Answer:
Steel is almost 2.9 times heavier the aluminium.
APPARENT MOTION- <span>the sensation of seeing movement when nothing actually moves in the environment, as when two neighbouring lights are switched on and off in rapid <span>succession.</span></span>
Background research is used to familiarize yourself with the topic or problem you're conducting your experiment on and help you form an educated guess explaining or solving it that will become your hypothesis.
Answer:In first case
Explanation:
When two People pull the spring, let the applied force by each person be 
and spring constant of spring be k
so Total force is 
total extension according to Hooke's law is

When they detach the spring and apply force in opposite direction then force on either side is F so net extension is

so in first case there will be more extension