Laws of Motion

Question

A monkey of mass 40 kg climbs on a rope (Fig. 5.20) which can stand a maximum tension of 600 N. In which of the following cases will the rope break: the monkey

(a) climbs up with an acceleration of 6 m s–2

(b) climbs down with an acceleration of 4 m s–2

(c) climbs up with a uniform speed of 5 m s–1

(d) falls down the rope nearly freely under gravity?

(Ignore the mass of the rope).


Answer

Case (a):
Mass of the monkey, m = 40 kg
Acceleration due to gravity, g = 10 m/s 
Maximum tension that the rope can bear, Tmax = 600 N 
Acceleration of the monkey, a = 6 m/s2 , upward
Using Newton’s second law of motion, equation of motion is
                   T – mg = ma 

∴                          T = m(g + a
                              = 40 (10 + 6) 
                               = 640 N 
Since T > Tmax, the rope will break in this case.
Case (b)
Acceleration of the monkey, a = 4 m/s2 downward 
Using Newton’s second law of motion, the equation of motion is, 
                                  mg – ma 

∴                                        T = (g – a
                                            = 40(10 – 4)  
                                            = 240 N 
Since T < Tmax, the rope will not break in this case.
Case (c)
The monkey is climbing with a uniform speed of 5 m/s.
Therefore, its acceleration is zero, i.e., a = 0. 
Using Newton’s second law of motion, equation of motion is, 
                     T – m= ma 

                     T – mg = 0
 
∴                           m
                               = 40 × 10  
                               = 400 N 
Since T < Tmax, the rope will not break in this case.
Case (d) 
When the monkey falls freely under gravity, acceleration will become equal to the acceleration due to gravity, i.e., a = 
Using Newton’s second law of motion, we can write the equation of motion as:
                          mg – T = m
∴                                 T = m(g – g) = 0 
Since T < Tmax, the rope will not break in this case. 

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