# Holt Physics Chapter 4

### Holt Physics Chapter 4

Force
An action exerted on an object which may change the object’s state of rest or motion. (Interaction which changes the motion of an object -Ex. Person hitting a baseball.) It is a vector because it has both magnitude and direction.
SI Unit of Force
Newton
Contact Forces
Forces which result from physical contact between two objects.
Field Forces
No physical contact between two objects. (Ex. Gravitational force or Electrical field)
Free-body Diagram
A diagram used to identify forces affecting the motion of a single object.
Newtons 1st Law
Object at rest remains at rest, object in motion stays in motion (with constant velocity) unless the object experiences a net external force.
Inertia
Tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction. (Tendency of an object NOT to accelerate.)
Ff
The forward force.
Fr
Acts in the opposite direction, due to friction and air resistance.
Fg
The downward gravitational force.
Net Force
A single force whose external effects on a rigid body are the same as the effects of several actual forces action on the body. (Vector sum of all forces acting on an object.)
Mass and Inertia
Inertia of an object is proportional to the object’s mass. (Greater mass = less acceleration)
Equilibrium
The state in which the net force on an object is zero. (Objects at rest or moving with constant velocity.)
Newton’s 2nd Law
The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the object’s mass. (Literally –> F=ma, net force = mass x acceleration)
Newton’s 3rd Law
If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite direction.