Gedanken Physics

To those who wonder WHY ?

Diagrams and inspiration from ‘Thinking Physics’ by

Lewis Carroll Epstein

Exercise the dog

Dr 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 him

b) Ti the back

c)Sideways

d) In any direction

d) Time is the important bit make sure you answer the question asked

Rolling Balls

As a ball rolls down a hill :

a) Its speed increases and acceleration decreases

b)Its speed decreases and acceleration increases

c) Both increase

d)Both remain constant

e) Its speed increases and acceleration remains constant

Rolling 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 decrease

a) Component of Fg decreases as slope decreases

Acceleration 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 ?

a)Zero

b)9.8ms-2

c) -9.8ms-2

d)Between 0 and 9.8 ms-2

c)

KINK

Water is shooting out of the tube. The end of the pipe is bent into a figure 6.

( ignore gravitational effects )

a)The water shoots out in a curved arc

b)The water shoots out in a straight line

Elephant and FeatherAn elephant and a feather fall

out of a tree. Which encounters the greatest force of air friction in falling to the ground ?

a)The elephant

b)The feather

c) Both the same

Strong ManThe strong man is pulling

the spring apart. Is there a net force on the spring ?

a)Certainly

b)No

No Force, forces cause accelerations. There is a TENSION though which results from forces acting in different directions

Tension1kg

1kg

1kg

9.8 N

What happens ?

Block a accelerates at 4.9 m s-2 and block b accelerates at 9.8 m s-

2

Horse 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 force

b) The horse pulls harder on the buggy than the buggy pulls on the horse

c) The horse pulls before the buggy has time to pull on the horse

d) 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

Horse and BuggyAcceleration 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.

Energy

A 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 has

a)Half as much kinetic energy as the first

b) The same kinetic energy as the first

c) Twice as much kinetic energy as the first

d)4 times as much kinetic energy as the first

More Bricks

Brick 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

Furniture removals

3m

6m

The drums have a mass of 100 kg. How much force is applied to move the drums up the slope ?

a)1960N

b)980N

c) 490N

d)98N

e)Can’t say

c) 490 N

How much energy do the drums gain ?

Speed

An initially stationary rock falls vertically for one second, what is its average speed during that second ?

a) 0 m/s

b) 1 m/s

c) 9.8 m/s

d) 4.9 m/s

Rocks againIf the rock now falls for 2 s what would be its average speed during the seconds ?

9.8 m s-1

Sketch a speed time graph of the motion and calculate the distance travelled.

19.6 m

The carpenter

The carpenter drops the hammer and in the first second it drops one storey. In more second it will be :

a)Two stories from the top

b)3 stories from the top

c) 4 stories from the top

d)16 stories from the top

c) Sketch a graph

SplatA 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

StoppingA 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

Hurricane

The force exerted on a house by a 120mph hurricane wind is …

a)Equally

b)Twice

c) Thrice

d)Four times as strong as the force exerted on the same house by a 60mph gale

d) Mass of air doubles and speed doubles

SmushA 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 ?

a)0%

b) 25 %

c) 50 %

d)75%

e) 100 %

c)

Tackle

Mighty 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 ?