21. A uniform bar \( A B \) of length 6 as been placed on a horizontal smooth table of width \( 5 a \) as shown in the figure. Length \( 2 a \) of the bar is overhanging. Mass of the bar is \( 4 m \). An insect of mass \( 3 m \) is sitting at the end \( A \) of the bar. The insect walks along the length of the bar to reach its other end \( B \).

Un dipôle ohmique de résistance \( 3300 \Omega \) est traversé par une tension de 900 mV . Calculer l'intensité qui traverse se dipôle.

1. Calculer la tension à ses bornes 2. On triple la tension à ses bornes, quelle est alors l'intensité qui la traverse?

Un conducteur de résistance \( 65 \Omega \) est traversé par un courant de \( 0,1 A \). 1. Calculer la tension à ses bornes 2. On triple la tension à ses bornes, quelle est alors l'intensité qui la traverse?

EXERCISES It is known that the flow of fluid \( (\mathrm{Q} \) rate) is a fluid volume passing per unit time, based on the pressure gradient \( (\mathrm{P} / \ell) \) where \( (p) \) is the pressure between the ends of the tube flow stream, \( (\ell) \) is the length of pipe flow, also depends on the viscosity of the liquid \( (\eta) \) and the radius of the tube \( (r) \), by using the concept of compatibility between the units and dimensions theory derive a mathematical formula for flow rate depending on the variables mentioned above.

Una palla di massa \( m_{1}=24 \mathrm{~g} \), che viaggia alla velocità \( v_{1} \), urta elasticamente una palla ferma di massa \( m_{2}=\frac{1}{2} m_{1} \). Dopo l'urto, la seconda palla va a colpire elasticamente una terza palla ferma, di massa \( m_{3} \). Tutti gli urti avven- gono lungo la stessa retta. Quale deve essere la massa \( m_{3} \) affinché la sua velocita dopo l'urto sia uguale a \( v_{1} \) ?