Cannon Drop
In this scene, Captain Jack Sparrow joins us after a brave and dangerous rescue mission, saving Elizabeth Swann from drowning in the waters of Port Royal. After towing Ms. Swann to the safety of a nearby dock and bringing her back to consciousness, Jack is immediately surrounded by British troops, lead by Commodore Norrington and Governor Weatherby Swann. After tending to his shocked daughter, Governor Swann orders the troops to shoot Jack. Elizabeth recognizing the lunacy of the situation asks Norrington, “do you really intend to kill my rescuer?”. The Commodore then comes his senses and tells the soldiers to lower their guns.
Norrington then begins to have a conversation with Jack and uncovers that he is in fact a pirate and orders his arrest. Jack then takes Elizabeth hostage and escapes Norrington and his men by kicking the lever suspending cannon above the dock. Jack then grabs the rope which is being pulled by the cannon and is pulled to the top of the crane (motion occurs at 2:23 and ends at 2:30) . When this escape act is analyzed it is apparent that the laws of Physics exist in this scene, making it an example of “good physics”.
The question involved is does the cannon produce enough force to pull Jack to the top of the crane? A man of Jack Sparrow’s size and build would most likely weigh in at approximately 150 pounds or 68 kilograms. The cannon which is pulling Jack towards the top of the crane would have an approximate mass of 5000 kiliograms. The force of gravity acting on both the cannon and Jack are below.
Fg on Jack= mg
=68(9.8)
= 666 N
Fg on cannon= mg
= 5000(9.8)
= 4.9x10^4 N
Before Jack hits the lever which releases the tension on the rope that is holding the cannon in place, a number of Newton’s laws are in effect. These forces can be viewed in Free Body Diagram number 1 at the bottom of the page.
Since there is no external unbalanced force on either Jack or the cannon, according to Newton's first law they will both stay at rest. Until the lever is released the objects will not begin their motion. When Jack does hit the lever, the force of tension within the rope holding the cannon in place, is released, causing the cannon to fall towards the Earth or in this case the dock and propelling Jack up to the top of the crane. As a result of the lever being released the forces acting on Jack and the cannon are altered. The force of gravity acting on Jack and the cannon remain constant. What has changed is the force propelling Jack up.
When the cannon is falling to Earth it tows the rope which once held it in position. Since the cannon and the rope are physically attached to each other they are both falling to Earth with a force of gravity of 4.9x10^4 N. When Jack grabs the rope he exerts a tension force equal to that of the gravitational force of the cannon. As one can see from the second Free Body Diagram (bottom of page), the force of tension Jack has applied is greater than his gravitational force. According to Newton’s second law, whenever an external unbalanced force exists on an object, the object will accelerate in the direction of the net force such that acceleration is proportional to net force and inversely proportional to the mass of the object. This is exactly what is happening to Jack. When calculated his net force is 4.83x10^4 N [up].
Since the tension force is greater the his gravitational force he will be accelerated up towards the top of the crane. It is true that this motion will occur, however, Jack will be going fast enough where he would be killed or seriously injured and this is something which was not portrayed in the movie.
Norrington then begins to have a conversation with Jack and uncovers that he is in fact a pirate and orders his arrest. Jack then takes Elizabeth hostage and escapes Norrington and his men by kicking the lever suspending cannon above the dock. Jack then grabs the rope which is being pulled by the cannon and is pulled to the top of the crane (motion occurs at 2:23 and ends at 2:30) . When this escape act is analyzed it is apparent that the laws of Physics exist in this scene, making it an example of “good physics”.
The question involved is does the cannon produce enough force to pull Jack to the top of the crane? A man of Jack Sparrow’s size and build would most likely weigh in at approximately 150 pounds or 68 kilograms. The cannon which is pulling Jack towards the top of the crane would have an approximate mass of 5000 kiliograms. The force of gravity acting on both the cannon and Jack are below.
Fg on Jack= mg
=68(9.8)
= 666 N
Fg on cannon= mg
= 5000(9.8)
= 4.9x10^4 N
Before Jack hits the lever which releases the tension on the rope that is holding the cannon in place, a number of Newton’s laws are in effect. These forces can be viewed in Free Body Diagram number 1 at the bottom of the page.
Since there is no external unbalanced force on either Jack or the cannon, according to Newton's first law they will both stay at rest. Until the lever is released the objects will not begin their motion. When Jack does hit the lever, the force of tension within the rope holding the cannon in place, is released, causing the cannon to fall towards the Earth or in this case the dock and propelling Jack up to the top of the crane. As a result of the lever being released the forces acting on Jack and the cannon are altered. The force of gravity acting on Jack and the cannon remain constant. What has changed is the force propelling Jack up.
When the cannon is falling to Earth it tows the rope which once held it in position. Since the cannon and the rope are physically attached to each other they are both falling to Earth with a force of gravity of 4.9x10^4 N. When Jack grabs the rope he exerts a tension force equal to that of the gravitational force of the cannon. As one can see from the second Free Body Diagram (bottom of page), the force of tension Jack has applied is greater than his gravitational force. According to Newton’s second law, whenever an external unbalanced force exists on an object, the object will accelerate in the direction of the net force such that acceleration is proportional to net force and inversely proportional to the mass of the object. This is exactly what is happening to Jack. When calculated his net force is 4.83x10^4 N [up].
Since the tension force is greater the his gravitational force he will be accelerated up towards the top of the crane. It is true that this motion will occur, however, Jack will be going fast enough where he would be killed or seriously injured and this is something which was not portrayed in the movie.