A small object is traveling in a straight line through space. A student wants to change the model so the object's path bends toward a nearby planet. Which change to the model would most likely cause the object's path to bend? A) Decreasing the object’s speed so it moves more slowly B) Turning off the light source that represents the Sun C) Increasing the size of the object without changing anything else D) Adding a gravitational pull from the planet toward the object

What has to change for a straight path to become a curved path

An object keeps moving in a straight line at constant speed unless a net force acts on it. To make the path bend, the model must include an unbalanced force that pulls the object sideways relative to its current motion, causing acceleration and a change in direction.

Checking each option against Newton’s first law

• A) Decreasing speed: Moving more slowly does not automatically make the path curve. Without a sideways net force, the object still travels in a straight line.
• B) Turning off the Sun light source: Light is not what causes planetary attraction in this context. Removing light does not create a force that bends the path.
• C) Increasing the object’s size: If nothing else changes, size alone does not add a sideways force. The motion direction would stay the same.
• D) Adding gravitational pull: Gravity from the planet provides an attractive force toward the planet. That force produces an acceleration toward the planet, so the velocity direction changes and the trajectory curves.

Why gravity bends the path

The gravitational force points toward the planet: $$F_g = \frac{G m_1 m_2}{r^2}$$ This force creates acceleration toward the planet: $$a = \frac{F_g}{m}$$ Because this acceleration is not along the object’s original straight-line direction, it changes the direction of the velocity, producing a curved path toward the planet.