We hear an echo due to

At uniform speed the acceleration is

An infinite combination of resistors, each having resistance $R = 4 \Omega$, is given below. What is the net resistance between the points A and B? (Each resistance is of equal value, $R = 4 \Omega$)

An electric circuit is given below. $V_1 = 1 V$ and Resistance $R = 1000 \Omega$.

The current through the resistance $R$ is very close to $1$ mA and the voltage across point A and B, $V_{AB} = 1 V$. Now the circuit is changed to :

where value of $V_2 = 5 V$. The internal resistances of both the batteries are $0.1 \Omega$. The current through the resistance $R$ is about :