Download Sci_ch9_lesson_2_notes

Lesson 2 Forces and Motion
A force is any push or pull from one object to another. Newtons (N) and pounds (lb) are units of force.
A spring scale is used to measure force. Most forces occur when an object touches another, but forces
an act without touching. (compass needle)
Lift, thrust, and drag are forces acting on a plane. Lift lifts the plane upward. Thrust pushes the plane
forward. Drag slows the plane down by pulling back on it.
Gravity:
Gravity depends on the mass of an object. The greater the mass, the greater the gravity. Even small
objects have gravity. They are just too small to feel the gravitational pull. Planets and stars are huge and
have a much greater gravitational pull.
Friction:
Friction is a force that opposes the motion of one object moving past another. Friction depends on the
surfaces of two objects and how hard the objects are pushed together.
Air resistance:
When an object moves through air, the air hits the object and slows it down. Liquids also exert
resistance.
Newton’s First Law of Motion
An object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted
upon by an unbalanced force.
Balanced forces are forces that act on an object without changing its motion. When an object is
still, the forces acting on it are balanced. Forces are also balanced when objects’ velocity does not
change. Like a bus traveling straight at a constant speed.
Unbalanced forces are forces that change an objects motion.
Newton’s Second Law of Motion
The unbalanced force on an object is equal to the mass of the object multiplied by its acceleration:
F=mxa
As acceleration increases, force increases. To go faster, it takes more force.
As mass increases, force increases. To move something heavier, it takes more force.
Newton’s Third Law of Motion
All forces occur in pairs, and these two forces are equal in strength and opposite in direction.
The first action is called the action force. The push of the second object back on the first is the reaction
force. Example: two people on skates