Chapter 3, Topic 7
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# Free Fall

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What is free fall?
It is when an object is falling without air resistance.

All free-falling objects experience:
Constant acceleration of 10m s⁻²

#### Technique to Answer Free Fall Questions

When the skydiver jumps out of the plane:

• Air resistance less than the weight of the skydiver
• The resultant force is positive
• Since F(resultant force)=ma, the acceleration is positive
• As the velocity increases the air resistance increases
• Therefore the skydiver is falling with a decreasing acceleration.

When the skydiver is falling in the air:

• As the speed of the falling skydiver increases, the air resistance increases
• the air resistance increases until it is equal to the weight of the skydiver.
• Resultant force decreases to zero.
• Since F(resultant force)=ma, the acceleration is 0 m s⁻² and the skydiver is falling at constant velocity, also known as terminal velocity.

When the skydiver opens his parachute:

• When the parachute opens, the air resistance is greater than the weight of the skydiver.
• Resultant force becomes negative.
• Since F(resultant force)=ma, the acceleration is negative and the skydiver is falling at a deceleration until he reaches another terminal velocity with much lower value and lands safely.

### Classic Questions of Free Fall

#### 1. Describe how an object released from rest (subjected to air resistance) will eventually fall with constant velocity.

• Immediately upon release, gravitational force acting on the object causes it to accelerate at 10 m s⁻².
• As the object gains speed, the upward air resistance increases. This will cancel out the downward gravitational force and decrease the acceleration.
• Eventually, the upward air resistance increases to a point where it equals the downward gravitational force.
• The resultant force acting on the object is now zero. Therefore the acceleration is zero and it continues falling with constant velocity (known as terminal velocity).

#### 2a. Which parachutist will reach terminal velocity faster, the parachutist weighing 100kg or 150kg? Why?

• The 100kg parachutist.
• This is because it will be faster for the force of air resistance to be equal to his weight of 100kg than the 150kg parachutist.

#### 2b. Which parachutist will reach the ground faster? Why?

• The 150kg parachutist.
• As the 150 kg parachutist is heavier than the 100kg parachutist, there will be more force acting on the 150 kg parachutist, causing him to fall at a higher speed and thus, reach the ground faster than the 100 kg parachutist. 