Understanding Gravity and Speed of Falling Bodies

The question asks how gravity affects the speed of bodies that are falling. Let's analyze the concept of gravity and its influence on motion.

Gravity is a fundamental force that attracts any objects with mass towards each other. For objects near the Earth's surface, gravity pulls them downwards towards the center of the Earth. This downward pull is what causes objects to fall.

When an object falls under the influence of gravity, it experiences a constant acceleration (assuming negligible air resistance). This acceleration is approximately $9.8 \, \text{m/s}^2$ near the Earth's surface and is directed downwards. Acceleration is the rate of change of velocity. Since the object is falling downwards and the acceleration due to gravity is also downwards, the velocity of the falling object increases over time in the downward direction.

Speed is the magnitude of velocity. As the downward velocity increases, the speed of the falling object also increases.

Analyzing the Options:

• increases: This aligns with our understanding of gravity causing acceleration, which leads to an increase in speed for a falling object.
• decreases: Gravity causes acceleration downwards. For a falling object moving downwards, this increases speed, not decreases it. Decreasing speed would imply deceleration or motion against the direction of force.
• adds: While gravity adds to the velocity, saying it "adds their speed" is not the standard or most appropriate term. Gravity increases the speed.
• causes: Gravity does cause their speed to change, but the blank requires a word that describes *how* it causes the speed to change in this context (falling bodies). 'Increases' is more specific and accurate for falling bodies.

Therefore, the most appropriate word to fill in the blank is "increases", as gravity causes falling bodies to accelerate downwards, thus increasing their speed.

Revision Table: Gravity's Effect on Speed

Additional Information: Gravity and Acceleration

The acceleration due to gravity is often denoted by $g$. Near the Earth's surface, $g \approx 9.8 \, \text{m/s}^2$. This means that for every second an object falls, its speed increases by approximately $9.8 \, \text{m/s}$ (ignoring air resistance). The velocity of a falling object after time $t$, starting from rest, can be described by the equation $v = v_0 + gt$, where $v_0$ is the initial velocity (0 if starting from rest). Since $g$ is positive and $t$ increases, $v$ (and hence speed) increases.

If an object is thrown upwards, gravity still acts downwards, causing deceleration (decreasing speed) until it reaches its peak, after which it starts falling, and gravity then causes it to accelerate (increasing speed).