The motion picture flickers in the dim light of an elementary school classroom. It shows a happy scene - children playing baseball. Suddenly, a man who is watching tries to strike up a conversation with one of the players. The man talks baseball. Then he says a friend gave him a transistor radio. Wouldn't the boy like it? Wouldn't he come and look at it?
The film stops. The lights go on. The Jacksonville police officer showing the film--he is known to the youngsters as Officer Friendly - asks the class: ''What would you do?''
Hands shoot up. The boys and girls give their ideas. A few relate similar instances of strangers approaching them. Then Officer Friendly starts the film again and the children hear the message: Don't talk to strangers. Don't accept anything. Tell the incident to an adult you know and trust.
The film, ''Better Safe Than Sorry,'' is part of the Danger-Stranger program presented each year to 100,000 Duval County schoolchildren in Florida by the four members of the Officer Friendly unit of the Jacksonville Police. Its aim is to give children an awareness of possible dangers and the courage to protect themselves.
''The children respond because the narrators are youngsters like themselves, '' says Sgt. Emmett Lee, coordinator of the Officer Friendly unit. ''The advice is practical and straight from the shoulder and it comes from their peers, not from an adult who tries to scare them or preach at them. It's something they can accept and relate to.''
The young narrators explain at the beginning of the film that the incidents to be shown don't happen very often but that as children get older, ''it's time for us to watch out for ourselves - to use our heads and think before we act.''
Some of the situations and the advice given:
* A man and woman in a car offer a ride, saying: ''We know your parents.'' Keep walking. Say, ''I'm sorry but my mom doesn't want me to take a ride from anyone.'' It's a hard choice. They seem like nice people--but ''better safe than sorry.'' Never get in a car. Never hitchhike.
* A car follows beside the curb as a youth walks along. Turn and walk the other way. If the car turns, too, go at once to a nearby store, house, people. Don't be afraid to ask for help. Don't go places alone. Go with a friend.
* At home when parents are away, someone phones and asks questions, supposedly for a survey: ''How old are you? Are you home alone?'' Never give out information to anyone you don't know. Never admit you are home alone. Never let anyone into the house. Before parents leave, ask for a number to call or place to go if you need help.
* A girl is shopping when a stranger comes up, talks nicely, puts an arm around her. This is not someone being friendly like parents or relatives. If grabbed, get away, scream, get help from clerks, shoppers. Don't be afraid to make a scene.
The film--from Film Fair Communications, 10900 Ventura Boulevard, Studio City , Calif. 91604 -- is shown by police and civic groups nationwide.
Another aspect of Jacksonville's Danger-Stranger program is training children to look upon police officers as friends to turn to in a crisis. Officer Friendly appears in full uniform and explains his role in finding lost children, investigating accidents, arresting people ''who break the law or try to hurt children or their parents,'' and finds stolen articles, such as bikes.
The police officer is presented as a family person with a caring attitude toward children, not the traditional authority figure children were once taught to fear.
A ''Safe House'' program is an important adjunct to Danger-Stranger. More than 2,500 homes display window cards which children are taught to recognize as places where trustworthy people trained to call police are eager to help in any emergency. The cards bear the police phone number on the front and step-by-step instructions on the back for the precise reporting of an incident. A code number known only to caller and police protects the program from abuse.
M. A. Rogers, who has been an Officer Friendly for seven of her 10 years on the police force, says it's so commonplace for parents and children to tell her they were protected by something taught in the Danger-Stranger program that she doesn't keep track of it.
''Once a school principal said her daughter was approached by two strange men in a car but the child knew what to do and got the help she needed,'' Officer Rogers says. ''That's gratifying.''
Children frequently send drawings and thank-you letters. Among those posted at police headquarters:
''Thank you for sharing all the nice things. I hope we can share again.''
''We thank you for the movie. Thank you for letting us see your police car. We will not talk to strangers.''
With a stick drawing of a little boy walking toward a police officer: ''Little boy lost will tell police his address. The police will help him.''
With a clown holding out candy, these words: ''Clever disguise. I will not take his candy.''
The children are given police badges, certificates, and coloring books which repeat all the safety rules concerning traffic, bicycles, and personal protection. Parents are asked to review the book with their offspring.
The Officer Friendly unit was founded in 1970 by the consolidated Office of the Sheriff and Jacksonville Police. The unit also trains and supervises adult school crossing guards and the school safety patrol program, conducts a Police Youth recreational program, and recently initiated an antidrug program for older children.
Officer J. F. Wiggins received a three-page thank-you from Neil Nichols and Greg Miller of University Christian School saying: ''Dear Joe, I know that when some of my classmates are offered any kind of drugs, because of you they will think twice about accepting.''
There are about 250 Officer Friendly programs throughout the country. Costs are borne by the various police departments. Educational materials are underwritten by grants from the Sears-Roebuck Foundation. Many communities also have variations of the Safe House program.
''The name of the game is education and prevention,''says Sergeant Lee. ''We deal with hardened criminals that we never were able to reach, and that's sad. If we can teach our children not to break the law -- and to save themselves from harm -- we will have done something constructive.''