Transcript Document

GCSE Forces
W Richards
Worthing High School
Distance, Speed
and Time
D
Speed = distance (in metres)
time (in seconds)
S
T
1) Dave walks 200 metres in 40 seconds. What is his speed?
2) Laura covers 2km in 1,000 seconds. What is her speed?
3) How long would it take to run 100 metres if you run at 10m/s?
4) Steve travels at 50m/s for 20s. How far does he go?
5) Susan drives her car at 85mph (about 40m/s). How long does it
take her to drive 20km?
Distance-time graphs
2) Horizontal line =
40
4) Diagonal line
downwards =
30
Distance
(metres)
20
10
0
Time/s
20
1) Diagonal line =
40
60
80
100
3) Steeper diagonal line =
40
Distance
(metres)
30
20
10
0
Time/s
20
40
60
80
100
1) What is the speed during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the speed during the last 40 seconds?
4) When was the object travelling the fastest?
Speed vs. Velocity
Speed is simply how fast you are travelling…
This car is travelling at a
speed of 20m/s
Velocity is “speed in a given direction”…
This car is travelling at a
velocity of 20m/s east
Acceleration
V-U
Acceleration = change in velocity (in m/s)
(in m/s2)
time taken (in s)
A
T
1) A cyclist accelerates from 0 to 10m/s in 5 seconds. What is her
acceleration?
2) A ball is dropped and accelerates downwards at a rate of 10m/s2
for 12 seconds. How much will the ball’s velocity increase by?
3) A car accelerates from 10 to 20m/s with an acceleration of 2m/s2.
How long did this take?
4) A rocket accelerates from 1,000m/s to 5,000m/s in 2 seconds.
What is its acceleration?
Velocity-time graphs
1) Upwards line =
80
Velocity
m/s
4) Downward line =
60
40
20
0
10
2) Horizontal line =
20
30
40
50
3) Steeper line =
T/s
80
60
Velocity
m/s
40
20
0
T/s
10
20
30
40
1) How fast was the object going after 10 seconds?
2) What is the acceleration from 20 to 30 seconds?
3) What was the deceleration from 30 to 50s?
4) How far did the object travel altogether?
50
Balanced and unbalanced forces
Consider a camel standing on a road.
What forces are acting on it?
Reaction
These two forces would be equal –
we say that they are BALANCED.
The camel doesn’t move anywhere.
Weight
Balanced and unbalanced forces
Reaction
What would happen if we took the
road away?
Weight
Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Weight
Balanced and unbalanced forces
What would happen if we took the
road away?
The camel’s weight is no longer
balanced by anything, so the camel
falls downwards…
Balanced and unbalanced forces
1) This animal is either
________ or moving
with _____ _____…
3) This animal is getting
_______….
2) This animal is getting
_________…
4) This animal is…
Balanced and unbalanced forces
Force and acceleration
If the forces acting on an object
are unbalanced then the object will
accelerate, like these wrestlers:
Force (in N) = Mass (in kg) x Acceleration (in m/s2)
F
M
A
Force, mass and acceleration
1) A force of 1000N is applied to push
a mass of 500kg. How quickly does
it accelerate?
F
2) A force of 3000N acts on a car to
make it accelerate by 1.5m/s2. How
heavy is the car?
3) A car accelerates at a rate of
5m/s2. If it weighs 500kg how
much driving force is the engine
applying?
4) A force of 10N is applied by a boy
while lifting a 20kg mass. How
much does it accelerate by?
M
A
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is _______ so he
_______ downwards.
2) As his speed increases his air
resistance will _______
3) Eventually the air resistance will be
big enough to _______ the
skydiver’s weight. At this point
the forces are balanced so his
speed becomes ________ - this is
called TERMINAL VELOCITY
Terminal Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly ________,
causing him to start _____ ____.
5) Because he is slowing down his air
resistance will _______ again until
it balances his _________. The
skydiver has now reached a new,
lower ________ _______.
Velocity-time graph for terminal velocity…
Parachute opens –
diver slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
Weight vs. Mass
Earth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
W
Weight = Mass x Gravitational Field Strength
(in N)
(in kg)
(in N/kg)
M
G
1) What is the weight on Earth of a book with mass 2kg?
