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What Is Physics?

We have seen that part of physics is a study of motion, including accelerations,

which are changes in velocities. Physics is also a study of what can causean object

to accelerate. That cause is a force,which is, loosely speaking, a push or pull on the

object. The force is said to acton the object to change its velocity. For example,

when a dragster accelerates, a force from the track acts on the rear tires to cause

the dragster’s acceleration. When a defensive guard knocks down a quarterback, a

force from the guard acts on the quarterback to cause the quarterback’s backward

acceleration. When a car slams into a telephone pole, a force on the car from the

CHAPTER 5

Force and Motion—I

5-1NEWTON’S FIRST AND SECOND LAWS

After reading this module, you should be able to...

5.01Identify that a force is a vector quantity and thus has

both magnitude and direction and also components.

5.02Given two or more forces acting on the same particle,

add the forces as vectorsto get the net force.

5.03Identify Newton’s first and second laws of motion.

5.04Identify inertial reference frames.

5.05Sketch a free-body diagram for an object, showing the

object as a particle and drawing the forces acting on it as

vectors with their tails anchored on the particle.

5.06Apply the relationship (Newton’s second law) between

the net force on an object, the mass of the object, and the

acceleration produced by the net force.

5.07Identify that only externalforces on an object can cause

the object to accelerate.

●The velocity of an object can change (the object can accel-

erate) when the object is acted on by one or more forces

(pushes or pulls) from other objects. Newtonian mechanics

relates accelerations and forces.

●Forces are vector quantities. Their magnitudes are defined

in terms of the acceleration they would give the standard kilo-

gram. A force that accelerates that standard body by exactly

1 m/s^2 is defined to have a magnitude of 1 N. The direction of

a force is the direction of the acceleration it causes. Forces

are combined according to the rules of vector algebra. The

net force on a body is the vector sum of all the forces acting

on the body.

●If there is no net force on a body, the body remains at rest if

it is initially at rest or moves in a straight line at constant

speed if it is in motion.

●Reference frames in which Newtonian mechanics holds are

called inertial reference frames or inertial frames. Reference

frames in which Newtonian mechanics does not hold are

called noninertial reference frames or noninertial frames.

●The mass of a body is the characteristic of that body that

relates the body’s acceleration to the net force causing the

acceleration. Masses are scalar quantities.

●The net force on a body with mass mis related to the

body’s acceleration by

which may be written in the component versions

.

The second law indicates that in SI units

1 N 1 kgm/s^2.

●A free-body diagram is a stripped-down diagram in which

onlyonebody is considered. That body is represented by

either a sketch or a dot. The external forces on the body are

drawn, and a coordinate system is superimposed, oriented

so as to simplify the solution.

Fnet,xmax Fnet,ymay and Fnet,zmaz

Fnet

: ma:,

:a

Fnet

:

Key Ideas

Learning Objectives