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Thursday, January 7, 2016

Physics 9702 Doubts | Help Page 231

  • 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




3 comments:

  1. 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?

    ReplyDelete
  2. in 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?

    ReplyDelete
    Replies
    1. a force causes an acceleration AND an acceleration means a change in velocity.

      So, 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.

      Delete

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