You are on page 1of 34

# EDEXCEL IGCSE / CERTIFICATE IN 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’ December 4th 2012

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 Hooke’s 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

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. The word ’normal’ means that this force acts at 90° to the surfaces of the bodies.

normal reaction forces

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 attractive motion

molecules contact

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

4N object 6N 4N

object

6N resultant = 10N

object The original vectors are called COMPONENT vectors. The final overall vector is called the RESULTANT vector. 4N object 6N object 6N 4N

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

2N

TRIPLE ONLY

Determine the resultant force in the cases below:
1. 4N 6N 10N 2.

3N

4N 1N

3.

3N

4.

6N 4N

5. 4N

4N 7N
2N

There is no resultant 4N case force in this

TRIPLE ONLY

Resultant force and motion
Resultant force Zero Effect on the motion of an object Object’s velocity stays the same including staying stationary Object accelerates Object decelerates

In the direction the object is moving In the opposite direction in which the object is moving

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 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
450 450

2
3 4

509
541 570

59
91 120

450
450

5
6

601
629

151
179

Force against extension graph
Force (N) 0 0

Extension (mm)

Hooke’s law
Hooke’s 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.

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 against extension graph if the elastic limit is exceeded
Force (N)

elastic limit

0 0

Extension (mm)

Stretching an elastic band
Force

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. (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

4. 5.
6. 7.

DOUBLE ONLY

Forces & Shape
Notes questions from pages 12 to 22 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) 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.

2.

3. 4.

5.
{"found_math": false, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 0, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0, "katex": 0, "math-container": 0, "wp-katex-eq": 0, "align": 0, "equation": 0, "x-ck12": 0, "texerror": 0}