The killing of Lee Rigby - and his family's subsequent appeal for calm - caused swings in the national mood as expressed through Twitter, according to scientists working to measure just that.
Their computer programme, Emotive, analyses the emotional content of postings on the microblogging platform.
The team, from Loughborough University, say Emotive can scan up to 2,000 tweets a second and detect eight human emotions: anger, disgust, fear, happiness, sadness, surprise, shame and confusion.
The team's leader, Prof Tom Jackson, said posts on the site give a very accurate real-time record of how the platform's users are feeling.
Drummer Rigby, 25, was hacked to death near Woolwich Barracks in south east London as he returned to base on May 22.
Prof Jackson told the BBC: "Following the murder of solider Lee Rigby in Woolwich there was an outpouring of sadness and disgust through Twitter.
"Across the country people expressed their emotions at this unprovoked attack, with some using the incident to incite racial hatred against Muslims.
"Two days after his murder his family appealed for calm, stating that their son would not have wanted his name to be used as an excuse to carry out attacks against others.
"This appeal had an almost immediate effect, leading to an outpour of positive sentiment."
The team claim that the programme could have further-reaching effects, like use in public order or preventing crime. They said it could able to guide national policy on the best way to react to major incidents
Prof Jackson said: "Twitter is a very concise platform through which users express how they feel about a particular event, be that a criminal act, a new government policy or even a change in the weather.
"Through the computer program we have created we can collate these expressions of feelings in real time, map them geographically and track how they develop."
The team says the system could also be rolled out to analyse tweets anywhere in the world.