Physics

Chapter 4 Forces and Newton’s Laws of Motion

The Concepts of Force and Mass

A force is a push or a pull.

Contact forces arise from physical contact . Action-at-a-distance forces do not require contact and include gravity and electrical forces.

The Concepts of Force and Mass

Arrows are used to represent forces. The length of the arrow is proportional to the magnitude of the force.

15 N

5N

The Concepts of Force and Mass

Mass is a measure of the amount of “stuff” contained in an object.

Newton’s First Law of Motion

Newton’s First Law

 F  0

Newton’s First Law of Motion

The net force on an object is the vector sum of all forces acting on that object. The SI unit of force is the Newton (N).

Individual Forces Net Force

4N

10 N

6N

Newton’s First Law of Motion

Individual Forces

Net Force

5N

3N

64

4N

Newton’s First Law of Motion

Inertia is the natural tendency of an object to remain at rest in motion at a constant speed along a straight line.

The mass of an object is a quantitative measure of inertia.

SI Unit of Mass: kilogram (kg)

Newton’s First Law of Motion

An inertial reference frame is one in which Newton’s law of inertia is valid. All accelerating reference frames are noninertial.

Newton’s First Law of Motion

Mathematically, the net force is written as  F



where the Greek letter sigma denotes the vector sum.

Newton’s Second Law of Motion

Newton’s Second Law

 a



 F

m



  F  ma

Newton’s Second Law of Motion

SI Unit for Force

 m  kg  m kg   2   2 s s 

This combination of units is called a newton (N).

Newton’s Second Law of Motion

Newton’s Second Law of Motion

A free-body-diagram is a diagram that represents the object and the forces that act on it.

Newton’s Second Law of Motion

The net force in this case is:

275 N + 395 N – 560 N = +110 N

and is directed along the + x axis of the...