Transcript Gedanken Physics

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
decreases
b) Its speed decreases and acceleration
increases
c) Both increase
d) Both remain constant
e) 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 Feather
An 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 Man
The 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
Tension
What happens ?
1kg
1kg
1kg
9.8 N
Block a accelerates at
4.9 m s-2 and block b
accelerates at 9.8 m s-2
Horse and Buggy
a) 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 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.
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
The drums have a mass of
100 kg. How much
force is applied to
move the drums up the
slope ?
3m
6m
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 again
If 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
Splat
A 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
Stopping
A 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
Smush
A 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 ?