2) What is the weight on Earth of an apple with mass 100g?
3) Dave weighs 700N. What is his mass?
4) On the moon the gravitational field strength is 1.6N/kg. What will
Dave weigh if he stands on the moon?
Friction
1) What is friction?
2) Give 3 examples where it is annoying:
3) Give 3 examples where it is useful:
4) What effect does friction have on the surfaces?
Stopping a car…
Tiredness
Too many
drugs
Thinking
distance
Poor
visibility
Wet roads
Icy roads
Tyres/brakes
worn out
Too much
alcohol
Stopping
distance
Driving too
fast
Work done
When any object is moved around work will need to be
done on it to get it to move (obviously).
We can work out the amount of work done in moving an
object using the formula:
Work done = Force x distance moved
in J
in N
W
in m
F
D
Kinetic energy
Any object that moves will have kinetic energy.
The amount of kinetic energy an object has can be found
using the formula:
Kinetic energy = ½ x mass x velocity squared
in J
in kg
KE =
½
in m/s
mv2
Some example questions…
1) A 70kg boy is running at about 10m/s. What is his
kinetic energy?
2) A braking force of 1000N is applied by a driver to
stop his car. The car covered 50m before it stopped.
How much work did the brakes do?
3) What is the kinetic energy of a 100g tennis ball being
thrown at a speed of 5m/s?
4) A crane is lifting a 50kg load up into the air with a
constant speed. If the load is raised by 200m how
much work has the crane done? (The answer isn’t
10,000J)
Pressure – some basic ideas
Describe and draw experiments for each of the following
ideas:
1) Pressure increases with depth
2) Pressure is the same in all directions at a certain depth
Pressure
Pressure depends on two things:
1) How much force is applied, and
2) How big (or small) the area on which this force is
applied is.
Pressure can be calculated using the equation:
F
Pressure (in N/m2) = Force (in N)
Area (in m2)
P
A
Some example questions…
1) A circus elephant weighs 10,000N and can stand on one foot.
This foot has an area of 50cm2. How much pressure does he
exert on the floor (in N/cm2)?
2) A 50kg woman copies the elephant by standing on the heel of one
of her high-heeled shoes. This heel has an area of 1cm2. How
much pressure does she exert on the floor?
Work through q2 on p58
Extension task:
Atmospheric pressure is roughly equivalent to 1kg pressing on every
square centimetre on our body. What does this equate to in
units called Pascals? (1 Pascal = 1N/m2)
Hydraulic systems
Hydraulic systems
Pressure is constant throughout this liquid
Hydraulic systems
Basically, a smaller force
on piston A will produce
a larger force on piston
B because the pressure
of the liquid is constant.
Magic!
1) If the area of piston A is 5cm2 and piston B is 20cm2 how much
will the force be multiplied by?
2) If the area of the slave piston is ten times bigger than the
master piston what force will be needed to lift an object weighing
1000N?
3) A force of 10N is used to lift a weight of 50N. What is the ratio
of the master piston area to the slave piston area?
Pressure and Volume in gases
Pressure
Volume
Pressure x
volume
Pressure and volume in gases
Conclusion
When we multiplied the pressure of a gas by its volume we
found that the answer was always __ _______.
In other words, if you DECREASE the volume you _______
the pressure and so on.
“One goes up, the other goes down”
Pressure and Volume in gases
This can be expressed using the equation:
Initial Pressure x Initial Volume = Final Press. x Final Vol.
PIVI = PFVF
1) A gas has a volume of 3m3 at a pressure of 20N/m2. What will the
pressure be if the volume is reduced to 1.5m3?
2) A gas increases in volume from 10m3 to 50m3. If the initial
pressure was 10,000N/m2 what is the new pressure?
3) A gas decreases in pressure from 100,000 Pascals to 50,000
Pascals. The final volume was 3m3. What was the initial volume?
4) The pressure of a gas changes from 100N/m2 to 20N/m2. What is
the ratio for volume change?