LOs: • Show familiarity with the idea of the mass of a body • State that weight is a force • Distinguish between mass and weight • Recall and use the equation W = mg • Recall and use the relation between force, mass and acceleration (including the direction), F = ma • Demonstrate understanding that weights (and hence masses) may be compared using a balance Supplement • Demonstrate an understanding that mass is a property that ‘resists’ change in motion • Describe, and use the concept of, weight as the effect of a gravitational field on a mass

Task Define what mass and weight are. Describe the differences between them.

Mass from youtube Eureka episode

Mass & Weight from youtube Eureka episode

Mass The mass of an object is how much matter it is composed of. It is measured in kilograms [kg]. Mass is independent of gravity. It is the same anywhere in the Universe. Mass is also known as the resistance to move. - This is called INERTIA.

Weight

The force of gravity on an object is its weight. It is measured in Newtons [N].

Weight is a vector and a force. Its force depends upon 2 things.

gravitational acceleration

mass

Weight is caused by the gravitational field of Earth on a mass. F = mg

m is mass

g is acceleration due to gravity

ie) Lorries are harder to jump start than cars. Weight is a force -> We can compare weights using a balance. Hence we can compare masses.

Everything at rest will stay at rest unless there is a force applied to it.
Everything in motion will stay in motion unless there is a force applied to it.

Newton's 2nd law

Force = mass x acceleration

The bigger the force acting on the object, the bigger the acceleration it gives to the object.

Quantity

Symbol

SI unit

force

F

The unit of weight (force) is the Newton, N

mass

m

The unit of mass is the kilogram, kg

acceleration

a

The unit of acceleration is the metres per second squared, m/s2

Figure 3.10 on your text page 31. The increasing speed of a falling ball image shows the acceleration due to the gravity acting on it.

Question A A car of mass 1000Kg moving at 15 m/s hits a barrier and stops in a time of 0.1 s.

(a). Calculate its deceleration.

(b). What force would the car experience?

Question B A hovercraft moves on a cushion of air which is trapped underneath it. The trapped air reduces friction.

(a) The hovercraft starts from rest and as it starts the propeller produces a forward force F of 22,000 N. The mass of the hovercraft is 25,000 kg. Calculate the initial acceleration of the hovercraft. You may assume there is no friction.

(b) Some time later, the hovercraft reaches a steady speed, even though the force F is unchanged. Suggest, in terms of the forces acting on the hovercraft, why the speed is now constant?

For every action, there is an equal and opposite reaction.

1.5 Forces

1.5.5 Scalars and vectors Supplement • Understand that vectors have a magnitude and direction • Demonstrate an understanding of the difference between scalars and vectors and give common examples • Add vectors by graphical representation to determine a resultant • Determine graphically the resultant of two vectors

The resultant vector must have magnitude and direction.

2 Vectors in the same direction; The resultant force is the addition of two vectors acting in the same direction.

2 Vectors in the opposite direction; The resultant force is the addition of two vectors acting in towards the bigger force's direction.

Addition of non-parallel vectors using the parallelogram method

The resultant force is represented by the diagonal of the parallelogram. It also gives us the direction of the resultant force.

Addition of non-parallel vectors using the tip-to-tail method

The resultant force is found by joining the start point of a force to the end point of another force. A triangle is thus formed and the resultant force can be measured.

1.5.1 Effects of forces LOs • Recognise that a force may produce a change in size and shape of a body • Plot extension/load graphs and describe the associated experimental procedure • Describe the ways in which a force may change the motion of a body • Find the resultant of two or more forces acting along the same line • Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line • Understand friction as the force between two surfaces which impedes motion and results in heating • Recognise air resistance as a form of friction

A force is a push or pull in a particular direction. A force causes acceleration since it changes in the speed or direction of an object. A force is measured in Newtons[[N]. Forces usually cannot be seen but their effects can. Forces usually acts in pairs. Forces can push or pull in any direction and more than one force can act on an object at a time. Ex) A force of 10N is roughly the amount of force the Earth's gravity pulls on a 1 kg mass.

