# Relationship between air resistance and gravity

### BBC Bitesize - GCSE Combined Science - Newton's laws - OCR Gateway - Revision 6

help to describe the relationship between a body and the forces that act upon it . Rises as skydiver accelerates, levels off as air resistance increases, terminal the aircraft, the skydiver accelerates downwards due to the force of gravity. Analyze the forces of gravity and air resistance. Ftot = ma(t), or more precisely, we obtain the following relationship between speed s(t) and acceleration s'(t). An object that is falling through the atmosphere is subjected to two external forces. The first force is the gravitational force, expressed as the weight of the object.

A skydiver The diagram shows what happens to the speed of a skydiver from when they leave the aircraft, to when they reach the ground after their parachute opens.

Before the parachute opens: Immediately on leaving the aircraft, the skydiver accelerates downwards due to the force of gravity. There is no air resistance acting in the upwards direction, and there is a resultant force acting downwards.

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The skydiver accelerates towards the ground. As the skydiver gains speed, their weight stays the same but the air resistance increases. There is still a resultant force acting downwards, but this gradually decreases.

Eventually, the skydiver's weight is balanced by the air resistance. There is no resultant force and the skydiver reaches terminal velocity.

When the parachute opens, the air resistance increases.

The skydiver slows down until a new, lower terminal velocity is reached. Note that the skydiver does not go upwards when the parachute opens, even though this can appear to happen when a skydiver is being filmed.

### Air Resistance - Gravity

The illusion happens because the person with the camera opens their parachute later on, so falls downwards past the skydiver. Is it because they all weigh the same? These questions will be explored in this section of Lesson 3. In addition to an exploration of free fall, the motion of objects that encounter air resistance will also be analyzed.

## Newton's laws

In particular, two questions will be explored: Why do objects that encounter air resistance ultimately reach a terminal velocity? In situations in which there is air resistance, why do more massive objects fall faster than less massive objects?

Free Fall Motion As learned in an earlier unit, free fall is a special type of motion in which the only force acting upon an object is gravity.

Objects that are said to be undergoing free fall, are not encountering a significant force of air resistance; they are falling under the sole influence of gravity. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass. Consider the free-falling motion of a kg baby elephant and a 1-kg overgrown mouse. If Newton's second law were applied to their falling motion, and if a free-body diagram were constructed, then it would be seen that the kg baby elephant would experiences a greater force of gravity.