How Forces Affect Motion · Lesson 4 of 8
Newton’s First Law of Motion
“Objects do not need a net force to keep moving; they need one only to change their velocity.”
• State Newton’s first law in terms of net force. • Connect zero net force with zero acceleration. • Distinguish rest, constant velocity, and changing velocity. • Explain inertia. • Draw position-time and velocity-time graphs for zero net force.
Suppose a puck is sliding over a surface. On a rough floor it soon stops; on smooth ice it travels much farther. If every source of resistance could be removed, there would be no reason for its velocity to keep decreasing. This shift in thinking replaces the idea that force maintains motion with the deeper idea that net force changes motion.
An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon the object.
Meaning Of Constant Velocity
Velocity includes both speed and direction. Constant velocity therefore means constant speed in an unchanged direction. The path is a straight line. An object moving around a curve at constant speed does not have constant velocity because its direction changes continuously.
Rest is the special case of constant velocity equal to zero. When net force is zero, a resting object remains at rest and a moving object continues with its existing constant velocity. The law does not prefer rest over motion.
Net Force And Acceleration
Acceleration is the rate at which velocity changes. If net force is zero, acceleration is zero. Zero acceleration does not mean zero velocity; it means velocity is not changing. This is one of the most important distinctions in the chapter.
Inertia
Inertia is the tendency of an object to resist a change in its state of rest or uniform straight-line motion.
Inertia is not a force. It is a property revealed when an object resists acceleration. A book on a table does not begin moving without a net force. A moving trolley does not stop instantly merely because the push ends; a backward net force such as friction is required to reduce its velocity.
Balanced Forces On A Moving Object
Imagine a box moving right while a person pushes it right with 20 N and friction acts left with 20 N. The forces are balanced, so the net force is zero. The first law says the box continues with constant velocity. It does not gradually stop as long as the two forces remain equal and opposite.
Problem
A trolley moves east at 3 m s⁻¹. The driving force is 60 N east and resistance is 60 N west. Predict its motion.
- 1.Add the horizontal forces with direction: +60 N − 60 N = 0 N.
- 2.Zero net force means zero acceleration.
- 3.Zero acceleration means the velocity does not change.
- 4.The trolley continues east at 3 m s⁻¹, provided the forces remain balanced.
Motion Graphs When Net Force Is Zero
For an object at rest, position remains unchanged as time passes, so the position-time graph is horizontal. Its velocity remains zero, so the velocity-time graph lies along the time axis. For an object moving with constant non-zero velocity, position changes by equal amounts in equal time intervals. Its position-time graph is a straight inclined line, and its velocity-time graph is horizontal above or below the time axis depending on direction.
A horizontal velocity-time line does not mean the object is stationary. Its vertical value gives the velocity. Only a line at zero velocity represents rest. Likewise, the slope of a position-time graph represents velocity: zero slope means rest, while a constant non-zero slope means constant velocity.
Problem
An object moves in a straight line with constant velocity while no net force acts on it. Describe both graphs.
- 1.Use the first law: zero net force gives zero acceleration.
- 2.The velocity therefore remains constant, so draw a horizontal line on the velocity-time graph at the stated non-zero velocity.
- 3.Constant velocity means position changes by equal amounts in equal times.
- 4.Draw a straight inclined line on the position-time graph.
- 5.Use a steeper position-time line for a larger speed in the chosen positive direction.
Common Misconceptions
“No net force” is not the same as “no forces.” Several forces may balance. “No acceleration” is not the same as “no motion.” A spacecraft far from significant external forces may move at high constant velocity with zero acceleration. Finally, a force is not required to keep constant velocity; a net force is required to change velocity.
Quiz
Which description best matches Newton’s First Law Of Motion?
Which description best matches Inertia?
Which term matches this description: An object at rest remains at rest, and an object in motion continues to move with constant velocity, unless a net force acts upon the object.
Which term matches this description: Inertia is the tendency of an object to resist a change in its state of rest or uniform straight-line motion.
Which statement is a key takeaway from this lesson?
Practice Problems
- An object moves with constant velocity. What is its acceleration and net force?
- Explain why a horizontal line above zero on a velocity-time graph represents motion, not rest.
- Compare the position-time graphs of an object at rest and an object moving uniformly.
- Give a real situation in which forces act but the net force is zero.
Key Takeaways
• Zero net force gives zero acceleration. • Zero acceleration means constant velocity, not necessarily rest. • Constant velocity requires unchanged speed and direction. • Inertia is a tendency, not a force. • Force changes velocity; it is not required to preserve constant velocity.
The next law quantifies how much acceleration a non-zero net force produces.