The answer to this question is A.
 
        
             
        
        
        
If the object is moving in a straight line with constant speed, 
that's a description of " acceleration = zero ".
Zero acceleration means zero net force on the object.
NO net force is 'required' to keep an object moving in a straight line 
at constant speed.  In fact, if there IS any net force on the object, 
then either its speed or its direction MUST change ... there's no way 
to avoid it. 
None of this depends on the object's mass, or on the speed or direction 
of its motion.
        
             
        
        
        
Answer:
a.  ,
, 
Explanation:
From the information given;
Using the work-energy theorem
ΔKE = W = 
K = 
∴

Since  and r_1 = 4, and r_2 = 2 (from the missing diagram which is attached below)
 and r_1 = 4, and r_2 = 2 (from the missing diagram which is attached below)
Then;


 
        
             
        
        
        
Answer:

Given:
Mass (m) = 3.0 kg
Uniform speed (v) = 20 m/s
Length of string (r) = 40 cm = 0.4 m
To Find:
Tension in the string (T)
Explanation:
Tension (T) is the string will be equal to centripetal force ( ).
).

Substituting value of m, v & r in the equation:


Tension in the string (T) = 3 kN