Newton's 3rd law Solutions

A propeller drives a boat.

•Action = propeller exerts force on water •Reaction = water exerts force on propeller

A spider hangs from its web attached to the ceiling

•Action = spider exerts force on web •Reaction = web exerts force on spider

Michael Johnson comes out of the starting blocks at the beginning of the 100 m dash

•Action =MJ's foot exerts force on starting block •Reaction =starting block exerts force on MJ's foot

Action:

"Object A" exerts a force on "Object B"

Reaction:

"Object B" exerts a force on "Object A"

You get extremely angry and hit your fist against the wall. •

Action = your fist exerts a force on the wall •Reaction = the wall exerts a force on your fist

Is the following statement true or false: "When a book is at rest lying on a table, the downward force of gravity on the book is equal and opposite to the upward force of the table on the book. This is an example of Newton's third law." Explain your reasoning.

No! False! This is not an example of Newtons 3rd Law! When considering the downward force of gravity, the corresponding reaction force is the force that the book exerts upwards on the Earth. When it comes to the normal force, the action-reaction pair can be stated as follows: Table exerts upward force on book while book exerts downward force on table. Not all equal and opposite forces constitute an action-reaction pair, as this case clearly shows. Action-reaction forces never act upon the same object.

Can a dog wag its tail without the tail in turn wagging the dog? Explain .

No. If the dog exerts a force on the tail then the tail must exert a force back on thedog

According to Newton's third law, each team in a tug-of-war pulls with equal force on the other team. What, then, determines which team will win?

The team that wins is the team that can supply the greatest force on the ground.

Your weight is the result of the gravitational force of the earth acting upon your body. What is the corresponding reaction force?

The upward force that your body exerts on the Earth

Why can you exert greater force on the pedals of a bicycle if you pull up on the handlebars?

Without pulling up on the handlebars, the downward force on the pedals is due to your weight only. But if you pull up on the handlebars, then according to Newton's 3rd law the handlebars exert a downward force on you. This downward force gets transmitted to the to the pedals, which acts on them in addition to the force of gravity.

A cannon recoils when fired.

•Action = cannon exerts force on cannonball •Reaction = cannonball exerts force on cannon