EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 1-2 Forces

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EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 1-2Forces
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
December 4th 2012
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Edexcel Specification

• Section 1 Forces and motion
• c) Forces, movement, shape and momentum
• describe the effects of forces between bodies
  such as changes in speed, shape or direction
• identify different types of force such as
  gravitational or electrostatic
• distinguish between vector and scalar quantities
• understand that force is a vector quantity
• find the resultant force of forces that act along
  a line
• understand that friction is a force that opposes
  motion
• describe experiments to investigate how extension
  varies with applied force for helical springs,
  metal wires and rubber bands
• understand that the initial linear region of a
  force-extension graph is associated with Hookes
  law
• describe elastic behaviour as the ability of a
  material to recover its original shape after the
  forces causing deformation have been removed.

Red type Triple Science Only
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Force

• 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.

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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.

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• 2. Normal reaction or contact
• This is the repulsive force that stops two
  touching bodies moving into each other.
• The word normal means that this force acts at
  90 to the surfaces of the bodies.
• It is caused by repulsive molecular forces.

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• 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.

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• 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.

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• 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.

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• 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.

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Choose appropriate words to fill in the gaps
below A _____ is a push or a pull. A force can
cause an object to ___________ or change
shape. Force is measured in _______ (N) with a
newtonmeter. There are many types of force.
________ force occurs when two bodies touch each
other. Friction is a force that _______ the
_______ of one body relative to another. It is
caused by the _________ forces between
___________.
force
accelerate
newtons
contact
motion
opposes
attractive
molecules
WORD SELECTION
attractive
newtons
force
accelerate
motion
contact
opposes
molecules
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Vectors and Scalars
TRIPLE ONLY

• 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.

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Representing Vectors
TRIPLE ONLY

• An arrowed straight line is used.
• The arrow indicates the direction and the length
  of the line is proportional to the magnitude.

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Addition of vectors
TRIPLE ONLY
The original vectors are called COMPONENT
vectors. The final overall vector is called the
RESULTANT vector.
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Resultant force
TRIPLE ONLY

• 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.
• This overall force is called resultant force.
• In the example opposite, 5N is the resultant
  force of the 3N and 2N forces.

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Determine the resultant force in the cases below
TRIPLE ONLY
3.
4.
5.
3N
7N
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Resultant force and motion
TRIPLE ONLY
Resultant force Effect on the motion of an object
Zero Objects velocity stays the same including staying stationary
In the direction the object is moving Object accelerates
In the opposite direction in which the object is moving Object decelerates
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Examples 1 2
TRIPLE ONLY

• The box will move when the mans push force is
  greater than the friction force.

• The plane will accelerate provided that the
  engine force is greater than the drag force.

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Examples 3 4
TRIPLE ONLY

• The brakes exert a resultant force in the
  opposite direction to the cars 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.

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TRIPLE ONLY
Choose appropriate words to fill in the gaps
below A single force, called _________ force,
can be used to replace a _______ of forces that
act on a body. If the resultant force is _____
then the body will either remain at _____ or
continue to move at a constant ________. If the
resultant force is in the same _________ as an
objects motion, the object will __________. A
car is decelerated when the braking force acts in
the _________ direction to the cars motion.
resultant
number
zero
velocity
rest
direction
accelerate
opposite
WORD SELECTION
rest
direction
number
zero
opposite
accelerate
velocity
resultant
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Changing shape

• Force can 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.

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• Brittle materials such as glass do not change
  shape easily and break before noticeably
  stretching.
• Resilient materials do not break easily.

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• Elastic materials return to their original shape
  when the forces on them are removed.
• Plastic materials retain their new shape.

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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.

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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
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Force against extension graph
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Hookes law

• Hookes law states that the extension of a spring
  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.

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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

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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.
• Hookes law is no longer obeyed by the spring if
  its elastic limit is exceeded.

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Force against extension graph if the elastic
limit is exceeded
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Stretching an elastic band
An elastic band does not obey Hookes law.
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Choose appropriate words to fill in the gaps
below Hookes law states that when a wire or
spring is _________ the increase in length or
_________ is proportional to the load ______
applied. This law is not obeyed if the spring is
taken beyond its ______ limit after which it will
become _____________ stretched. A ________ band
does not obey Hookes law. A graph illustrating
Hookes law will have a line that is ___________
and passes through the _______.
stretched
extension
force
elastic
permanently
rubber
straight
origin
WORD SELECTION
elastic
stretched
extension
permanently
straight
origin
rubber
force
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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 (330mins) - 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.
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Forces ShapeNotes questions from pages 4 and
12 to 22
TRIPLE ONLY

1. (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)
2. Explain the difference between vectors and
   scalars quantities and give two examples of each.
   (see pages 4 and 13)
3. State what is meant by Hookes law and explain
   how a graph can be drawn to verify that a spring
   obeys this law.
4. What is meant by elastic limit?
5. Sketch a graph showing how the loading force
   varies with extension when extending an elastic
   band.
6. Answer the questions on pages 21 22.
7. Verify that you can do all of the items listed in
   the end of chapter checklist on page 21

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Forces ShapeNotes questions from pages 12 to 22
DOUBLE ONLY

1. (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)
2. State what is meant by Hookes law and explain
   how a graph can be drawn to verify that a spring
   obeys this law.
3. What is meant by elastic limit?
4. Sketch a graph showing how the loading force
   varies with extension when extending an elastic
   band.
5. Answer questions 1, 2, 6 and 9 on pages 21 22.