall of a steel ball photo-electrically as shown below. The ball passes P and Q at time t1 and t2 after release.
What is the acceleration of free fall?
Tried using COE, but I still have no idea how to do.
3) The minute hand of a large clock is 3.0 m long. What is its mean angular speed?
Why isn't it found using 2pi/T?
4) A car of mass m has an engine which can deliver power P. What is the minimum time in which the car can be accelerated from rest to a speed v?
v = u + at
Given u = 0,
v = at
t = v/a
For minimum t, a = maximum.
I have no idea what follows.5) A solid steel sphere is held just below the surface of water and released. Its terminal velocity v is measured. If the experiment were repeated in places with different values of g (the acceleration of free fall), a graph of v against g would most closely resemble
I need the proper explanation for this.
6) The door of a working refrigerator is left open. After some hours, the temperature of the room in which the refrigerator is placed is
a unchanged, because the refrigerator absorbs as much heat as it gives out
b lower, because the refrigerator will extract heat from the room
c unchanged, because the refrigerator is thermostatically controlled
d higher, because the refrigerator gives out more heat than it absorbs
e lower, because the coolant in the refrigerator will evaporate much more rapidly
7) A light spring has a mass of 0.20 kg suspended from its lower end. A second mass of 0.10 kg is suspended from the first by a thread. The arrangement is allowed to come into static equilibrium and then the thread is burned through. At this instant, what is the upward acceleration of the 0.20 kg mass? (take g as 10 m s-2)
I tried and got 10 which I know is incorrect.
8) A body of mass 2 kg is moving on a horizontal surface with a speed of 1.41 m s-1 in a north-easterly direction. A force of 0.2 N acting in a westerly direction is applied to the body for 10 s. If friction is negligible, the body is then moving with a speed of
a) 0.41 m s-1 in a north-easterly direction
b) 1.00 m s-1 in a northerly direction
c) 1.41 m s-1 in a north-westerly direction
d) 2.24 m s-1 in a direction 63.4 ° east of north
e) 2.41 m s-1 in a north-easterly direction
I know that the direction will be northerly, but how do you get 1.00?
9) The force F required to extend a sample of rubber by a length x was found to vary as shown in the diagram.
What is the energy stored in the rubber for an extension of 5m?
I calculated it to be 100, but it's wrong. =/
10) An object of mass m passes a point X with a velocity v and slides up a frictionless incline to stop at point Y which is at a height h above X.
A second object of mass ½ m passes X with a velocity of ½ v.
To what height will it rise?
I worked it out and i got 1/8h, but the answer is 1/4h. Am I right or is the answer wrong?
1/2 mv^2 = mgh ------ (1)
1/2 (1/2m) (1/2v)^2 = mg(xh) ------ (2)
Comparing (1) & (2),
1/2 (1/2m) (1/4v^2) = mg(1/8h)
11) The formula for the period of a simple pendulum is T = 2p (l/g). Such a pendulum is used to determine g.
The fractional error in the measurement of the period T is ±x and that in the measurement of the length l is ±y. The fractional error in the calculated value of g is no greater than?a) x + y
b) x - y
c) 2x - y
d) 2x + y
e) xy -
1. F = ma + Weight of object.
6. (d) Don't believe can try at home. =)
11. The equation is wrong. period of pendulum is 2 * pi * sqrt(L/g)
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Originally posted by ThunderFbolt:
1. F = ma + Weight of object.
6. (d) Don't believe can try at home. =)
11. The equation is wrong. period of pendulum is 2 * pi * sqrt(L/g)
Thanks a bunch! :D
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for qn3, the equation is correct. but did u use the wrong value for T? is it in seconds? u used 1 min or 1hour?
for qn 4, u should know that work done on an object = change in its kinetic energy.
since it start from rest, change in KE = 1/2mv^2- 0 = Pt ( since P = Work done / T)
t= (1/2mv^2)/P
for qn 10, i will first simplify the expression.
1/2mv^2= mgh
h= v^2/2g - (1)
let h' be the height of the new object will reach
h' = (1/2v)^2/2g = v^2/8g
h'/h = (v^2/8g)/(v^2/2g) = 1/4h. -
Q1) Answered by ThunderFbolt
Q2) Use s = ut + 1/2 a t^2?
Can I see the choices available?Q3) It is. What did u use as T>
Q4) Not a very well phrased question... Don't think time is minimum...
It's using COE
Pt = 1/2 m v^2And... you can't use v = u + at because it is not a constant acceleration!!!
Q5) fluid resistance is proportional to v (or v^2 at very high speeds, but here, we use proportional to v)
So, at terminal velocity, fluid resistance is equal to weight,
i.e. at terminal velocity, weight will be a constant times velocity
Hence, we can see that v varies linearly with gHence answer D
Q6) d, because fridge is inefficient.
