Question

on his way to class, a student on a skateboard is accelerating on a downhill stretch. which of the following statements is true? (select all that apply.) the skateboarders acceleration is perpendicular to the net force. the skateboarder is moving in the direction of the net force. the skateboarders acceleration is in the direction of the net force. the skateboarder is subjected to a net external force in the direction of motion.

Explanation:

Step1: Recall Newton's second law

According to Newton's second - law $\vec{F}_{net}=m\vec{a}$, where $\vec{F}_{net}$ is the net force, $m$ is the mass and $\vec{a}$ is the acceleration. The direction of the net force is the same as the direction of the acceleration.

Step2: Analyze each statement

1. The skateboarder's acceleration is perpendicular to the net force. This is false as $\vec{F}_{net}=m\vec{a}$ implies they are in the same direction.
2. The skateboarder is moving in the direction of the net force. This is not always true. The direction of motion (velocity) and the direction of the net force are not necessarily the same. For example, an object can be moving upwards while the net force (due to gravity) is downwards when it is on its way up.
3. The skateboarder's acceleration is in the direction of the net force. This is true according to $\vec{F}_{net}=m\vec{a}$.
4. The skateboarder is subjected to a net external force in the direction of motion. Since the skateboarder is accelerating downhill, there must be a net external force in the direction of motion (down - hill). This is true.

Answer:

The skateboarder's acceleration is in the direction of the net force.
The skateboarder is subjected to a net external force in the direction of motion.