Physics · Grades 7–10

Balanced forces and inertia

Recognize that zero net force does not require zero velocity.

Learn the idea

Combine all forces on the same object as vectors. Equal opposite forces on that object can cancel.

With zero resultant force, an object remains at rest or continues in uniform straight-line motion in an inertial frame.

Real moving toys often slow because a force acts, not because motion uses up an internal supply of force.

Do not cancel a force on one object against a force on a different object when finding a resultant.

Equal opposing forces on the same object sum to zero.

0 N

Constant velocity implies zero acceleration and zero net force for constant mass.

Zero

Which situation is possible with zero net force?
1. Turning with changing velocity
2. Increasing speed in a straight line
3. Motion at constant velocity
Why does a toy eventually slow on a rough floor after a push?
Which forces can be added directly to find one cart's net force?
1. Only the force the cart exerts on the floor
2. All forces acting on that cart
3. Forces on unrelated objects

Motion at constant velocity
Zero resultant force allows rest or uniform straight-line motion.
Friction and other resistive forces change its velocity.
Credit an external force rather than a force being used up.
All forces acting on that cart
A resultant must refer to forces on the chosen object.