Chapter #5 – Lecture notes

When an object changes position over time, when compared to a reference point, the object is said to be in motion. The reference point is the object that appears to stay in place. The most common reference point is the Earth’s surface.

The rate at which an object moves is called its speed. Speed depends on the distance traveled and the time it takes to travel that distance. The SI unit of speed is the meters per second (m/s).

Speed can be calculated using the formula,

Speed = Distance
Time

The speed of an object in a particular direction is the objects velocity. Speed and velocity do not mean the same thing. Velocity must include direction. Velocity changes as speed or direction changes. Velocities can be combined. The result you get is called the Resultant Velocity. Resultant velocity can be calculated two ways. If they are traveling in the same direction you add them, if they are traveling in opposite directions you subtract them.

Acceleration is the rate at which velocity changes. An object can accelerate by changing speed, changing direction, or both.

Acceleration can be calculated using the formula,

Acceleration = Final Velocity – Starting Velocity
                    Time

Acceleration in which velocity increases is called Positive Acceleration.  Acceleration in which velocity decreases is called Negative Acceleration or Deceleration.  Acceleration that occurs in circular motion is called Centripetal Acceleration.  Circular motion has continuous acceleration.

A force is defined as a push or a pull. All forces have both size and direction. The SI unit of force is the Newton (N). Force is always exerted by one object on another object. Forces can be combined. The result is called the net force. Net force can be calculated two ways. If the forces are in the same direction add them, if they are in opposite directions subtract the smaller force from the larger force.

Forces can be balanced or unbalanced. Unbalanced forces produce a change in motion, balanced forces do not.

Friction is a force that opposes the motion of objects in contact. Friction is caused by the "hills and valleys" that are present on all surfaces. The amount of friction produced depends on certain factors such as the roughness of the surfaces, and the force pushing the surfaces together. The greater the force, the greater the amount of friction produced.

There are 4 basic types of friction:

1. Sliding friction – which requires a lot of force to overcome.

2. Rolling friction – which requires a small force to overcome.

3. Fluid friction – which is a force that opposes the motion of objects moving through a gas or a liquid. This requires a small force to overcome.

4. Static friction – when a force is applied to an object, but the object does not move static friction is produced. The object does not move because the static friction balances the force applied. As soon as the object moves, static friction disappears.

Friction can be helpful or harmful, because of this it is sometimes necessary to increase or decrease friction. To decrease friction you can use a lubricant, switch from sliding friction to rolling friction, or make the surfaces in contact smoother. To increase friction you can, make the surfaces rougher or increase the force pushing the surfaces together.

Gravity is a force of attraction between objects due to their masses. All matter has mass, gravity is a result of mass, so all matter experiences gravity. Gravitational force pulls objects towards each other. The Earth’s gravitational force is the largest force that acts on us.

This law states that all objects in the universe attract each other through gravitational force. The size of the force depends on the objects masses and the distance between them. Gravitational force increases as mass increases, and decreases as distance increases. Weight is a measure of the force of gravity acting on an objects mass. Remember, mass and weight is not the same thing. Mass is the amount of matter in an object.