FORCEChapter 4

Force: A push or a pull

Describes why objects move

Defined by Sir Isaac Newton

Newton’s Laws

Newton’s First Law:An object at motion stays in motion unless

acted upon by a net force

In other words: no net force means no change in motion.

Newton’s 2nd Law: F = maMeasured in Newton’s

○ 1 N = 1 kg * m/s2

Newton’s 3rd Law:When an object exerts a force on a second

object, the second exerts a force equal in magnitude but opposite in direction to the first.○ Action-reaction forces

Types of Forces

Gravity: considered to be straight downward.Symbolized Fg or WAlso called Weight

Fg =

Normal Force: force perpendicular to the surface of contact between two surfaces.The force of contact between 2 surfaces.

Net Force

Net force is the sum of all forces acting on an object in a particular direction.

Fnet = mass of object * actual acceleration of the object in that direction

No acceleration means net force = 0 NConstant velocity, object at restEquilibrium

Free-Body Diagrams

Simple sketches that show all forces acting on an object.

You and a friend push a large box across the floor. You push with a force of 100 N, your friend pushes with a force of 85 N. The force of friction on the box is 40 N. The box has a mass of 60 kg. Draw a free body diagram showing all forces acting on the box. Determine the net force acting on the box. What will be the acceleration of the box? How far will the box have moved after 3 seconds?

Finding net force/acceleration

A 14 kg box is being pulled up a ramp inclined at 27 degrees above the horizontal with a force of 140N. The force of friction acting on the box is 35 N. What will be the acceleration of the box?

Homework

Pg. 124 #1-2 Practice A Pg. 128 #1-3 Practice B Pg. 129 #1-5 Section Review

Friction: the force that opposes motion between 2 surfaces.