Basic definitions
Steps Newton's First Law (NI)
must have a resultant
If an object changes velocity (or direction) then it
constant
1. A bodywill remain at rest or at a force acting on it. If an object is stationary or moving at a
constant velocity velocity the forces must be balanced.
2. Providing that there is no So if an object is at rest and begins to move, it has changing in velocity
and so according to NI it must have resultant force acting on it.
resultant force acting on it.
If an object is moving at a constant speed (not changing direction), then
according to NI, the forces are balanced.
Steps Newton's Second Law (N2)
This is very similar to NI. Use this to explain why the magnitude of
2
1. The acceleration of an object is acceleration might change. For example a body is accelerating at 2ms
proportional to the resultant and begins to accelerate at 3ms-2. According to NI there must be a
force acting on it. resultant force as there is an acceleration. According to N2 the resultant
2. F = ma force may have increased as the acceleration has increased. Could
changing the mass and leaving the force constant have the same effe0
Steps Newton's Third Law (N3)
Make sure you spend time on the Newton's Third Law section of this
1. When BodyA exerts a force on pack. This is a very tricky one.
Body B, Body B exerts an equal In a question always replace Body A and Body B with the
and opposite force on Body A. objects/gas/liquid in the question.
Steps Principle of conservation of momentum
An external force is any force which does not come from the two bodies
1. The total momentum before an
So for example a force exerted by Body A on Body B is fine, but air
event equals the total
resistance or friction are not. Space is therefore a good example of wher
momentum after an event
conservation of momentum does apply as there is no friction.
2. provided that there are no
external forces When doing a calculation using conservation of momentum, your
assumption is that there are no external forces acting.
Steps Principle of moments
1. The sum of the clockwise For the beam as long as the two
moments equals the sum of the clockwise moments add up to the one anticlockwise moments the bean
anticlockwise moments will be in equilibrium. If the clockwise moments don't equal the
2. If the system is in equilibrium anticlockwise moments then the beam won't be in equilibrium and will
start tip (rotate).
,Newton's Third Law
How do I know this is what the question's asking me about?
You see the key words-
Pairs of Forces,Two bodies interacting.
Steps Theory
When Body A exerts a force N3 is the easiest of Newton's Laws to get wrong (l always do if I
on Body B, Body B exerts an don't apply the steps). At the time of writing even GCSE bitesize
equal and opposite force on page has it wrong (have a look after completing these
Body A. questions). So tread carefully!
Newton's third law applies if- Learn the five criteria-
1. Forces are of the same 1. The most obvious- the forces are the same size
magnitude 2. Opposite is a key word. Not different directions- opposite.
2. Forces act in opposite 3. That means if one force is a contact force, the other has to
directions be a contact force. If one is a gravitational force, the other
3. Forces are of the same is also a gravitational force.
kind 4. Anothertricky one. An object may have two forces acting
4. Forces act on different on it, but as they're acting on the same
body, they are not
bodies a N3 pair.
5. Forces arise in pairs 5. If you have one force there must
be a corresponding one
on a different body.
Application to an exam question- (first read the question
and try and do the steps)
For question one the correct answer is B, for question 2
the answer is D.
For question 3 you already have the definition for
Newton's First Law.
For 3b) looking at the checklist, the two which are the
same are magnitude
which are different are opposite direction, and act and type, and the ones
on different bodies.
For b)iii) think about what needs to change in the
sentence and what
should stay the same. The
The car exert and upward gravitational force
of 1200N on the
Earth,
, apply steps 1 to 5-
Steps Read the question and try to
When Body A exerts a force on Body
B, Body B exerts an equal and
opposite force on Body A.
Newton's third law applies if-
1. Forces are of the same
magnitude
2. Forces act in opposite directions
3. Forces are of the same kind
4. Forces act on different bodies
5. Forces arise in pairs
Application to an exam question- (first
read the question and try and do the
steps)
First we need to state Newton's Third
Law, this is simply- "When Body A
exerts a force on Body B, Body B exerts
an equal and opposite force on Body
The rest is applying the five steps and
seeing whether or not they apply.