EXERCISE: Fill in the blanks below 1. A force can cause a stationary object to start moving. 2. A force can cause a moving object to increase speed. 3. A force can cause a moving object to decrease speed. 4. A force can cause a moving object to change its direction of motion. 5. A force can represented by arrows.

Resultant force:

The resultant force is the single force that has the same effect as two or more forces acting on it. The combination of all the forces acting on an object is the net/resultant force.

The net force is equal to the sum of the two forces when two forces act in the same direction on an object. The net force is equal to the difference of the two forces when two forces act in opposite directions on an object.

Balanced forces;

If two forces of equal strength act on an object in opposite direction, resulting in a net force of zero and no change of motion, forces are balanced. Two or more opposite forces acting on an object and if their effects cancel each other and they do not cause a change in an object's motion is known as balanced forces.

If the effects of the forces don't cancel each other, if one force is stronger than others, the forces are unbalanced. Unbalanced forces cause a change in motion; speed or direction or both

For an object with zero acceleration, the different forces acting on it are balanced or add up to zero; the resultant(or net force) is zero. Fig 3.6 b on your text page 29.

Falling through the air

Describe a speed against time graph for a falling parachutist. Explain what the terminal velocity is. Skydiving Task: Plot a speed against time graph ofa falling parachutist in the animation.
( Graph of terminal velocity during a parachute jump from absorblearning.com)

Terminal velocity

As a skydiver jumps out of a plane she accelerates because gravity pulls her down. The air resistance increases as her speed increases and so she starts to slow down. She reaches a terminal velocity - but at this speed she would most probably splat when she hits the ground. By using a parachute, she increases her air resistance and so lower her terminal velocity so that it is safer to land.

Notice that a parachutist's weight is constant. When air resistance equals weight, the forces are balanced and the parachutist reaches a steady speed. This is called as the terminal velocity. The acceleration is upwards when she opens her parachute, then she reaches at the new terminal velocity.

Fig 3.13 and 3.14 on your text page 33.

Supplement • Describe qualitatively motion in a circular path due to a perpendicular force (F = mv 2/r is not required)

Circular motion

Any object moving along a circular path is changing direction as it goes. An object following a circular path is acted on by a force at right angles to its velocity. Fig A and B on your text page 48.

Supplement

Interpret extension/load graphs

State Hooke’s Law and recall and use the expression F = kx

Recognise the significance of the term ‘limit of proportionality’ for an extension/ load graph

Recall and use the relation between force, mass and acceleration (including the direction)

Describe qualitatively motion in a curved path due to a perpendicular force (F = mv2/r is not required)

Homework Plot a graph showing extension vs load using a spring. Add the line of the best fit and determine/explain the relationship between independent and dependent variables. Do workbook page 19 - 21.

Supplement • State Hooke’s Law and recall and use the expression F = k x, where k is the spring constant • Recognise the significance of the ‘limit of proportionality’ for an extension-load graph

Hooke's Law

The extension of a spring is directly proportional to the force applied to it, provided the elastic limit is not exceeded.

F = -kx

F is the restoring force of a spring. x is displacement from equilibrium k is spring constant [ unit: N/m ]

Point 1: Hooke's law holds only within certain limitations.
Point 2: Negative indicates the direction of the force

Example 1:
A mass m stretches a spring a distance x. How far will 5m stretch it? 5 times
Example 2:
A 300g mass stretches a spring 50cm. What is the spring constant? F = -kx, (300g x 10m/s2 ) = -k (50cm), (0.3kg x 10m/s2) = -k (0.5m), k= -6 N /m
Example 3:
How far will a force of 6 x 103N stretch a spring with spring constant 3 x 104 N /m?

Hooke’s law ( The extension of a spring is directly proportional to the force applied to it ) is valid only up to the proportionality limit of a material. Beyond this limit, Hooke’s law no longer applies. Beyond the elastic limit, the material remains deformed even when the stress is removed.

