Chapter 5

Matter in Motion

Motion is a change in position.

A reference point is a place or object used for comparison to determine if something is in motion. (see pg 118)

Speed is the change in position of an object per unit of time (the “rate of change” in position).

-is always positive

-is nondirectional

speed = distance

time

v = d / t

Velocity is the speed of an object and its direction.

-may be a negative depending

on direction.

Instantaneous speed is the speed of an object at any given instant.

Average speed is the total distance traveled divided by the total time of travel.

total distance

total time

Acceleration is the rate at which velocity changes.

Acceleration refers to:

  • Increasing speed
  • Deceleration (decreasing speed)

A negative acceleration!

  • Changing direction

Acceleration formula:

final velocity - initial velocity

acceleration = time of change

Vf - Vi

a = t

(practice problems)

Centripetal acceleration occurs to objects traveling in circular motion (even if it is at constant speed).

  • The direction of the object (and therefore its velocity)is constantly changing, so the object is constantly accelerating!

A force is a push or a pull.

  • forces have sizedirection

Forces are measured in units called newtons (N).

Two forces can combine so that they add together, or subtract from each other. They may also cancel each other out.

The overall force on an object after all forces are added together is called the net force.

Unbalanced forces acting on an object will change the object’s motion.

Equal forces acting in opposite directions are called balanced forces.

(balanced forces will NOT change the motion of an object!)

Friction is a force that opposes motion between two surfaces that are in contact.

Types of friction:

  • static friction is friction when an object is not moving.

(static friction disappears as soon as an object starts moving)

  • kinetic friction is friction between moving surfaces.

-sliding friction

-rolling friction

-fluid friction

Gravity is a force of attraction between objects due to their masses.

All objects have mass. Gravity is a result of mass. Therefore, all objects have and are affected by gravity.

The Law of Universal Gravitation:

The force of gravity acts between all objects in the universe.

  • The force of gravity varies with mass and distance.

Differences in mass

Differences in distance

Weight is a measure of the force of gravity on an object

(use newtons).

-may change depending on location.

Mass is the amount of matter in an object (use grams).

-does not change.

mass / weight
measures / amount of matter / force of gravity
unit / kg / N
what affects it? / does not
change / may change
due to location

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