Transcript P4: Explaining Motion

P4: Explaining Motion
Linking together
forces, energy and
motion
Speed:
• How quickly an object
covers a set distance
• Speed (m/s) = distance (m)
time (s)
Instantaneous
speed… speed at a given
point
Average speed…
d
txs
dirty toes smell!
distance of whole journey
divided by total time takenwhich takes into account
slowing down and speeding
up.
Showing speeds with distance time
graphs
Distance/
metres
Graph for a 100m sprint:
Can you find where the athlete…
1. Slows down
2. Speeds up
3. Moves at a constant
speed?
Copy the graph then
show a much faster
runner’s graph on the
same axes
Time/ seconds
Key features:
• Flat = stationary
• Straight diagonal
= constant speed
• Increasingly
steeper curve =
speeding up
• Decreasingly
steep curve =
slowing down
Gradient = speed
Finding the speed using the
gradient of distance- time graphs
Distance/
metres
Change in y/ change in x = speed
(Gradient)
40/2 = 20 m/s
70
Change in distance=70-30= 40 m
30
Time for this change to occur= 6-4=2 s
4
6
Time/ seconds
Velocity:
• Simply the speed of an object at a point of
time PLUS the direction in which it travels
(vector quantity)
• Units are the same as speed: m/s
The cheetah’s speed for the overall
chase was 85 km/ h;
its’ velocity reached 113 km/ h
running north after 10.4 seconds.
If the cheetah then ran south at the same
speed the velocity would be negative
e.g. -113 km/ h running south
Showing acceleration with velocitytime graphs
• Acceleration is a change in the velocity
(either speeding up OR slowing down OR changing direction)
Is the roller coaster
accelerating?
Velocity
(m/ s west)
Constant
Velocity
(level)
Low positive
acceleration
High
negative
acceleration
High positive
Acceleration
(steep)
Time taken (s)
Where would you show a stationary object on this graph?
Forces
• Newton’s 3rd Law: “For every action there
will be an equal but opposite reaction”
• Rephrased…when one object exerts a
force on another, it always experiences a
force in return
Describe and
explain what
happens when
the balloon
deflates
If the person jumps to shore
what happens to the boat?
Examples…In which direction will be the reaction force?
Wings push downwards;
Weight of cork pulls
towards
the centre of the
Earth
Feet push backwards
The magnet attracts the Iron objects
(add both arrows)
stationary
Interactions at
surfaces
moving
The Sumo wrestler exerts a force
downwards which deforms the
floor surface slightly causing an
equal force upwards (the reaction
of the surface)
For two surfaces sliding past each
other…each surface experiences a
force in the direction which prevents
(or tends to prevent) relative
movement; this interaction is called
friction… in the case of the bear not
very much!
Resultant forces
• The overall effect of all
the forces acting on an
object taking into account
their sizes and directions.
In flight there are 4 main forces to consider!
Which force must be largest to slow the plane down?
e.g. For a pendulum
swinging back…
Momentum
• If there is a resultant force then there will be a change of
momentum in the direction of the force
• Momentum (kg m/s) = mass (kg) x velocity (m/s)
M
mxv
Mom mashes vegetables
• The change in momentum varies according to the resultant force
and the time this force acts
• Change in (kg m/s) = resultant force (N) x time force acts (s)
momentum
Draw your own revision triangle
for this equation and make up
your own mnemonic
Changing momentum
• If there is no change in momentum the object stays at
rest OR if moving stays at a constant speed in a
straight line
• During collisions a big change in momentum occurs but
since…
Change in (kg m/s) = resultant force (N) x time force
momentum
acts (s)
If we increase the time over which the force acts then the
resultant force will be smaller (the change in
momentum is unchanged!)
• This is the principle used in crash helmets, air bags,
seat belts, climbing ropes and crumple zones on cars
Driving and counter forces
Driving force
Counter force
(complete this section for
each one)
Boat engine
Cyclists’ legs pedalling
Table tennis bat hitting ping
pong ball
Using the car
diagram below
show the force
arrows for when
the car is
(a) speeding up
(b) slowing down
and
(c) at a constant
speed
Wind blowing a sail on a
yacht
Air resistance
Counter forces
friction
Thrust from
engine:
driving force
Changing Energy
Work done (J) = force (N) x distance (m)
• Work is being done on the rock
opposite so the energy of the rock
increases
• Work is being done by the person so
the energy of the person decreases
Change in energy (J) = work done (J)
• The falling rock will gain kinetic
energy and lose gravitational potential
energy
Gravitational Potential Energy
Change in
GPE (J)
= weight (N) x vertical height
difference (m)
Show all the forces
acting on the man
(left) and make the
resultant force
clear.
Calculate the
change in GPE
800N
man
2.5 m fall
In which position does the
baseball have the greatest
potential energy?
Kinetic Energy
• Value depends on the speed of an object and
the mass
• Speed has a much greater effect than mass
KE (J) = ½ mass (kg) x velocity2 ((m/s)2)
A force which causes the object to speed up
increases the kinetic energy.
Use this statement to explain why the speed limit
in towns is 30 mph {“It’s 30 for a reason!”}
Conservation of energy
(A) Kinetic
energy 100
kJ (useful)
Energy input will equal the
energy output BUT the gain
in kinetic energy of an object
will be less than the work
done on the object because
of the forces of friction and
air resistance.
Energy input
from fuel 150kJ
(B) Heat and sound
energy dissipated due
to friction and air
resistance (wasted)
50 kJ