It's actually Friction.
I just did the test and got it right.
 
        
                    
             
        
        
        
1) At the moment of being at the top, the piston will not only tend to push the penny up but will also descend at a faster rate at which the penny can reach in 'free fall', in that short distance. Therefore, at the highest point, the penny will lose contact with the piston. Therefore the correct answer is C.
2) To solve this problem we will apply the equations related to the simple harmonic movement, hence we have that the acceleration can be defined as

Where,
a = Acceleration
A = Amplitude
 = Angular velocity
= Angular velocity
From a reference system in which the downward acceleration is negative due to the force of gravity we will have to



From the definition of frequency and angular velocity we have to




Therefore the maximum frequency for which the penny just barely remains in place for the full cycle is 2.5Hz
 
        
             
        
        
        
Answer:The frequency of the echo is slightly decreased
Explanation:
Given
speed of boat 
cliff is  away
 away
when boat is still , suppose t is the time taken by the echo to reach observer on the boat 
But as soon as boat starts moving  the distance between cliff and boat decreasing and time for echo to reach observer also decreases 
and we know 
therefore frequency of the echo slightly decreased.
 
        
             
        
        
        
gamma radiation and heat flares from the sun, they use refelective gold sheets 
 
        
             
        
        
        
1. First blank is A. Conductors
Second blank is D. Insulators
2. C. Heat