(3) The frictional force exerted by the floor on the box
        
             
        
        
        
A) use v=u+at for both
First section, v=27, u=0, a=2.4. You should get 11seconds.
Second section, v=0, u=27, a=-1.3. You should get 21seconds. 
This means that the total time is 22seconds. 
b) You can either use s=ut+0.5at^2 or v^2=u^2+2as. Personally, I would use the second one as you are not relying on your previous answer. 
First section, v=27, u=0, a=2.4. You should get 152m. 
Second section, v=0, u=27, a=-1.3. You should get 280m. 
This makes your overall displacement 432m.
        
             
        
        
        
Answer:
The photoelectric effect is a phenomenon in which the photoelectrons are emitted from the metal when an incident electromagnetic wave hits the metal. The incident light should have a threshold frequency to meet the work function of the metal
 
        
             
        
        
        
The answer to your question would be C