Question

current attempt in progress
as the text discusses, the conservation of linear momentum is applicable only when the system of objects is an isolated system. which of the systems listed below are isolated systems?

1. a ball is dropped from the top of a building. the system is the ball.
2. a ball is dropped from the top of a building. the system is the ball and the earth.
3. a moving billiard ball collides with a stationary billiard ball on a frictionless pool table. the system is the moving ball.
4. a car slides to a halt in an emergency. the system is the car.
5. a space probe is moving in deep space where gravitational and other forces are negligible. the system is the space probe.

only 5 is an isolated system.
only 2 and 5 are isolated systems.
only 1 and 3 are isolated systems.
only 3 and 5 are isolated systems.
only 4 and 5 are isolated systems.

Explanation:

An isolated system is one on which no external forces act.

• For case 1: The ball is acted upon by gravitational force (external force as the system is just the ball), so not isolated.
• For case 2: The ball - earth system: Gravitational force is internal (force between ball and earth which are part of the system). But when the ball is dropped, air resistance (if any, though not mentioned but in real - world near - earth scenarios, we assume some approximation) is an external force. However, if we consider ideal conditions (no air resistance), the only force is the internal gravitational force between ball and earth.
• For case 3: The moving ball is acted upon by the normal force from the table (external, as the system is just the moving ball) and the force due to collision with the stationary ball (if we consider before collision, the stationary ball is external).
• For case 4: The car is acted upon by frictional force (external, as the system is the car) which stops it.
• For case 5: In deep space with negligible gravitational and other forces, no external forces act on the space - probe (system is the space - probe).

Answer:

Only 2 and 5 are isolated systems.