CHAPTER 5

Section 1: Matter in Motion

  1. Even though you may not see anything moving around you, motion IS occurring. The object moving or the amount of movement may be so small that the unaided eye can’t see it.

Observing Motion by Using a Reference Point

  1. Any object movement that you observe is actually observable because you are watching the object in relation to another object that is staying in place.
  1. A reference point is an object that appears to stay in place.
  1. Motion occurs when an object changes position over time relative to a reference point.
  1. An object’s motion will be in a certain direction.
  1. A direction can be described with terms such as north, south, up, down, left, right, etc.
  1. Earth’s surface is a common reference point for determining motion.
  1. Any common, non-moving object can be used as a reference point. Examples: trees, buildings, rock formations.
  1. Moving objects can occasionally also be reference points. Example: You are in a moving car and a motorcycle passes you going in the opposite direction.

Speed Depends on Distance and Time

  1. Speed is defined as the distance traveled by an object divided by the time it took for the object to cover that distance.
  1. The SI unit for speed is meters per second (m/s) or kilometers per hour (km/h).
  1. Most of the time, an object does not travel at a constant speed, so when you determine speed, you are finding an average.
  1. Formula for average speed:

Average speed = total distance

total time

  1. When graphing speed, the distance traveled is on the y-axis. The time it took to travel the distance is on the x-axis.(DRY MIX)
  1. The slope of the line created by graphing distance versus time is the average speed of the object.

Velocity: Direction Matters

  1. Velocity is defined as the speed of an object in a particular direction.
  1. Velocity has 2 components: speed and direction. An example of a velocity would be 28 km/h south.
  1. Velocity should be viewed as the rate of change in an object’s position.
  1. If an object’s speed and direction do not change, the object is said to have constant velocity.
  1. On a distance vs. time graph, constant velocity will have a consistent slope evidenced by a straight line.
  1. Velocity changes if speed changes and/or if direction changes.
  1. Velocities can be combined.
  1. When you combine velocities that are in the same direction, add them together to get the resultant velocity.
  1. When you combine two velocities that are in opposite directions, subtract the smaller from the larger. The resultant velocity goes in the direction of the larger velocity.

CHAPTER 5

Section 1: Matter in Motion (Continued)

Acceleration

  1. In Science, acceleration is defined as the rate at which velocity changes.
  1. Acceleration occurs whenever direction and/or speed changes.
  1. An increase in velocity is positive acceleration.
  1. A decrease in velocity is negative acceleration, or deceleration.
  1. The faster the velocity changes, the greater the acceleration is.
  1. To calculate average acceleration, use the following formula:

average acceleration = final velocity – initial velocity

time it takes to change velocity

  1. The units on acceleration are meters per second per second (m/s/s) or meters per second squared (m/s2).
  1. When graphing acceleration, velocity is on the y-axis and time is on the x-axis.
  1. When velocity is increasing on the graph, your acceleration is positive. When velocity is decreasing on a graph the acceleration is negative.
  1. An object traveling in a circle is always changing direction. This means the velocity is always changing.
  1. Centripetal acceleration occurs with circular motion.

Chapter 5

SECTION 2: Balanced & Unbalanced Forces

  1. In Science, a force is a push or a pull.
  1. All forces have size and direction.
  1. Forces change the acceleration of an object by changing its speed and/or direction.
  1. The unit on force is a Newton (N).

Forces Acting On Objects

  1. All forces have to have objects to act on.
  1. Even though forces may be acting on an object, it does not mean the object will move.
  1. In order for motion to occur, forces have to be unbalanced. In other words, the forces acting on the object cannot be equal from all directions if motion is occurring.

Determining Net Force

  1. Usually more than one force is acting on an object.
  1. Net force is defined as the combination of all forces acting on an object.
  1. Forces acting on an object in the same direction are added to determine net force.
  1. Forces acting in opposite directions are subtracted to determine net force.

Balanced and Unbalanced Forces

  1. If the net force on an object is 0 N, the forces are balanced and there will not be a change in the object’s motion or non-motion.
  1. If the net force is greater than 0 N, the forces are unbalanced and there will be a change in the object’s motion or non-motion.
  1. An object will continue to move once the unbalanced force is removed.

Chapter 5

SECTION 3: Friction

  1. Unbalanced forces are needed to affect an object’s motion.
  1. Friction is a force that can slow an object’s motion.
  1. Friction is defined as a force that opposes motion between objects that are in contact.

Sources Of Friction

  1. Friction occurs because the surfaces of objects are covered with microscopic hills and valleys. These hills and valleys hang up on each other as objects come in contact.
  1. The rougher the surface is, the greater the size of the hills and valleys and the greater the friction will be.
  1. Smooth surfaces have very little height to the hills and depth to the valleys. Consequently, smooth surfaces tend to get hung up on each other less as they come in contact and friction is decreased.
  1. The amount of friction is also determined by the amount of force that is pushing the surfaces in contact.
  1. The greater the force pushing the object’s together the greater the friction.
  1. Objects with greater mass are being pulled on with a greater gravitational pull, so they will be more “smushed” together. This increases friction.
  1. There are 2 types of friction
  2. kinetic friction – occurs between moving surfaces
  3. static friction – occurs when a force is applied to an object, but the object doesn’t move because the force of friction is equal to or greater than the force applied
  1. Static friction can become kinetic friction and kinetic friction can become static friction.

9. Friction can be both harmful and helpful.

a. harmful – friction from wind and water can cause erosion

b. helpful – friction in brakes helps bring the car to a stop

  1. Ways to reduce friction:
  2. lubricants
  3. smooth rough surfaces
  1. Ways to increase friction:
  2. increase the force pushing the objects together
  3. roughen surfaces

Chapter 5:

SECTION 4: Gravity

  1. Gravity is defined as a force of attraction between objects.
  1. The force of gravity can change an object’s motion by changing its speed and/or direction.

The Effects Of Gravity On Matter

  1. Gravity is the result of mass.
  1. The strength of a gravitational pull is determined by an object’s mass and the distance between the objects.
  1. All objects have mass, therefore all objects exert a gravitational pull.
  1. Only objects with mass great enough to exert a very strong gravitational pull can exert a force great enough to change an object’s motion.
  1. Earth is just such an object. Its mass is great enough to exert a gravitational pull strong enough to affect motion.
  1. Earth’s gravitational pull causes objects to fall to the surface toward Earth’s center.
  1. In order to lift an object, we must exert a force great enough to overcome gravity’s pull.

Newton and the Study of Gravity

  1. Newton summarized his ideas on gravity in the law of universal gravitation.
  1. This law describes the relationship between gravitational force, mass, and distance.

The Law of Universal Gravitation

  1. This law states that all objects in the universe attract each other through a gravitational force that is determined by masses of objects and the distance between them.
  1. As mass increases, gravitational force increases.
  1. As the distance between the objects increases, the gravitational force decreases.

Weight as a Measure of Gravitational Force

  1. Weight is the force with which gravity pulls on an object.
  1. The greater an object’s mass, the greater the force with which gravity pulls. Therefore, the greater the mass, the greater the weight.
  1. Mass does not change, weight does.
  1. Because weight is a force, the unit on weight is a Newton (N).