Transcript newtons laws

Dynamics and Space
Newton’s laws
Learning Outcomes
• Applications of Newton’s laws and balanced forces to explain
constant velocity, making reference to frictional forces.
• Calculations involving the relationship between unbalanced force,
mass and acceleration for situations where more than one force
is acting.
• Calculations involving the relationship between work done,
unbalanced force and distance / displacement.
• Calculations involving the relationship between weight, mass and
gravitational field strength during interplanetary rocket flight.
• Newton’s second law and it’s application to space travel including
rocket launch and landing.
• Newton’s third law and its application to explain motion resulting
from a ‘reaction force’.
• Use of Newton’s laws to explain free-fall and terminal velocity.
Lesson 1
• Define the term friction and give
examples of how to increase and
decrease frictional forces.
The Force of Friction
• A force of friction is a force acting
between two surfaces in contact which
opposes the motion of the surfaces (i.e.
tries to stop them moving).
Reducing Friction
• As friction always stops things moving
it is important to reduce it if we want
to move things.
• We are now going to look at ways of
reducing friction.
Demonstrations
Reducing Friction
(copy out note or make your own mindmap)
•
1.
2.
3.
4.
There are four main ways in which we can reduce
friction:
Streamlining – changing the shape to reduce air
resistance e.g. a car with a ‘flatter shape’
Rollers – adding rollers or wheels e.g. a trolley with
wheels moves easier than one without.
Lubrication – adding a layer of liquid or air between
two surfaces e.g. oiling hinges to stop them
‘squeaking’.
Smoothing – making a surface smoother e.g. adding
wax to skis
Increasing Friction
• Friction can also be useful to us:
1. When a car applies its brakes it
increases friction to slow down.
2. Parachutes are used to increase air
resistance to slow things down.
Highest Skydive EVER
• 2nd Highest Parachute Jump
Homework
• Research a machine or animal that has
been specially adapted to become more
streamlined.
• Present your findings in one of the
following ways:
– a short essay in your homework jotter
– an information leaflet
– A poster
Lesson 2
• Define the term friction and give
examples of how to increase and
decrease frictional forces.
Balanced Forces
• Today we are going to look at the effect
of two opposite but equal forces acting
on an object.
• Tug of war
Balanced Forces
• Balanced forces are equal forces acting in opposite
directions.
• They are equivalent to no force at all acting on an
object.
• When an object is stationary, or travelling at a
constant velocity, all forces on it are balanced.
• (HINT: this is often an exam question – they will tell
you an object is travelling at constant velocity with a
thrust force of say 100N. They will then ask you what
the force of friction would be. This would be the
same as the thrust, in this case 100N.)
Balanced Forces: Motion
• When a car is moving at constant speed, the
forces acting on it are said to be balanced.
• In the car below, force of engine = friction
Other examples include:
Plane
• Note there are also balanced vertical
forces, lift = weight.
Other examples include:
Boat
• Upthrust = weight
Newton’s First Law of Motion
• Newton’s first law states that if an
object is stationary or moving at a
constant speed then the forces acting
on it are balanced.
Newton’s First Law in action
• Newton’s first law shows us that when
you move at constant speed it has the
same effect as not moving at all.
• You only feel a force if you are speeding
up (accelerating) or slowing down
(decelerating).
In an airplane
• As the plane takes off you are pushed
back as the plane speeds up.
• The forces are not balanced.
In an airplane
• As the plane lands you are pushed
forward as the plane applies its brakes
and slows down.
• The forces are not balanced.
In an airplane
• When in mid-air, going at a constant speed and a
constant altitude, without any turbulence you would
feel exactly as you would if you were on the ground
• The forces are balanced. You can do anything that
you could do if you were on land.
• Harlem Shake Frontier Flight 157
What about now?
• Even now as you sit in
the class (in the UK) you
are actually travelling at
around 1000 km/h (622
mph) as the Earth
rotates on its axis.
• You don’t feel this
effect as it is a constant
speed.
• (Earth travels at around
1600 km/h at the
Equator).
Motion of a spaceship
• The motion of a spaceship is
consistent with Newton’s First Law.
• Space is a vacuum so there are no
frictional forces acting.
• The ship continues moving at the same
speed in the same direction until it
comes under the gravitational
influence of a planet.
Homework - podcast
• Using iTunes, search for the free
podcasts called “stuff you should know”
(from howstuffworks.com).
• Find the episode from 21 Feb 2013 on
‘What would happen if the Earth
stopped spinning’.
• This will be discussed in a future lesson.
Lesson 3
(usually two periods)
• Investigate the relationship between
unbalanced force, mass and
acceleration.
• Carry out calculations involving the
relationship between unbalanced force,
mass and acceleration for situations
where more than one force is acting.
Newton’s Second Law of Motion
• Newton’s first law of motion looked at
how balanced forces affected the
velocity of an object.
• We are now going to look at how
unbalanced forces change the velocity
of an object e.g. acceleration.
Newton’s Second Law of Motion
experiment 1
•
•
A light gate is used to measure acceleration of a trolley.
Hanging masses are used for the unbalanced forces.
•
Note: the mass of the trolley is kept constant. Only the masses on the hanger
are changed causing and unbalanced force (due to gravity acting on the masses).
Results
Force (N)
1
2
3
4
5
6
Acceleration (m/s2)
Newton’s Second Law of Motion
Conclusion: As the Force on a body
increases so does the acceleration.
In other words
a~F
Newton’s Second Law of Motion
• We are now going to investigate the
effect of changing the mass of an
object being accelerated by an
unbalanced force.
Newton’s Second Law of Motion
experiment 2
•
•
A light gate is used to measure acceleration of a trolley.
The hanging mass is kept constant therefore there is an unbalanced force.
