Physics 9702 Doubts | Help Page 231
Question 1088:
[Work, Energy and Power]
The pump of a water pumping system
uses 2.0 kW of electrical power when raising water. The pumping system lifts 16
kg of water per second through a vertical height of 7.0 m.
What is the efficiency of the
pumping system?
A 1.8% B 5.6% C
22% D 55%
Reference: Past Exam Paper – November 2013 Paper 13
Q17
Solution 1088:
Answer: D.
Input (electrical) power = 2.0 kW
As the water is raised, it gains
gravitational potential energy (= mgh).
Power = Energy / time = mgh / t
The pumping system lifts 16 kg of
water per second. So, (m/t) = 16 kg s-1.
Output Power = (m/t)gh = 16 × 9.81 ×
7.0 = 1098.72 W
Efficiency = Output power / Input
Power = (1098.72 / 2000) × 100% = 55%
Question 1089:
[Dynamics]
(a) Define force.
(b) A resultant force F acts on an object of mass 2.4 kg. The
variation with time t of F is shown in Fig. 2.1.
The object starts from rest.
(i) On Fig. 2.2, show quantitatively
the variation with t of the acceleration a of the object. Include appropriate
values on the y-axis.
(ii) On Fig. 2.3, show
quantitatively the variation with t of the momentum p of the object. Include
appropriate values on the y-axis.
Reference: Past Exam Paper – June 2013 Paper 22 Q2
Solution 1089:
(a) Force is defined as the rate of change of momentum.
(b)
(i)
{Force is constant and
equal to 8.4N from t=0 to t=2. This
corresponds to a constant acceleration of a = F/m = 8.4/2.4 = 3.5ms-2.
Then, at t=2, the resultant force becomes zero and so is the acceleration. For
t=2 to t=4, acceleration remains zero since F is 0.}
Horizontal line on graph from t = 0
to t about 2.0s ± ½ square,
a>0
Horizontal line at a=3.5 on graph
from 0 to 2s
Vertical line at t=2.0s to a=0 or sharp
step without a line
Horizontal line from t=2s to t=4s
with a=0.
(ii)
{Since the object starts
from rest, the initial velocity is 0 at t = 0.
Acceleration, a = 3.5ms-2
for 2s, so [since the object is accelerating from rest for 2s, at time t
= 2s, its velocity is not zero, but is given as follows]
velocity v at t = 2 is v =
u + at = 0 + 3.5(2) = 7ms-1
Momentum, p = mv = 2.4×7 = 16.8Ns. So, a straight line with +ve gradient.
From t=2 onwards, a=0 (but
a = 0 does not mean momentum p is zero since p depends on the velocity, not
acceleration. Acceleration = 0 does not mean that velocity = 0. It only means
that velocity is no changing), so v is constant and so is p. So, straight line (horizontal)
at 16.8Ns.}
Straight line and +ve gradient starting at (0,0) and finishing at (2, 16.8)
Then
a horizontal line from 16.8 for t = 2.0 to 4.0
Question 1090: [Physical Quantities and Units]
The maximum theoretical power P of a wind turbine is given by the
equation
P = kρAvn
where ρ is the density of air, A is the area swept by the turbine
blades, v is the speed of the air and k is a constant with no units.
What is the value of n ?
A 1 B
2 C 3 D 4
Reference: Past Exam Paper – June 2014 Paper 12 Q1
Solution 1090:
Answer: C.
P = kρAvn
vn = P / kρA
Power P = Work done / time = Force ×
distance / time
Consider the units:
[Power P]: [Force F] × [distance s]
/ [time t] = kgms-2 m s-1 = kgm2s-3
k has no units
[kρA]: kgm-3 m2 = kgm-1
[v]: ms-1
So, [vn]: kgm2s-3
/ kgm-1 = m3s-3 = [ms-1]3
Thus, n =3
in solution 1089,as you said that momentum depends on velocity.I know you are applying the formula P=mv as mass is constant so velocity stays the same.But at the same time,force is rate of change of momentum.it means that momentum is also dependent on force so why us used the firsst formula rather than the second one?
ReplyDeletein solution 1089> force is rate of change of momentum so force is 0 there so why can not we use this formula rather than P=mv?
ReplyDeletea force causes an acceleration AND an acceleration means a change in velocity.
DeleteSo, when the force is NOT zero (just as initially), there is an acceleration, thus a change in velocity and a change in momentum (since p = mv).
When the force is zero, there is no acceleration and NO CHANGE IN VELOCITY. also, NO CHANGE in momentum.
'NO CHANGE IN MOMENTUM' means that the value of the momentum does not change. i.e. if it was, say, 5Ns, the momentum does not change - its value remains 5 Ns. It DOES NOT mean that the momentum is zero.