(I) A hiker determines the length of a lake by listening for the echo of her shout reflected by a cliff at the far end of the lake. She hears the echo 2.5 s after shouting. Estimate the length of the lake.

425 m

Explanation:

From the question,

v = 2x/t Equation 1

Where v = speed of sound in air, x = Length of the lake, t = time.

make x the subject of the equation

x = vt/2 Equation 2

Given: t = 2.5 s

Constant: v = 340 m/s.

Substitute these values into equation 2

x = 340(2.5)/2

x = 425 m.

Hence the length of the lake is 425 m

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

                                              ⇒      

here   k is the proportionality constant whose value is 1.

                                                            [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.