Forces and acceleration

Terminal Velocity1 September, 2011ObjectivesBe able to describe why things fall at different rates, define terminal velocity and calculate weight.HSW: AF2 Understanding the applications and implications of scienceUsed before in: lesson 5 braking forcesWill use again in:PLTS: Team workers - collaborate with others to work towards common goalsUsed before in: lesson 3 speed velocity accelerationWill use again in:KeywordsTerminal velocity, Drag, Surface area, Air resistance, Gravity, Acceleration, Weight, Newtons 2nd Law.OutcomesAll 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 Newtons 2nd Law.

Amazing JumpAmazing jump Why do we fall? Why do we fall at different rates? How can we change how fast we fall? Does gravity matter?WeightWeight = 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/kgOn the moon it is 1/6th of this why?

Falling objects.

10N20NWhat is the weight?w = m x gw = 1 x 10 = 10Ng=10N/kgWhat is the weight?What rate will they fall?This will be their acceleration.How do we work that out?Force = mass x accelerationSoacceleration = Force / massIn our case,acceleration = weight / massacceleration = 10N/1kg10m/s2 acceleration = 20N/2kg10m/s2 1kg2kgThey fall at the same rate!They both fall at 10m/s2 even though their masses are different!ALL OBJECTS FALL AT THE SAME RATE.REALLY?THAT DOESNT SOUND RIGHTLETS PROVE IT.ButThis 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.

g=10N/kgacceleration = 10N/1kg10m/s2 1kgInitially forces are unbalanced Object accelerates in direction of force(down)As the object goes faster it hits intomore air the air resistance increases The resultant downward force is less.So it speeds up less quickly The acceleration decreases.1kgacceleration 10N/1kg10m/s2 It is less1kgThe air resistance increases even moreuntil it is the same as the weight.The resultant = 0NThere is no accelerationTERMINAL VELOCITYTerminal velocity of a skydiver

9Teacher notesThe terminal velocity for a skydiver is around 60m/s, but varies with factors such as the weight and the shape of person.Velocitytime graph of skydiver

parachute10Teacher notesThis animated and interactive graph provides an opportunity for students to apply their knowledge of velocitytime / speedtime 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.

Terminal Velocity Practical Timeball bearinginsulating tapeviscous fluidTotal dist travelled /mDistance interval /mTime to fall through distance interval/ sMean velocity over distance interval / m s-1Tube 1Tube 2Tube 1Tube 20.10.10.20.10.30.10.40.10.50.10.60.10.70.10.80.10.90.1AimTo observe how objects fallat different rates when droppedthrough different fluids.Set up:Results:We will draw a velocity time graph for each tube and compare

equals air resistanceTerminal Velocity gravity mass quicker2a) 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.Copy and Complete (hint keywords below in blue box):

Terminal VelocityAs 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)

2b) (i) If the skydiver has a mass of 60kg what will her weight be? Dont 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 QuizAll : 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 Newtons 2nd Law?