Q7) Before thread burned, upwards force by spring = 0.3 * 10 = 3 N
(draw the free body diagram of the spring)After thread burned, upwards force is still 3N, downwards force become 2N, so resultant upwards force on the 0.2kg mass is 1N
Since F = ma...................................
Q8) Resolve original velocity into north (vertical) and east (horizontal)
Remember what I said about working in perpendicular vectors?
Also, north easterly means centre between north and east, so angle is 45 degrees.Then apply F=ma to find a in the west direction
Use kinematics eqn v = u + at to find final velocity (and direction)Then use pygathoras theorem if need [actually no need ;)]
Q9) It's definitely less than 100
Remember, Energy stored is the area under the F-x graph
Q10) 1/2 (1/2m) (1/2v)^2 = mg(xh) ------ (2)
RHS forgot to use 1/2 m
Q11) Answered by ThunderFbolt
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Originally posted by eagle:
Q1) Answered by ThunderFbolt
Q2) Use s = ut + 1/2 a t^2?
Can I see the choices available?Q3) It is. What did u use as T>
Q4) Not a very well phrased question... Don't think time is minimum...
It's using COE
Pt = 1/2 m v^2And... you can't use v = u + at because it is not a constant acceleration!!!
Q5) fluid resistance is proportional to v (or v^2 at very high speeds, but here, we use proportional to v)
So, at terminal velocity, fluid resistance is equal to weight,
i.e. at terminal velocity, weight will be a constant times velocity
Hence, we can see that v varies linearly with gHence answer D
Q6) d, because fridge is inefficient.
Q7) Before thread burned, upwards force by spring = 0.3 * 10 = 3 N
(draw the free body diagram of the spring)After thread burned, upwards force is still 3N, downwards force become 2N, so resultant upwards force on the 0.2kg mass is 1N
Since F = ma...................................
Q8) Resolve original velocity into north (vertical) and east (horizontal)
Remember what I said about working in perpendicular vectors?
Also, north easterly means centre between north and east, so angle is 45 degrees.Then apply F=ma to find a in the west direction
Use kinematics eqn v = u + at to find final velocity (and direction)Then use pygathoras theorem if need [actually no need ;)]
Q9) It's definitely less than 100
Remember, Energy stored is the area under the F-x graph
Q10) 1/2 (1/2m) (1/2v)^2 = mg(xh) ------ (2)
RHS forgot to use 1/2 m
Q11) Answered by ThunderFbolt
2)
a 2h / (t2 – t1)
b h / (t22 – t12)
c h2 / (t2 – t1)
d 2h / (t22 – t12)
e h / 2(t2 – t1)3) I used 2pi / 60s. OH. Minute hand, means period is actually 3600s?
4) AH, overlooked that part, thanks lavastar and eagle. :D
5) Okay, understood!
6) So it's supposed to be a common sense question? (-_-)
7) UNDERSTOOD!
8) Ahhh, didn't realised that 45deg part.
9) Oh yeah, no wonder I was thinking since it's a curve, should I just calculate it this way. I see I see.
10) Oh ya!
YAY. Thanks. xD
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For 6, just think like this. Energy is required for the fridge to suck in hot air (supposedly inside the fridge) and blast the heat out at the back. So heat produced > heat absorbed. However, cos now fridge is open...
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Originally posted by iamapebble:
2)
a 2h / (t2 – t1)
b h / (t22 – t12)
c h2 / (t2 – t1)
d 2h / (t22 – t12)
e h / 2(t2 – t1)3) I used 2pi / 60s. OH. Minute hand, means period is actually 3600s?
4) AH, overlooked that part, thanks lavastar and eagle. :D
5) Okay, understood!
6) So it's supposed to be a common sense question? (-_-)
7) UNDERSTOOD!
8) Ahhh, didn't realised that 45deg part.
9) Oh yeah, no wonder I was thinking since it's a curve, should I just calculate it this way. I see I see.
10) Oh ya!
YAY. Thanks. xD
I think this reply is too late for your test :(
Anyway...Q2) Answer is d
Have to use speed at P is at1
Then sub this into
h = ut + 0.5 a t^2, where u = at1, and t = t2-t1
reararnge to get answerQ3) Yup
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Originally posted by eagle:
I think this reply is too late for your test :(
Anyway...Q2) Answer is d
Have to use speed at P is at1
Then sub this into
h = ut + 0.5 a t^2, where u = at1, and t = t2-t1
reararnge to get answerQ3) Yup
Hahaha, it's isn't too late for promos/alevels though. Thanks! I think I understand already.