Elastic limit:
•The material no longer shows elastic behaviour (ie The spring does not return to original size when stretching force is removed)
•The material is permanently deformed ie The spring is larger or longer than originally
•The material is weaker as the above effects are caused by fracture of some atomic bonds

Deformation of materials is also critical in vehicle design though in practice the terms stress( the force per square metre ) and strain ( the extension per unit length ) are more commonly used instead of simply force and extension; stress is directly proportional to strain

LOs:1.3 Mass and weight• Show familiarity with the idea of the mass of a body

• State that weight is a force

• Distinguish between mass and weight

• Recall and use the equation W = mg

• Recall and use the relation between force, mass and acceleration (including the direction), F = ma

• Demonstrate understanding that weights (and hence masses) may be compared using a balance

Supplement• Demonstrate an understanding that mass is a property that ‘resists’ change in motion• Describe, and use the concept of, weight as the effect of a gravitational field on a massTaskDefine what mass and weight are. Describe the differences between them.MassThe mass of an object is how much matter it is composed of. It is measured in

kilograms[kg].Mass is independent of gravity. It is the same anywhere in the Universe.

Mass is also known as the resistance to move. - This is called INERTIA.

## Weight

Theofforcegravityon an object is its weight. It is measured inNewtons[N].Weight is a vector and a force. Its force depends upon 2 things.

- gravitational acceleration
- mass

Weight is caused by the gravitational field of Earth on a mass.F = mg

ie) Lorries are harder to jump start than cars.

Weight is a force -> We can compare weights using a balance. Hence we can compare masses.

The Mass vs Weight song by Mr. Edmonds YouTube

Force Gravity and Weight from BBC GCSE BITESIZE

Gravity and Weight from Newton's homepage

Planet from BBC BITESIZE

Everything at rest will stay at rest unless there is a force applied to it.Newton's 1st law (Inertia)Everything in motion will stay in motion unless there is a force applied to it.

Newton's 2nd law## Force = mass x acceleration

The bigger the force acting on the object, the bigger the acceleration it gives to the object.FmaQuestion A

A car of mass 1000Kg moving at 15 m/s hits a barrier and stops in a time of 0.1 s.

(a). Calculate its deceleration.

(b). What force would the car experience?

Question B

A hovercraft moves on a cushion of air which is trapped underneath it. The trapped air reduces friction.

(a) The hovercraft starts from rest and as it starts the propeller produces a forward force F of 22,000 N. The mass of the hovercraft is 25,000 kg.

Calculate the initial acceleration of the hovercraft. You may assume there is no friction.

(b) Some time later, the hovercraft reaches a steady speed, even though the force F is unchanged. Suggest, in terms of the forces acting on the hovercraft, why the speed is now constant?

Click the link and solve the Quizz on Force, Gravity, Mass, Weight.

For every action, there is an equal andNewton's 3rd lawoppositereaction.1.5 Forces1.5.5 Scalars and vectors

Supplement• Understand that vectors have a magnitude and direction• Demonstrate an understanding of the difference between scalars and vectors and give common examples• Add vectors by graphical representation to determine a resultant• Determine graphically the resultant of two vectorsScalar and vector quantities

The resultant vector must have magnitude and direction.Addition of parallel vectors.2 Vectors in the same direction; The resultant force is the a

dditionof two vectors acting in thesamedirection.2 Vectors in the opposite direction; The resultant force is the addition of two vectors acting in t

owardsthe bigger force's direction.

The resultant force is represented by the diagonal of the parallelogram. It also gives us the direction of the resultant force.Addition of non-parallel vectors using the parallelogram method

The resultant force is found by joining the start point of a force to the end point of another force. A triangle is thus formed and the resultant force can be measured.Addition of non-parallel vectors using the tip-to-tail method1.5.1 Effects of forces

LOs

• Recognise that a force may produce a change in size and shape of a body

• Plot extension/load graphs and describe the associated experimental procedure

• Describe the ways in which a force may change the motion of a body

• Find the resultant of two or more forces acting along the same line

• Recognise that if there is no resultant force on a body it either remains at rest or continues at constant speed in a straight line

• Understand friction as the force between two surfaces which impedes motion and results in heating

• Recognise air resistance as a form of friction

## Force:

A force is a p

ushor pullin a particular direction.A force causes a

ccelerationsince it changes in the speedor direction of an object.A force is measured in

Newtons[[N].Forces usually cannot be seen but their effects can.

Forces usually acts in pairs. Forces can push or pull in any direction and more than one force can act on an object at a time.

