A ball of mass 0.4 kg flies through the air at low speed, so that air resistance is negligible.

(a) What is the net force acting on the ball while it is in motion?

(b) Which components of the ball's momentum will be changed by this force?

(c) What happens to the x component of the ball's momentum during its flight?

(d) What happens to the y component of the ball's momentum during its flight?

(e) What happens to the z component of the ball's momentum during its flight?

(f) In this situation, why is it legitimate to use the expression for average y component of velocity, ${\mathit{v}}_{\mathbf{a}\mathbf{v}\mathbf{g}\mathbf{,}\mathbf{x}}\mathbf{=}\frac{\mathbf{(}{\mathbf{v}}_{\mathbf{i}\mathbf{x}}\mathbf{+}{\mathbf{v}}_{\mathbf{f}\mathbf{x}}\mathbf{)}}{\mathbf{2}}$, to update the y component of position?

• Mass,$m=0.4\text{\hspace{0.33em}kg}$
• The average velocity expression is,$v_{avg,x}=\frac{v_{ix}+v_{fx}}{2}$

Average speed is the ratio of distance covered in a particular period of time.

(a)

Acceleration due to gravity$a=g=-9.8\text{\hspace{0.17em}\hspace{0.17em}m}/{\text{s}}^{\text{2}}$

The net force will be calculated as,

$\begin{array}{l}F=ma\\ F=0.4\times \left(-9.8\right)\\ F=-3.92\text{\hspace{0.33em}N}\end{array}$

The net force is –3.92 N.

(b)

Thenet force does have only the y-direction component, which means that only the y component will change.

(c)

Thenet force does not have only an x-direction component, which means that only it does not change. It remains constant.

(d)

Thenet force does not have an x and z component, so that momentum in that directions, it does not change and in the y-direction has a negative value. So it will be deceased.

(e)

Thenet force does not have only a z-direction component, Due to which there is no Z moment change in the component. It remains constant.

(f)

The given expression $v_{avg,x}=\frac{(v_{ix}+v_{fx})}{2}$in the question does not valid for the motion at a constant rate. The speed of the ball is small compared to the speed of light.