Rolling resistance

Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface. It is mainly caused by non-elastic effects; that is, not all the energy needed for deformation (or movement) of the wheel, roadbed, etc. is recovered when the pressure is removed. Two forms of this are hysteresis losses (see below), and permanent (plastic) deformation of the object or the surface (e.g. soil). Another cause of rolling resistance lies in the slippage between the wheel and the surface, which dissipates energy. Note that only the last of these effects involves friction, therefore the name "rolling friction" is to an extent a misnomer.

In analogy with sliding friction, rolling resistance is often expressed as a coefficient times the normal force. This coefficient of rolling resistance is generally much smaller than the coefficient of sliding friction.

Any coasting wheeled vehicle will gradually slow down due to rolling resistance including that of the bearings, but a train car with steel wheels running on steel rails will roll farther than a bus of the same mass with rubber tires running on tarmac. Factors that contribute to rolling resistance are the (amount of) deformation of the wheels, the deformation of the roadbed surface, and movement below the surface. Additional contributing factors include wheel diameter, speed,load on wheel, surface adhesion, sliding, and relative micro-sliding between the surfaces of contact. The losses due to hysteresis also depend strongly on the material properties of the wheel or tire and the surface. For example, a rubber tire will have higher rolling resistance on a paved road than a steel railroad wheel on a steel rail. Also, sand on the ground will give more rolling resistance than concrete.

CRR

CRR may refer to:

Repurchase agreement

A sale (and) repurchase agreement, also known as a (currency) repo, RP, or sale and repurchase agreement, is a transaction concluded on a deal date t_D between two parties A and B:

(i) A will on the near date t_N sell a specified security S at an agreed price P_N to B

(ii) A will on the far date t_F (after t_N) re-purchase S from B at a price P_F which is already pre-agreed on the deal date.

If we assume positive interest rates, the repurchase price P_F can be expected to be greater than the original sale price P_N.

The (time-adjusted) difference (P_F-P_N)/P_N*(t_F-t_N)/365 is called the repo rate; it can be interpreted as the interest rate for the period between near date and far date.

Ambiguity in the usage of the term repo

The term repo has given rise to a lot of misunderstanding: there are two types of transactions with identical cash flows

(i) a sell-and-buy-back as well as

(ii) a collateralized borrowing.

The sole difference is that in (i) the asset is sold (and later re-purchased), whereas in (ii) the asset is instead pledged as a collateral for a loan: in the sell-and-buy-back transaction the ownership and possession of S are transferred at t_N from a A to B and in t_F transferred back from B to A; conversely, in the repo only the possession is temporarily transferred to B whereas the ownership remains with A.