It happened on the B3081 near Verwood - he had to be cut free by firefighters.
Knifepoint Robbery In Hampshire
Detectives are investigating a knifepoint robbery in Southsea.
A 23-year-old man was walking along Granada Road at 1.15am on Thursday March 21, when he was approached by a man who was wearing a dark-coloured, hooded jacket and had a scarf around his face.
The man showed the victim a knife and told him to face a nearby tree, before stealing the victim's grey, Nike rucksack, which contained a laptop, wallet and coursework.
The thief then made off in the direction of Canoe Lake.
DC Pete Bambury said: "Fortunately this type of crime is not common; however it has left the victim very shaken.
"I would like to hear from anyone who may have seen somebody fitting the description of the thief in the area around the time of the incident, or from anyone who may have been offered a laptop for sale."
Anyone with information is asked to call DC Pete Bambury at Fratton CID on 101, or to call the anonymous Crimestoppers number on 0800 555 111.
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