Answer: Social loafing
Explanation: Social loafing can be simply defined as the tendency of individuals in a group to provide or contribute less than they will normally provide if they were to work as an individual.
When Joel pulled the rope, he exerted a force of 100, when Frank also pulled the rope, he exerted a force of 100 pound. So one will expect that their combined exertion force will be 200, but working together as a group will require them to provide less individual force than they would have provided if they where to pull the rope individually. This is a good illustration of social loafing.