Ex) A force of 10N is roughly the amount of force the Earth's gravity pulls on a 1 kg mass.

EXERCISE: Fill in the blanks below

1. A force can cause a stationary object to start

moving.2. A force can cause a moving object to increase s

peed.3. A force can cause a moving object to d

ecreasespeed.4. A force can cause a moving object to

changeitsdirectionof motion.5. A force can represented by arrows.

The resultant force is the single force that has the same effect as two or more forces acting on it.Resultant force:The combination of all the forces acting on an object is the net/resultant force.

The net force is equal to the

sumof the two forces when two forces act in thesamedirection on an object.The net force is equal to the

differenceof the two forces when two forces act inoppositedirections on an object.

If two forces of equal strength act on an object in opposite direction, resulting in a net force of zero and no change of motion, forces are balanced.Balanced forces;Two or more opposite forces acting on an object and if their effects cancel each other and they do not cause a change in an object's motion is known as balanced forces.

If the effects of the forces don't cancel each other, if one force is stronger than others, the forces are un

balanced.Unbalanced forces cause a c

hangein motion; speed or direction or bothFor an object with zero acceleration, the different forces acting on it are

balancedor add up to zero; the resultant(or net force) is zero.Fig 3.6 b on your text page 29.

Describe a speed against time graph for a falling parachutist. Explain what the terminal velocity is.Falling through the airSkydiving

Task: Plot a speed against time graph ofa falling parachutist in the animation.(

of terminal velocity during a parachute jump from absorblearning.com)Graph

As a skydiver jumps out of a plane she accelerates because gravity pulls her down. The air resistance increases as her speed increases and so she starts to slow down. She reaches a terminal velocity - but at this speed she would most probably splat when she hits the ground. By using a parachute, she increases her air resistance and so lower her terminal velocity so that it is safer to land.Terminal velocityNotice that a parachutist's weight is constant. When air resistance equals weight, the forces are balanced and the parachutist reaches a steady speed. This is called as the terminal velocity. The acceleration is upwards when she opens her parachute, then she reaches at the new terminal velocity.

Fig 3.13 and 3.14 on your text page 33.Supplement• Describe qualitatively motion in a circular path due to a perpendicular force (F = mv2/r is not required)

Any object moving along a circular path is changing direction as it goes. An object following a circular path is acted on by a force at right angles to its velocity.Circular motionFig A and B on your text page 48.SupplementHomeworkPlot a graph showing extension vs load using a spring. Add the line of the best fit and determine/explain the relationship between independent and dependent variables. Do workbook page 19 - 21.

Supplement• State Hooke’s Law and recall and use the expression F = k x, where k is the spring constant• Recognise the significance of the ‘limit of proportionality’ for an extension-load graph## Hooke's Law

Theextensionof a spring is directly proportional to theforceapplied to it, provided the elastic limit is not exceeded.## F = -

kxFis therestoringforce of a spring.is displacement from equilibriumxis spring constant [ unit: N/m ]kPoint 2: Negative indicates the direction of the force

A mass m stretches a spring a distance x. How far will 5m stretch it?

5 times

Example 2:

A 300g mass stretches a spring 50cm. What is the spring constant?

F = -kx, (300g x 10m/s2 ) = -k (50cm), (0.3kg x 10m/s2) = -k (0.5m), k= -6 N /m

Example 3:

How far will a force of 6 x 103N stretch a spring with spring constant 3 x 104 N /m?

Hooke’s law ( The

extensionof a spring is directly proportional to theforceapplied to it ) is valid only up to the proportionality limit of a material. Beyond this limit, Hooke’s law no longer applies. Beyond the elastic limit, the material remains deformed even when the stress is removed.Elastic limit:•The material no longer shows elastic behaviour (ie The spring does not return to original size when stretching force is removed)

•The material is permanently deformed ie The spring is larger or longer than originally

•The material is weaker as the above effects are caused by fracture of some atomic bonds

Image from physconcepts

Deformation of materials is also critical in vehicle design though in practice the terms stress( the force per square metre ) and strain ( the extension per unit length ) are more commonly used instead of simply force and extension; stress is directly proportional to strain