Terminal Velocity Objectives 9 November, 2018 Terminal Velocity Objectives Be able to describe why things fall at different rates, define terminal velocity and calculate weight. HSW: AF2 – Understanding the applications and implications of science Used before in: lesson 5 braking forces Will use again in: PLTS: Team workers - collaborate with others to work towards common goals  Used before in: lesson 3 – speed velocity acceleration Keywords Terminal velocity, Drag, Surface area, Air resistance, Gravity, Acceleration, Weight, Newton’s 2nd Law.

Outcomes All Must know that: The faster a body moves through a fluid the greater the frictional force which acts on it. Most Will explain what terminal velocity is and be able to use the equation: weight = mass × gravitational field strength. Some will remember the equation weight = mass × gravitational field strength and see how it links to Newton’s 2nd Law.

Amazing Jump…

Amazing jump Why do we fall? Why do we fall at different rates? How can we change how fast we fall? Does gravity matter?

Weight Weight = Mass  acceleration due to gravity The force due to gravity depends on the masses of the two objects being attracted to each other. On Earth the force is 10N/kg On the moon it is 1/6th of this – why?

ALL OBJECTS FALL AT THE SAME RATE. THAT DOESN’T SOUND RIGHT… Falling objects. acceleration = 10N/1kg 10m/s2 acceleration = 20N/2kg 10m/s2 ALL OBJECTS FALL AT THE SAME RATE. REALLY? THAT DOESN’T SOUND RIGHT… LET’S PROVE IT. 1kg They fall at the same rate! They both fall at 10m/s2 even though their masses are different! 2kg Force = mass x acceleration So acceleration = Force / mass In our case, acceleration = weight / mass What rate will they fall? This will be their acceleration. How do we work that out? What is the weight? What is the weight? w = m x g w = 1 x 10 = 10N 10N 20N g=10N/kg

But… This is only true if there are no other forces acting on the object except weight. What other force is there? Air Resistance (Drag/Friction)

Falling objects. 1kg Initially forces are unbalanced – acceleration = 10N/1kg 10m/s2 acceleration ≠ 10N/1kg 10m/s2 It is less 1kg Initially forces are unbalanced – Object accelerates in direction of force (down) 1kg 1kg As the object goes faster it hits into more air – the air resistance increases – The resultant downward force is less. So it speeds up less quickly – The acceleration decreases. The air resistance increases even more until it is the same as the weight. The resultant = 0N There is no acceleration TERMINAL VELOCITY g=10N/kg

Terminal velocity of a skydiver Teacher notes The terminal velocity for a skydiver is around 60m/s, but varies with factors such as the weight and the shape of person.

Velocity–time graph of skydiver Teacher notes This animated and interactive graph provides an opportunity for students to apply their knowledge of velocity–time / speed–time graphs to explain the descent of a skydiver. While it continues the theme of balanced forces, the graph could also introduce the idea of unbalanced forces leading to acceleration and deceleration. parachute

Terminal Velocity – Practical Time Results: Aim To observe how objects fall at different rates when dropped through different fluids. Total dist travelled /m Distance interval /m Time to fall through distance interval/ s Mean velocity over distance interval / m s-1   Tube 1 Tube 2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Set up: ball bearing insulating tape viscous fluid We will draw a velocity – time graph for each tube and compare

equals air resistance Terminal Velocity gravity mass quicker Copy and Complete (hint – keywords below in blue box): Terminal Velocity As objects move they have………………. When they move ………… they have more air resistance. When the air resistance (and friction) ……….. the force that is pushing them they will go at a constant speed. This is called the ……… ………... Weight is the force on an object due to………... Weight = …………. x gravity. (Gravity = 10m/s2) equals air resistance Terminal Velocity gravity mass quicker 2a) Copy the graph and then explain (i) why a sky diver reaches a maximum speed and (ii) why the maximum speed is a lot lower when the parachute opens. 2b) (i) If the skydiver has a mass of 60kg what will her weight be? Don’t forget to write the units. 2b) (ii) If there is a resultant force of 450N on the skydiver, how long does it take to reach 55.4m/s (124 mph) (Hint – you will need to use F=ma and equation for acceleration)

Outcomes Quiz All : What happens to the frictional force on the body when it moves through a fluid faster? Most: What is terminal velocity? How can I make my terminal velocity increase? If weight = mass × gravitational field strength, What is my weight if I am 80Kg? (g=10N/kg) Some: What is weight? What is Newton’s 2nd Law?