Presentation on theme: "Gedanken Physics To those who wonder WHY ?"— Presentation transcript:
1Gedanken Physics To those who wonder WHY ? Diagrams and inspiration from ‘Thinking Physics’ byLewis Carroll Epstein
2Exercise the dogDr Pisani exercises his dog on a 15 minute walk by throwing a stick that the dog chases and retrieves. To keep the dog running for the longest time as Dr Pisani walks which way should he throw the stick ?a) In front of himb) Ti the backc)Sidewaysd) In any directiond) Time is the important bit make sure you answer the question asked
3Rolling Balls As a ball rolls down a hill : Its speed increases and acceleration decreasesIts speed decreases and acceleration increasesBoth increaseBoth remain constantIts speed increases and acceleration remains constant
4Rolling Balls As a ball rolls down a hill : a) Its speed increases and acceleration decreasesb) Its speed decreases and acceleration increasesc) Both increased) Both remain constante) Both decreasea) Component of Fg decreases as slope decreases
5Acceleration at the top A stone is thrown straight up. At the top of its path its velocity is momentarily zero. What is its acceleration at this point ?Zero9.8ms-2-9.8ms-2Between 0 and 9.8 ms-2c)
6KINKWater is shooting out of the tube. The end of the pipe is bent into a figure 6.( ignore gravitational effects )The water shoots out in a curved arcThe water shoots out in a straight line
7Elephant and FeatherAn elephant and a feather fall out of a tree. Which encounters the greatest force of air friction in falling to the ground ?The elephantThe featherBoth the same
8Strong ManThe strong man is pulling the spring apart. Is there a net force on the spring ?CertainlyNoNo Force, forces cause accelerations. There is a TENSION though which results from forces acting in different directions
9Tension 1kg What happens ? 1kg 1kg 9.8 N Block a accelerates at 4.9 m s-2 and block b accelerates at 9.8 m s-2
10Horse and Buggya) The horse can’t pull the buggy because the horse pulls on the buggy and the buggy pulls on the horse with the same size of forceb) The horse pulls harder on the buggy than the buggy pulls on the horsec) The horse pulls before the buggy has time to pull on the horsed) Both buggy and horse pull with the same size of force but the horse is joined to the earth by flat hoofs whilst the buggy is free to roll on wheels
11Horse and Buggy Acceleration depends on the ratio Force / mass. Who has the most mass ?The horse has joined itself to the earth. The buggy therefore pulls on the horse and earth. The buggy is free to move.The buggy goes forward and the earth and horse go backward. The buggy goes forward 1 m then earth and the horse go backward c.a. 1x10-22 m.
12EnergyA 1kg brick is lifted to a given height and the dropped to the ground. An identical brick is then lifted twice as high as the first brick and then dropped to the ground. When the second brick hits the ground it hasHalf as much kinetic energy as the firstThe same kinetic energy as the firstTwice as much kinetic energy as the first4 times as much kinetic energy as the first
13More BricksBrick a is twice the mass of brick b, Both are released from the same height and hit the ground together.Explain which brick has the most kinetic energy
14Furniture removalsThe drums have a mass of 100 kg. How much force is applied to move the drums up the slope ?1960N980N490N98NCan’t say3m6mc) 490 NHow much energy do the drums gain ?
15SpeedAn initially stationary rock falls vertically for one second, what is its average speed during that second ?a) 0 m/sb) 1 m/sc) 9.8 m/sd) 4.9 m/s
16Rocks againIf the rock now falls for 2 s what would be its average speed during the seconds ?9.8 m s-1Sketch a speed time graph of the motion and calculate the distance travelled.19.6 m
17The carpenterThe carpenter drops the hammer and in the first second it drops one storey. In more second it will be :Two stories from the top3 stories from the top4 stories from the top16 stories from the topc) Sketch a graph
18SplatA bottle dropped from a balcony hits the ground at x ms-1. To double the speed explain how high the balcony should be .4 times as high
19StoppingA car is going at 10km / h. It comes to rest in 3m when the driver brakes.How far would it come to rest in if the car now travels at 40 km / h ?48 m
20Hurricane The force exerted on a house by a 120mph hurricane wind is … EquallyTwiceThriceFour times as strong as the force exerted on the same house by a 60mph galed) Mass of air doubles and speed doubles
21SmushA 1 kg mass travelling at 1 ms-1 smashes into a stationary 1kg mass. They stick together . What proportion of the original kinetic energy is turned into heat energy ?0%25 %50 %75%100 %c)
22TackleMighty Mike, 100kg, is running at 2 ms-1. Speedy Gonzales, 50 kg , is running at 4 ms-1, Pondorous Poncho , 200 kg , is running at 1 ms-1.Who is more likely to stop Mike in a tackle ?Who is more likely to break Mike’s bones ?Both stop him but who has most Ek ?