A man held his son upside down by his leg and used a shirt hanger to repeatedly hit him because the nine-year-old boy made some mistakes in his homework.
When the boy was finally released, he maintained he did not know the answers.
This further angered his 35-year-old father, who kicked him and started thrashing the boy again.
Last Thursday, the father was sentenced to four months' jail after pleading guilty in June to one count of ill-treating his son. The man cannot be named owing to a gag order to protect the boy's identity.
Before handing down the sentence, District Judge Eddy Tham said the acts of violence were "relentless and extreme".
He also said a message has to be sent to deter others from committing a similar offence.
The incident took place on Aug 4 last year. The man was alone with the boy in their Tampines flat after his wife went out to meet friends at 9pm. While he watched television in the living room, his son did his homework at a nearby dining table.
When the boy asked his father for help, the man told him to figure it out on his own.
The boy gave several wrong answers. This angered his father, who took a plastic hanger from a bedroom and yanked the boy off his chair, causing him to fall.
"The accused then grabbed the victim's left leg and held him up in an inverted position. The accused hit the victim with the hanger multiple times on his buttocks and legs," said Deputy Public Prosecutor Grace Chua. "With each hit, the accused raised his arm and the hanger up before swinging it down with his full strength on the victim."
The beating was captured by a closed-circuit television (CCTV) camera in the room. The boy was heard screaming in pain in the video, which was shown in court.
After he stopped hitting the boy, he yelled at him. The boy, lying on the floor in a daze, said he "really did not know" the answers, DPP Chua said. On hearing this, the father kicked his son with some force, then flung the hanger at him and repeatedly slapped the boy's buttocks.
A neighbour alerted the police after hearing the boy's cries. The boy told his mother about the beating when she returned around 1.15am.
She retrieved the CCTV footage and took the boy to KK Women's and Children's Hospital, where he was found with multiple bruises. He was discharged from hospital three days later, on Aug 8 last year.
The man is out on a $15,000 bail and was ordered to surrender himself at the State Courts on Feb 11 next year to begin serving his sentence.