Transcript IGCSE-12-Forces&Shape

EDEXCEL IGCSE PHYSICS 1-2
Forces & Shape
Edexcel IGCSE Physics pages 12 to 22
Content applying to Triple Science only is shown in
red type on the next slide and is indicated on
subsequent slides by ‘TRIPLE ONLY’
July 13th 2011
Edexcel IGCSE Specification
Section 1: Forces and motion
c) Forces, movement, shape and momentum
express a force as a push or pull of one body on another
identify various types of force (for example gravitational, electrostatic etc)
distinguish between vector and scalar quantities
appreciate the vector nature of a force
add forces that act along a line
understand that friction is a force that opposes motion
describe how extension varies with applied force for helical springs, metal
wires and rubber bands
recall that the initial linear region of a force-extension graph is associated with
Hooke’s law
associate elastic behaviour with the ability of a material to recover its original
shape after the forces causing deformation have been removed.
Red type: Triple Science Only
Force
newtonmeters
A force is a push or a pull.
A force can cause an object to:
– speed up
– slow down
– change direction
– change shape
Force is measured in
newtons (N).
Force is measured with a
newtonmeter.
Some types of force
1. Gravitational
This is the attractive force exerted
between bodies because of their
masses.
This force increases if either or both of
the masses is increased and
decreases if they are moved further
apart.
Weight is the gravitational force of the
Earth on an object.
Bathroom scales measure
weight.
A mass of 1kg weighs
about 10N
1 stone is about 63N.
2. Normal reaction or contact
This is the repulsive force that
stops two touching bodies
moving into each other.
normal reaction forces
The word ’normal’ means that
this force acts at 90° to the
surfaces of the bodies.
weight
It is caused by repulsive
molecular forces.
The two upward reaction forces
on the tyres balance the
downward weight of the car
3. Friction
This is the force that opposes motion.
The kinetic energy of the moving object is converted to
heat energy by the force of friction.
Friction is needed for racing
cars to grip the road
Friction is needed for
walking!
4. Air resistance or drag
This is the force that opposes the movement of objects
through air.
Drag is a more general term used for the opposition force
in any gas or liquid.
Objects are often streamlined to reduce this force.
streamlined car
a parachute
maximises drag force
5. Upthrust
This is the force
experienced by objects
when they are placed into a
fluid (liquid or gas).
An object will float on a
liquid if the upthrust force
equals its weight.
A hot air balloon rises when the
upthrust from the surrounding
air is greater than the balloon’s
weight.
6. Magnetic
Between magnets but
also the force that
allows electric motors to
work.
7. Electrostatic
Attractive and repulsive
forces due to bodies
being charged.
Electrostatic force causes the
girls’ hair to rise when they
touch the Van der Graaff
generator.
Choose appropriate words to fill in the gaps below:
force is a push or a pull. A force can cause an object to
A _____
accelerate
___________
or change shape.
newtons (N) with a newtonmeter.
Force is measured in _______
contact force occurs when
There are many types of force. ________
two bodies touch each other.
motion of one body
opposes the _______
Friction is a force that _______
attractive forces
relative to another. It is caused by the _________
molecules
between ___________.
WORD SELECTION:
newtons
opposes
accelerate
force
molecules contact
attractive
motion
TRIPLE ONLY
Vectors and Scalars
All physical quantities (e.g. speed and force) are
described by a magnitude and a unit.
VECTORS – also need to have their direction specified
examples: displacement, velocity, acceleration, force.
SCALARS – do not have a direction
examples: distance, speed, mass, work, energy.
TRIPLE ONLY
Representing Vectors
An arrowed straight
line is used.
The arrow indicates
the direction and the
length of the line is
proportional to the
magnitude.
Displacement 50m EAST
Displacement 25m at
45o North of East
TRIPLE ONLY
Addition of vectors 1
4N
object
4N
6N
6N
object
resultant = 10N
object
The original vectors are called COMPONENT vectors.
The final overall vector is called the RESULTANT vector.
4N
6N
6N
object
4N
object
resultant = 2N
object
TRIPLE ONLY
Resultant force
A number of forces acting on
a body may be replaced by a
single force which has the
same effect on the body as
the original forces all acting
together.
3N
2N
This overall force is called
resultant force.
In the example opposite, 5N
is the resultant force of the
3N and 2N forces.
5N
TRIPLE ONLY
Determine the resultant force in the cases below:
4N
6N
10N
1.
3.
3N
4.
2.
6N
4N
4N
7N
2N
3N
4N
1N
5.
4N
There is no resultant
4N case
force in this
TRIPLE ONLY
Resultant force and motion
Resultant force
Effect on the motion of
an object
Zero
Object’s velocity stays
the same including
staying stationary
Object accelerates
In the direction the
object is moving
In the opposite direction
in which the object is
moving
Object decelerates
TRIPLE ONLY
Examples 1 & 2
The box will move when
the man’s push force is
greater than the friction
force.
The plane will
accelerate provided that
the engine force is
greater than the drag
force.
TRIPLE ONLY
Examples 3 & 4
The brakes exert a
resultant force in the
opposite direction to
the car’s motion
causing the car to
decelerate.
Once released, the
glider moves at a near
constant velocity as it
experiences a nearly
zero horizontal
resultant force.
TRIPLE ONLY
Choose appropriate words to fill in the gaps below:
resultant force, can be used to replace
A single force, called _________
number of forces that act on a body.
a _______
zero then the body will either
If the resultant force is _____
rest or continue to move at a constant ________.
velocity
remain at _____
direction as an object’s
If the resultant force is in the same _________
motion, the object will __________.
