Newton's Laws help please!

I'm having difficulties with this question here:

a. A student draws a book lying on a bench. Student X says that these two forces (the force from the bench and the weight of the book) are equal and opposite therefore this is an example of Newton's third law pair of forces.
Student Y says that the book is at rest so the forces are balanced, which means that they are equal and opposite i.e. Newton's first law.
a) explain who is right.
b) the force exerted from the bench is the "normal reaction" force on the book. Explain why "normal" and why a "reaction" force.

For Newton's third law to be obeyed by two forces, the forces must satisfy ALL of these conditions:

1. have equal magnitude
2. act in opposite directions
3. be of the same type e.g. electrostatic, gravitational, push, normal contact etc
4. act on the two different objects
5. act for the same time period

1 and 2 are satisfied. However 3 is not satisfied as one force is a normal contact whereas the other force is weight, so is gravitational type. Thus these forces are not of the same type, meaning Newton's third law is not obeyed. 4 is not obeyed as weight is the gravitational force exerted by Earth on book while the normal contact force is exerted by the bench on the book. 5 is not really obeyed - take off the book from the bench and weight still acts on it, but normal contact force doesn't.


The book is not accelerating on the bench as it doesn't have a change in speed. So there's constant speed. By Newton's first law no resultant force causes no acceleration, so there must be no resultant force on the book. Now provided there are two forces, they must be equal in magnitude and opposite in direction to cancel out to give no resultant force, making them balanced. Since weight acts down and normal reaction force acts up, they are in opposite directions and, as they have equal magnitude, Newton's first law applies.


b. I suppose this part seems to be a case of the way things are defined in Physics.

Here's a definition of normal:

technical
(of a line, ray, or other linear feature) intersecting a given line or surface at right angles.

The bench is horizontal and the normal reaction force is vertical, so the normal reaction force is perpendicular to the bench in its direction.

Here's a definition of reaction in physics:

a force exerted in opposition an applied force.

Sorry, but for part b I don't think there's really that much for me to talk about.

For Newton's third law to be obeyed by two forces, the forces must satisfy ALL of these conditions:

1. have equal magnitude
2. act in opposite directions
3. be of the same type e.g. electrostatic, gravitational, push, normal contact etc
4. act on the two different objects
5. act for the same time period

1 and 2 are satisfied. However 3 is not satisfied as one force is a normal contact whereas the other force is weight, so is gravitational type. Thus these forces are not of the same type, meaning Newton's third law is not obeyed. 4 is not obeyed as weight is the gravitational force exerted by Earth on book while the normal contact force is exerted by the bench on the book. 5 is not really obeyed - take off the book from the bench and weight still acts on it, but normal contact force doesn't.


The book is not accelerating on the bench as it doesn't have a change in speed. So there's constant speed. By Newton's first law no resultant force causes no acceleration, so there must be no resultant force on the book. Now provided there are two forces, they must be equal in magnitude and opposite in direction to cancel out to give no resultant force, making them balanced. Since weight acts down and normal reaction force acts up, they are in opposite directions and, as they have equal magnitude, Newton's first law applies.


b. I suppose this part seems to be a case of the way things are defined in Physics.

Here's a definition of normal:

technical
(of a line, ray, or other linear feature) intersecting a given line or surface at right angles.

The bench is horizontal and the normal reaction force is vertical, so the normal reaction force is perpendicular to the bench in its direction.

Here's a definition of reaction in physics:

a force exerted in opposition an applied force.

Sorry, but for part b I don't think there's really that much for me to talk about.

You could actually argue that both conditions 4 and 5 are satisfied through the symettry of the problem - the earth exerts a force on the book but the force the book exerts on the bench is not gravity but actually is a contact force - if that wasn't true then the bench would equally not be able to exert a contact force on the book. Both N1 and N3 are satisfied and both students are correct - in fact, N1,2&3 are always satisfied in classical mechanics as 1 is just a special case of 2