Terminal Velocity D. Crowley, 2008 2018年10月23日 Terminal Velocity ◼ To understand terminal velocity Terminal Velocity ◼ What are the forces on a skydiver? How do these forces change (think about when they first jump out; during free fall; and when the parachute has opened)? ◼ What happens if the skydiver changes their position? ◼ The skydiver’s force (Fweight=mg) remains the same throughout the jump ◼ But their air resistance changes depending upon what they’re doing which changes the overall resultant force Two Most Common Factors that Affect Air Resistance ◼Speed of the Object ◼Surface Area Air Resistance ◼ More massive objects fall faster than less massive objects Since the 150-kg skydiver weighs more (experiences a greater force of gravity), it will accelerate to higher speeds before reaching a terminal velocity. Thus, more massive objects fall faster than less massive objects because they are acted upon by a larger force of gravity; for this reason, they accelerate to higher speeds until the air resistance force equals the gravity force. Skydiving ◼ Falling objects are subject to the force of gravity pulling them down – this can be calculated by W=mg Weight (N) = mass (kg) x gravity (N/kg) ◼ On Earth the strength of gravity = 9.8N/kg ◼ On the Moon the strength of gravity is just 1.6N/kg Positional ◼ What happens when you change position during free-fall? ◼ Changing position whilst skydiving causes massive changes in air resistance, dramatically affecting how fast you fall… Skydiving Stages ◼ Draw the skydiving stages ◼ Label the forces ◼ Draw correctly sized force arrows ◼ Write a brief sentence explaining the forces experienced by the skydiver during the descent Skydiving Stages ◼ Stage 1 – after just jumping from the plane the skydiver is not moving very fast – their weight is a bigger force than their air resistance, so they accelerate downwards Skydiving Stages ◼ Stage 2 – eventually the force of the air resistance has increased so much that it is the same size as the skydiver’s weight – the forces are balanced and the speed remains constant (this is terminal velocity) Skydiving Stages ◼ Stage 3 – when the chute opens air resistance increases dramatically: the air resistance force is much greater than the weight force, so the skydiver slows down Skydiving Stages ◼ Stage 4 – as the skydiver slows, the air resistance force from the chute is reduced, until it is the same size as the weight force – the forces are balanced and the speed remains constant (this is a new terminal velocity) Skydive Physics of Sky Diving James Bond and Terminal Velocity Terminal Velocity ◼ When vehicles and free-falling objects first move they have much more force accelerating them than resistance which is trying to slow them ◼ As speed increases resistance builds up – gradually reducing the acceleration ◼ Eventually the resistance forces is equal to the accelerating force and the object remains at a constant speed (terminal velocity) Velocity-Time Graph ◼ Can you annotate what the velocity-time graph shows for a parachute jump? Velocity-Time Graph Parachute opens – diver Speed slows down increases… Terminal velocity reached… Velocity New, lower terminal velocity Time reached Diver hits the ground.