A car is decelerated when
accelerate
opposite
the braking force acts in the _________
direction to the car’s
motion.
WORD SELECTION:
number rest direction zero opposite
velocity accelerate
resultant
Changing shape
Force can also change the shape
of an object.
A stretching force puts an object
such as a wire or spring under
tension.
A squashing force puts an object
under compression.
Brittle materials such as
glass do not change shape
easily and break before
noticeably stretching.
Resilient materials do not
break easily.
Elastic materials return to
their original shape when
the forces on them are
removed.
Plastic materials retain their
new shape.
Stretching Springs
Experimental procedure:
1. Place the weight holder only on
the spring and note the position
of the pin against the metre rule.
2. Add 1N (100g) to the holder
and note the new position of the
pin.
3. Calculate the extension of the
spring.
4. Repeat stages 1 to 3 for 2N,
3N, 4N, 5N and 6N. DO NOT
EXCEED 6N.
spring
weights
pin
metre rule
Typical results
Pin position
with holder
only (mm)
Added weight
or Force (N)
Pin position
with weight
(mm)
Extension
(mm)
450
1
480
30
450
2
509
59
450
3
541
91
450
4
570
120
450
5
601
151
450
6
629
179
Force (N)
Force against extension graph
0
0
Extension (mm)
Hooke’s law
Hooke’s law states that the extension of a
spring force is proportional to the force used to
stretch the spring.
‘Proportional’ means that if the force is doubled
then the extension also doubles.
The line on a graph of force against extension will
be a straight AND go through the origin.
Question
A spring of original length 150mm is extended by
30mm by a force of 4N. Calculate the length of
the spring if a force of 12N is applied.
12N is three times 4N
Therefore the new extension should be 3 x 30mm
= 90mm
New spring length = 150mm + 90mm
= 240mm
Elastic limit
Up to a certain extension if the force is
removed the spring will return to its
original length. The spring is behaving
elastically.
If this critical extension is exceeded,
known as the elastic limit, the spring
will be permanently stretched.
Hooke’s law is no longer obeyed by
the spring if its elastic limit is
exceeded.
The right hand
spring has been
stretched beyond
its elastic limit
Force (N)
Force against extension graph if the
elastic limit is exceeded
elastic limit
0
0
Extension (mm)
Force
Stretching an elastic band
An elastic band
does not obey
Hooke’s law.
0
0
Extension
Choose appropriate words to fill in the gaps below:
stretched the
Hooke’s law states that when a wire or spring is _________
extension is proportional to the load
increase in length or _________
force applied.
______
elastic
This law is not obeyed if the spring is taken beyond its ______
permanently
limit after which it will become _____________
stretched.
A ________
rubber band does not obey Hooke’s law.
A graph illustrating Hooke’s law will have a line that is
straight
origin
___________
and passes through the _______.
WORD SELECTION:
stretched elastic permanently extension
origin force rubber straight
Online Simulations
Effect of forces on motion using a space module Freezeray.com
Force combination balloon game - eChalk
Electric & Magnetic Forces - 'Whys Guy' Video
Clip (3:30mins) - Shows Charged Balloon &
Effect of a magnet on a TV screen.
Resultant of two forces - Fendt
Forces on objects immersed in liquids - NTNU
BBC KS3 Bitesize Revision:
What is a force
Balanced forces
Unbalanced forces
BBC AQA GCSE Bitesize Revision:
Resultant force
Types of forces
Vector Addition - PhET - Learn how to add vectors.
Drag vectors onto a graph, change their length
and angle, and sum them together. The
magnitude, angle, and components of each vector
can be displayed in several formats.
Representing vectors - eChalk
Vectors & Scalars - eChalk
Vector addition - eChalk
Vector Chains - eChalk
Fifty-Fifty Game on Vectors & Scalars - by KT Microsoft WORD
Vector addition - Explore Science
Stretching Springs - PhET - A realistic mass and
spring laboratory. Hang masses from springs and
adjust the spring stiffness and damping. You can
even slow time. Transport the lab to different
planets. A chart shows the kinetic, potential, and
thermal energy for each spring.
TRIPLE ONLY
Forces & Shape
Notes questions from pages 4 and 12 to 22
1.
2.
3.
4.
5.
6.
7.
(a) What is force? (b) Explain the meaning of the following types of
force: gravitational, normal reaction, drag, electrostatic and friction.
(see pages 12 to 17)
Explain the difference between vectors and scalars quantities and
give two examples of each. (see pages 4 and 13)
State what is meant by Hooke’s law and explain how a graph can
be drawn to verify that a spring obeys this law.
What is meant by ‘elastic limit’?
Sketch a graph showing how the loading force varies with
extension when extending an elastic band.
Answer the questions on pages 21 & 22.
Verify that you can do all of the items listed in the end of chapter
checklist on page 21
DOUBLE ONLY
Forces & Shape
Notes questions from pages 12 to 22
1.
2.
3.
4.
5.
(a) What is force? (b) Explain the meaning of the
following types of force: gravitational, normal reaction,
drag, electrostatic and friction. (see pages 12 to 17)
State what is meant by Hooke’s law and explain how a
graph can be drawn to verify that a spring obeys this
law.
What is meant by ‘elastic limit’?
Sketch a graph showing how the loading force varies
with extension when extending an elastic band.
Answer questions 1, 2, 6 and 9 on pages 21 & 22.