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2.4 Newton’s First Law

Syllabus
2024
Topic
2.4
Level
—

Learning objectives

2.4A—Describe the conditions under which a system’s velocity remains constantDescribe the conditions under which a system’s velocity remains constant.• The net force on a system is the vector sum of all forces exerted on the system.• Translational equilibrium is a configuration of forces such that the net force exerted on a system is zero. Derived equation:• Newton’s first law states that if the net force exerted on a system is zero, the velocity of that system will remain constant.• Forces may be balanced in one dimension but unbalanced in another. The system’s velocity will change only in the direction of the unbalanced force.• An inertial reference frame is one from which an observer would verify Newton’s first law of motion. AP Physics 1: Algebra-Based Course and Exam Description Force and Translational Dynamics UNIT 2 TOPIC 2.5 Newton’s Second Law | AP Physics 1: Algebra-Based Course and Exam Description

Zero net force means constant velocity

Add forces as vectors

The net force on a system is the vector sum of every external force exerted on it. Direction matters: forces pointing oppositely contribute opposite vector components.

Recognize equilibrium

∑iF⃗i=0\sum_i \vec F_i=0

Test each direction

Component test What it tells you
∑Fx=0\sum F_x=0 and ∑Fy=0\sum F_y=0 The net-force vector is zero, so the system is in translational equilibrium
∑Fx≠0\sum F_x\ne0, ∑Fy=0\sum F_y=0 Only the xx component of velocity changes
∑Fx=0\sum F_x=0, ∑Fy≠0\sum F_y\ne0 Only the yy component of velocity changes

Apply the test

Example: a crate moves right while the floor's upward normal force balances Earth's downward gravitational force. If a rightward pull is larger than the leftward resistive force, the vertical forces are balanced but the horizontal forces are not. The crate's vertical velocity stays constant while its horizontal velocity changes.

Interpret the result

Zero net force does not mean zero velocity or no forces. It means velocity is constant: the system may remain at rest or keep moving at constant speed in a straight line. This statement is verified in an inertial reference frame; a frame that accelerates relative to an inertial frame is not inertial.