Newton's Laws

State Newton's First Law.

A body will remain in its state of rest or motion at constant velocity unless a non-zero resultant/net force acts on it.

Define inertia

The tendency of a body to resist a change in its state of motion. It is a property, not a force. Greater mass = greater inertia.

Seatbelts and Newton's First Law?

When a car stops suddenly, passengers continue forward due to inertia. The seatbelt provides the external force to stop this motion.

State Newton's Second Law.

When a net force acts on an object, the object will accelerate in the direction of the force and the acceleration is directly proportional to the force and inversely proportional to the mass of the object.

State Newton's Third Law.

When object A exerts a force on object B, object B SIMULTANEOUSLY exerts an oppositely directed force of equal magnitude on object A.

Three properties of action-reaction pairs

1. Equal in magnitude, opposite in direction.

2. Act on different objects — they do NOT cancel.

3. Act simultaneously along the same line.

Define normal force.

The force or the component of a force which a surface exerts on an object with which it is in contact, and which is perpendicular to the surface.

Frictional force (F)

The force that opposes the motion of an object and which acts parallel to the surface.

Define static friction (fs).

The force that opposes the tendency of motion of a stationary object relative to a surface.

Define kinetic friction (fk)

The force that opposes the motion of a moving object relative to a surface.

Why is μs always greater than μk?

More force is needed to start an object moving than to keep it moving.

Lift at rest or constant velocity — scale reading?

N = mg (Fnet = 0; person feels normal weight).

Lift accelerating upward — scale reading?

N = mg + ma (feels heavier; N > mg).

Lift accelerating downward — scale reading?

N = mg − ma (feels lighter; N < mg).

Lift in free fall — scale reading?

N = 0 (weightless — both person and lift accelerate at g).