•
Note: only the mass of the trolley is changed.
Results
Mass (kg)
0.2
0.4
0.6
a (m/s2)
Newton’s Second Law of Motion
Conclusion: As the mass of a body
increases the acceleration decreases.
In other words
a~ 1
m
Defining Newton’s Second Law of
Motion
• Newton’s Second Law of Motion states that
the acceleration of an object is directly
proportional to the unbalanced force acting on
it,
• e.g.
a ~ F
• and inversely proportional to the mass of an
object.
• e.g.
a ~ 1
m
Newton’s Second Law of
Motion
• Combining the two proportions
• a ~ F and a ~ 1
m
• Gives us the following equation:
a=F
m
• This is re-written as:
Fun = ma
• Also known as Newton’s second Law
Problems involving several forces
1.
In all situations apply: Fun = ma (work out the
resultant unbalanced force)
2. Draw a sketch diagram for the whole system
including masses and external forces.
3. Indicate the direction of acceleration on
the object.
Example 1
• What is the mass
of an object if an
unbalanced force
of 20 N produces
an acceleration of
4 m/s2?
Example 2
• What is the
acceleration of a
600 kg car, when
the engine exerts
a force of 1700 N,
but the frictional
force is 800 N?
Example 3
• A 6 kg block is
dragged along with a
horizontal force of
16 N.
• If the block
accelerates at
2 m/s2, what is the
force of friction
acting on the block?
Lesson 4
• Carry out calculations involving the
relationship between work done,
unbalanced force and distance /
displacement.
Work Done
• Work done is a measure of the energy
transferred.
• It has the symbol Ew and is measured in
Joules (J).
• The work done to any object depends on
the “size of the force” being applied and
the “distance” it is being moved along.
Work done = Force x distance
Ew = F d
Example 1
Ew = ?
F = 600 N
d=4m
Example 2
Ew = 15 MJ
F=?
d = 50 km
Example 3
Ew = 6000 J
F = 25 N
d=?
2010
2008 Qu: 23
Lesson 5
• Describe the difference between mass and
weight.
• Investigate the relationship between mass
and weight.
• State what is meant by gravitational field
strength.
• Carry out calculations involving the
relationship between weight, mass and
gravitational field strength during
interplanetary rocket flight.
Mass and Weight
• The mass of an object is the quantity
of matter it contains.
• It is measured in kilograms (kg).
• The weight of an object is a force
caused by the pull of the Earth on an
object.
• It is measured in Newtons (N).
What to do
• Collect a 1-10N / 1-15N / 1-20N Newton
balance and a set of slotted 100g masses.
• Copy & complete the table below:
Mass (kg)
0.2
0.4
0.6
Your mass =
Weight (N)
Ratio of weight
mass
Relationship Between Mass and
Weight
• We now have an equation linking weight,
mass and g:
• Weight = g
mass
• This can be re-written as:
• weight = mass x g. Or in symbol form:
W = m g
What is meant by gravitational field
strength?
• The ratio of weight / mass is known as
the gravitational field strength.
• This is represented by the symbol ‘g’.
• The value of g on Earth is usually taken
as 10 N/kg.
• g is also known as the ‘weight per unit
mass’.
Example
• Copy and complete the table:
Object on
Earth
(g=10N/kg)
Brick
Concrete
block
Bag of
cement
Tonne of
sand
Mass (kg)
Weight (N)
3
100
500
1000
Gravitational Field Strength
• The value of ‘g’ varies as we move from one body to
another in our solar system.
• The bigger the body, the bigger the value of g.
• Our mass would remain the same and never change
if we went to these different bodies, however, our
weight does.
• We can work out our weight (N) on different bodies
using the equation W = mg as long as we have a value
for ‘g’ for that body.
Body
g (N/kg)
Mercury
Venus
Earth
4
9
10
Mars
Jupiter
Saturn
4
26
11
Uranus
Neptune
Pluto
Moon
12
12
4
1.6
Sun
270
Your Weight in N
(W = mg)
Lesson 6
• Use Newton’s second law and apply it to
space travel including rocket launch and
landing.
Newton’s 2nd Law (Fun = ma) and
space travel
•
When using Fun = ma in space travel we
follow the same rules as before:
1. In all situations apply: Fun = ma (work out the
resultant unbalanced force)
2. Draw a sketch diagram for the whole system
including masses and external forces.
3. Indicate the direction of acceleration on
the object.
 However, we now must consider gravity and
the force of weight. In other words, apply
W = mg acting down on the object
Example
• What is the
acceleration of a 700
kg helicopter, when
the engine exerts a
force of 9900 N
vertically upwards,
but the frictional
force is 800 N?
• Fun = Fup – Ffr – W
• W = mg = 700 x 10 =
7000N
• Fun = 9900 – 800 -7000
= 2000 N
• a = Fun
m
= 2100
700
= 3 m/s2
2006
2003 Qu: 22 (1 of 2)
2003 Qu: 22 (2 of 2)
Lesson 7
• State Newton’s third law.
• Apply Newton’s third law to explain
motion resulting from a ‘reaction force’.
• Use of Newton’s laws to explain freefall and terminal velocity.
Newton’s Third Law
•
•
If object A exerts a
force on object B then
object B exerts an equal
but opposite force on
object A.
In other words:
‘action and reaction are
equal and opposite’.
Lesson 7
Lesson 7
Balanced Forces: Parachute jump
• When a skydiver jumps from a plane he
will only accelerate for a short while.
• The air friction rapidly increases until
it equals the skydiver’s weight.
• The skydiver will then fall with a
uniform velocity called terminal
velocity.
Balanced Forces: Parachute jump
• Terminal velocity is the constant
velocity reached by an object once it is
no longer accelerating.
Sky diving
• What not to do!
2004