NEWTON'S LAWS

Newton’s laws of motion - laid out by English physicist and theorist, Isaac Newton - were developed as the basis of Newtonian physics and mechanics. Newton’s primary three laws can be applied in a wide range of everyday physics scenarios - they only become inconsistent in at a quantum scale, at extremely high speeds, or in very strong gravitational fields (these exceptions are not part of the MYP physics syllabus, therefore do not need to be worried about.)

NEWTON'S FIRST LAW

An object at rest will remain at rest, and an object in motion will remain in motion unless acted upon by an external force.

To clarify, if an object is still, it will remain still unless an outside force (a push or pull) acts upon the object. If an object is in motion, it will continue moving uniformly (no change in direction or speed), unless acted upon by an outside force. Newton’s first law is supported by the concept of inertia.

Inertia - the resistance of any physics object to any change in its velocity; this property results in the tendency of objects to continue moving at a constant speed in a straight line, given that no forces are acting on them

Net force - the vector sum of forces acting on a particle or body with mass

Newton's first law tells us that:

a force is not required to maintain an object's velocity
mass is directly proportional to inertia
- objects with greater mass tend to resist change more than objects with less mass; objects with greater mass possess greater inertia
external and internal forces are relative
- gravitational force acting on objects in the Earth's gravitational field is an external force; however, the gravitational force is an internal force for the Earth, since the Earth is the object possessing/producing that force

15 - 10 =

NEWTON'S SECOND LAW

The acceleration of any object is dependent on its net force & mass. This can be quantified with:

F = ma

In the above grid, we see a 16N force being exerted downwards (gravitational force, for example), a 16N force being exerted upwards (normal force, for example), an 18N push force and a 5N pull force (friction force, for example). Therefore, the net horizontal force is (18-5) 13N; the net vertical force is (16-16) 0. Therefore, the net force is 13N towards the left. This net force divided by the mass of the object experiencing it would give us the acceleration.

Newton's second law tells us:

a quantitative representation of force (including the SI unit of force)
the acceleration of an object is produced by the net force (ΣF) acting on it

NEWTON'S THIRD LAW

Every action has an equal and opposite reaction.

This law states that all forces come in pairs; this is to say that in nature, no single force can act on an object without either an external or internal force reaction. These pairs of forces are referred to as action-reaction pairs, and tend to be equal in magnitude and opposite in direction; they must also act on the same object. From this law, we also derive the concept of a normal force. For example, if a book is resting on a table, the force of gravity would be pulling the book downwards and in response, the table exerts an upward force to keep the book still.

Normal force - the perpendicular contact force exerted by one object in order to prevent the other object from passing through it

Action-reaction pairs do not cancel out, because the forces are acting on different systems. If the action-reaction pair was being exerted on or within the same object, they would be canceled out. But in the case of different objects, the forces do not cancel out.

Even when objects collide and break, there is still an action-reaction pair present. The weaker object (which ends up breaking) will still exert an equal and opposite force on the other object; the fact that it breaks is instead dependent on the material, design, pressure resistance, and other such properties of the object. The force applied will be reciprocated regardless of the physical structure of the other (breaking) object.