Notes for Chapter 3, Section 1—Forces (pg. 72-75)
8 Force—a push or a pull that may cause an object’s motion to change.
é Units: newtons (N) in the SI, pounds (lb) in English
é Forces must have a magnitude (size) and a direction
̶ Example: 16 N, up or 2.5 × 10³ N, right
é Types of Forces
̶ Contact Forces—require objects to be touching each other, such as friction, normal force or tension forces.
̶ Field Forces—do not require objects to be touching each other. Examples are gravity, magnetic force and electrostatic force.
8 Balanced Forces—occur when all of the forces acting on an object cancel each other out. When the forces on an object are balanced, the object’s motion (its velocity) will remain constant (i.e. it will not speed up, slow down or change direction)
é Examples—a tug of war that is tied, a book sitting on a desk, a train moving at a constant velocity.
8 Unbalanced Forces—occur when the forces acting on an object do not cancel each other out. When the forces on an object are not balanced, the object’s motion (its velocity) will change (i.e. it will speed up, slow down or change direction)
é Examples—a tug of war in which one team is pulling harder than the other team, a book falling to the floor, a train that is going around a curve in the track.
8 Net Force—the force that results when many forces are added together—like “net pay” is the amount of $ you get to take home and spend from your paycheck. A net force always causes an object’s velocity to change.
8 Friction—a force that opposes the motion or attempted motion of surfaces that are in contact with each other.
é Static Friction—force that resists the initiation of sliding motion between 2 surfaces that are in contact and at rest.
̶ Examples: a car parked on a hill, taking a step, a nail holding 2 pieces of wood together.
é Kinetic (or Sliding) Friction—force that opposes the movement of 2 surfaces that are in contact and are sliding over each other.
̶ Examples: a hockey puck sliding on the ice, a car skidding to a stop, an eraser erasing pencil marks.
é Static friction is always greater than kinetic friction—it is harder to get something sliding from rest than it is to keep the thing sliding once it is in motion.
é Friction can be an advantage—How?
é Friction can be a disadvantage—How?
8 Force Diagrams—Use arrows to show the magnitude (size) and direction of the forces acting on an object.
é Examples:
̶ A book at rest on a table Ft
Forces are balanced
Fg
̶ A piñata hanging from a string Fs
Forces are balanced
̶ Book moving across table with constant velocity
Ftable Forces are balanced
v
Ffriction Fapplied
Fgravity
̶ A rock dropped from a bridge (with air resistance)
Forces are NOT balanced—there is a net force in the downward direction.
Fair
v
Fgravity
̶ A book speeding up as it is pulled across a table
Forces are NOT balanced—there is a net force to the right.
Ftable
Ffriction v Fapplied
Fgravity
̶ A car skidding to a stop on the road
Froad
v Forces are NOT balanced—there
Ffriction is a net force to the left.
Fgravity
Now answer the section review questions on page 336.
For homework, read pages 344 to 351.