Lifting water bottles

 
Mrs Harvey's science class carried out three experiments with pulleys. In all of them a rope was tied to the handle of a two-litre bottle filled with water.
A spring scale was used to measure the force needed to pull the bottle one metre off the floor.
The three experiments were:
  • A rope only was used to pull the bottle.
  • One pulley was used with the rope.
  • Two pulleys were used with the rope.

Each experiment was tried three times. Here is the table showing the students' results.

The force is measured in Newtons.

  Trial 1 Trial 2 Trial 3 Average
Expt. 1 4.2 N 4.3 N 4.2 N 4.2 N
Expt. 2 3.8 N 3.7 N 3.7 N 3.7 N
Expt. 3 2.0 N 1.9 N 1.8 N 1.9 N

 
a)
In which experiment was the most force needed to lift the bottle? _______________
 
b)
Explain why using one pulley decreased the force needed to lift the bottle.