There are two types of units: base units and derived units. Base units are a set of units from which all other units are derived, whereas derived units are the units derived from base units.

There are six main base units: meters, kilogram, second, Ampere, Kelvin, moles, and candela (unit for luminous intensity). The rest of the units are classified as derived units and will contain a combination of these base units. The SI units for the four measurements listed in question are as follows:

Mass: 

Volume:  (Liters)

Weight:  (Newtons)

Density: 

This means that volume, weight, and density are derived units, whereas mass is a base unit. 

Derived units are derived from the six base units. A derived unit usually contains multiple base units that are combined by using multiplication and/or division; addition and subtraction of base units is not performed to obtain derived units.

The SI unit for force is Newtons. Newtons written in base units is ; therefore, units for force is always a derived unit. A unit can be either a derived unit or a base unit, but it can never be both. Recall that there are only six base units and numerous derived units; therefore, there are more derived units than base units. 

The SI base unit for time is seconds. Hours are derived by multiplying seconds by sixty. Hours are not a base unit.

Recall that the mass of a substance is usually measured in kilograms (), which is a type of base unit. Moles () is a unit used to measure the amount of substance. Moles are also a type of base unit; therefore, both the unit for mass () and the unit for amount of substance () are measures of base units. 

Scalar quantities are those that can be described with magnitude only, as opposed to vectors, which include both magnitude and direction components. Distance, speed, and time are all scalars. Displacement is not a scalar, as it involves both the distance and the direction moved from a starting point. Velocity also includes a direction component, and is therefore a vector quantity.

A vector quantity has has both magnitude and direction. By definition, only velocity satisfies these criteria.

Distance, angle, and speed have magnitude, but no direction.

The answer to this question lies within Newton's third law: for every action, there is an equal and opposite reaction. If the large ball applies a  force on the smaller ball, the smaller ball will apply a  force on the large ball in the opposite (negative) direction.

This is a conservation of energy question. The event is the collision. The initial energy can be calculated by gravitational potential energy:

The final total energy must be the same. The energy of the scattered pieces must add to 

When the rope is around the pulley, the acceleration is slower due to internal friction, thus the initial acceleration of the platform would be lower. Once the platform was in free fall, however, the acceleration due to gravity is constant at 9.8 m/s²; thus, the final acceleration of the platform is the same in both scenarios.

This is a question that helps you see how displacement and velocity compare to distance and speed, respectively. Remember that the runner starts and stops at the same point on the track, so her total displacement is zero feet.

Since , the runner's average velocity is . Since the runner's average speed is greater than her average velocity, we can conclude this is the correct answer. Her average speed refers to her distance, rather than displacement, and is greater than zero.

This is also important to remember! Since the runner changed direction on the track, she accelerated and decelerated accordingly based on her position on the track. She did not have a constant acceleration.

The three blocks must remain in contact as they move, so they will each have the same velocity and acceleration regardless of their different masses. So, .