Question

In which situation is the object experiencing unbalanced forces? A) A box resting on a horizontal floor. B) A car slowing as it reaches a traffic light. C) A car with its cruise control set to 30 km/h. D) A penny at terminal velocity after falling off of a building.

Answer 1

Answer:

B) A car slowing as it reaches a traffic light.

Explanation:

Newton's second law states that an object which experiences unbalanced forces has an acceleration, according to the equation:

$F=ma$

where

F is the net force acting on the object

m is its mass

a is its acceleration

When the forces are unbalanced, it means that the net force F is different from zero, so the acceleration of the object (a) is also different from zero.

Now let's analyze each situation:

A) A box resting on a horizontal floor. --> The object is at rest, so it is not accelerating. This means that the forces acting on it are balanced (net force = 0), so this is not the correct choice

B) A car slowing as it reaches a traffic light. --> The car is slowing down, this means that it has an acceleration: therefore, it is experiencing unbalanced forces. So, this is the correct choice.

C) A car with its cruise control set to 30 km/h. --> The car is moving at constant velocity, so it has zero acceleration. This means that the forces acting on it are balanced (net force = 0), so this is not the correct choice

D) A penny at terminal velocity after falling off of a building. --> The penny is at terminal velocity, which means that it is moving at constant velocity, so it has zero acceleration. This means that the forces acting on it are balanced (net force = 0), so this is not the correct choice.

Therefore, the correct choice is

B) A car slowing as it reaches a traffic light.

Answer 2

"Unbalanced forces" show themselves as a change in the speed
or direction of an object's motion.

The only choice where the speed or direction of motion is changing
is the car that's slowing down for the light.