Unit 11- Newtons Laws

What is linear kinetics?

The study of forces and their relationship to linear motion.

What is an internal force?

A force arising from interactions within the chosen system.

What is an external force?

A force applied to the system by something outside it.

Can a force be internal or external depending on the chosen system?

Yes. Classification depends on the system boundary.

Which forces determine acceleration of the whole system’s center of mass?

The net external forces.

What is Newton’s first law?

A body remains at rest or at constant velocity unless a net external force acts.

What is another name for Newton’s first law?

The law of inertia.

Does zero net force mean an object must be stationary?

No. It can move at constant velocity.

Does an object need a net force to keep moving at constant velocity?

No. A net force is needed to change its velocity.

Can multiple external forces act while acceleration is zero?

Yes, if the forces balance.

What is Newton’s second law?

ΣF = ma. Net external force equals mass times acceleration.

Which direction does acceleration point?

In the direction of the net external force.

At constant mass, what happens if net force doubles?

Acceleration doubles.

For the same net force, what happens if mass doubles?

Acceleration halves.

In one dimension, what happens when net force and velocity point in the same direction?

Speed increases.

In one dimension, what happens when net force opposes velocity?

Speed decreases.

An object moves left and the net force points left. Is it slowing down?

No. It speeds up to the left.

What does a net force perpendicular to velocity change instantaneously?

The direction of motion.

Can a net force change both speed and direction?

Yes, if it has components parallel and perpendicular to velocity.

How is Newton’s second law applied in two dimensions?

Separately along each axis using ΣFx = max and ΣFy = may.

When is acceleration constant along a particular axis?

When net force along that axis and mass remain constant.

What is Newton’s third law?

Interacting bodies exert equal-magnitude, opposite-direction forces on each other.

Do third-law force pairs act on the same body?

No. They act on different bodies.

A runner pushes backward on the ground. What is the reaction force?

The ground pushes forward on the runner.

A stationary gymnast’s upward ring forces balance his weight. Are these a third-law pair?

No. They are balanced forces acting on the same person.