A ball of mass 0.8 kg flies through the air at low speed, so that air resistance is negligible. What is the net force acting on the ball while it is in motion? \( \vec{F}_{\text {net }}=0.8 \mathrm{~kg} \cdot 9.81 \frac{\mathrm{m}}{\mathrm{s}} 2 \) Which components of the ball's momentum will be changed by this force? y only What happens to the \( x \) component of the ball's momentum during its flight? It doesn't change. \( \hat{v} \) What happens to the \( y \) component of the ball's momentum during its flight? It decreases. \( \hat{v} x \) What happens to the \( z \) component of the ball's momentum during its flight? It doesn't change. \( \hat{v} \) In this situation, why is it legitimate to use the formula for average \( y \) component of velocity, \( v_{\text {avg, }}=\frac{v_{y i}+v_{y f}}{2} \), to update the \( y \) component of position? Check all that apply. The ball's velocity changes at a constant rate because the net force on the ball is constant. This formula for average velocity is always valid. The ball's speed is small compared to the speed of light.

Let's break down the questions and answers step by step. 1. **Net Force Acting on the Ball:** The net force acting on the ball while it is in motion is due to gravity, as air resistance is negligible. The force can be calculated using the formula: \[ \vec{F}_{\text{net}} = m \cdot g = 0.8 \, \text{kg} \cdot 9.81 \, \text{m/s}^2 = 7.848 \, \text{N} \] This force acts downward. 2. **Components of the Ball's Momentum Changed by the Force:** The only component of the ball's momentum that will change due to the net force (which is gravitational) is the \( y \) component. Therefore, the answer is: - **y only** 3. **What Happens to the \( x \) Component of the Ball's Momentum:** Since there is no net force acting in the horizontal direction (assuming no air resistance), the \( x \) component of the ball's momentum remains constant. Thus, the answer is: - **It doesn't change.** 4. **What Happens to the \( y \) Component of the Ball's Momentum:** The \( y \) component of the ball's momentum will decrease due to the downward acceleration caused by gravity. Therefore, the answer is: - **It decreases.** 5. **What Happens to the \( z \) Component of the Ball's Momentum:** If we assume the ball is moving in a two-dimensional plane (x and y), and there is no force acting in the z-direction, the \( z \) component of the ball's momentum does not change. Thus, the answer is: - **It doesn't change.** 6. **Legitimacy of Using the Average \( y \) Component of Velocity Formula:** The formula for average \( y \) component of velocity, \( v_{\text{avg}} = \frac{v_{y_i} + v_{y_f}}{2} \), can be used under certain conditions. The correct reason(s) for its validity in this context are: - **The ball's velocity changes at a constant rate because the net force on the ball is constant.** - **The ball's speed is small compared to the speed of light.** (This is a general principle in classical mechanics, where relativistic effects can be ignored at low speeds.) The statement "This formula for average velocity is always valid" is not correct in all contexts, as it specifically applies to uniformly accelerated motion. In summary, the correct answers are: - Net force: \( 7.848 \, \text{N} \) downward - Changed momentum components: **y only** - \( x \) momentum: **It doesn't change.** - \( y \) momentum: **It decreases.** - \( z \) momentum: **It doesn't change.** - Validity of average velocity formula: **The ball's velocity changes at a constant rate because the net force on the ball is constant.** and **The ball's speed is small compared to the speed of light.**