1. Both forces are 300N so forces
are of the same magnitude
2. One force is forward, and the
other is backward, so they act
in opposite directions,
3. Both forces are frictional and so
are of the same kind.
4, One force is from the tyre on
the road, and the other is from
the road on the tyre, so their
acting on different bodies
5. And the forces have arisen as a
pair
And then as their all true we should
write a final conclusion
, try to apply steps 1 to 5-
Read the question and
Steps
When Body A exerts a force on Body B,
Body B exerts an equal and opposite force
on Body A.
Newton's third law applies if-
1. Forces are of the same magnitude
2. Forces act in opposite directions
3. Forces are of the same kind
4. Forces act on different bodies
5. Forces arise in pairs
Correct the work of a professional
Countless people have fooled themselves into
thinking they understand N3, including the
author of this article who I'm sure is
otherwise very competent, don't be among
them! Apply the five steps very carefully to
each scenario before reading on, otherwise
you'll fool yourself into thinking you could
have always get to right answer without
reading this.
For the first book, steps 1, 2, 3 and 5 all
(appear) to apply to the situation, but 4 does
not. The forces are acting on the same body.
Can you think of a N3 law pair for this
scenario? If you were pushing the book with
your hand then the N3 pair would be the
contact force exerted by your hand on the
book, and the contact force exerted by the
book on your hand,
For the second book example, steps 1, 2, and
5 all (appear) to apply but 3 and 4 do not.
Weight is a gravitational force, but the
reaction force is a contact force- they are not
of the same kind. And again, they are not
acting on different bodies, Think of an actual
N3 pair for this situation? The earth pulls the
book downwards with a gravitational force,
so the book must pull the earth upwards with
an equal (and opposite) gravitational force.
Steps Newton's First Law (NI)
must have a resultant
If an object changes velocity (or direction) then it
constant
1. A bodywill remain at rest or at a force acting on it. If an object is stationary or moving at a
constant velocity velocity the forces must be balanced.
2. Providing that there is no So if an object is at rest and begins to move, it has changing in velocity
and so according to NI it must have resultant force acting on it.
resultant force acting on it.
If an object is moving at a constant speed (not changing direction), then
according to NI, the forces are balanced.
Steps Newton's Second Law (N2)
This is very similar to NI. Use this to explain why the magnitude of
2
1. The acceleration of an object is acceleration might change. For example a body is accelerating at 2ms
proportional to the resultant and begins to accelerate at 3ms-2. According to NI there must be a
force acting on it. resultant force as there is an acceleration. According to N2 the resultant
2. F = ma force may have increased as the acceleration has increased. Could
changing the mass and leaving the force constant have the same effe0
Steps Newton's Third Law (N3)
Make sure you spend time on the Newton's Third Law section of this
1. When BodyA exerts a force on pack. This is a very tricky one.
Body B, Body B exerts an equal In a question always replace Body A and Body B with the
and opposite force on Body A. objects/gas/liquid in the question.
Steps Principle of conservation of momentum
An external force is any force which does not come from the two bodies
1. The total momentum before an
So for example a force exerted by Body A on Body B is fine, but air
event equals the total
resistance or friction are not. Space is therefore a good example of wher
momentum after an event
conservation of momentum does apply as there is no friction.
2. provided that there are no
external forces When doing a calculation using conservation of momentum, your
assumption is that there are no external forces acting.
Steps Principle of moments
1. The sum of the clockwise For the beam as long as the two
moments equals the sum of the clockwise moments add up to the one anticlockwise moments the bean
anticlockwise moments will be in equilibrium. If the clockwise moments don't equal the
2. If the system is in equilibrium anticlockwise moments then the beam won't be in equilibrium and will
start tip (rotate).
,Newton's Third Law
How do I know this is what the question's asking me about?
You see the key words-
Pairs of Forces,Two bodies interacting.
Steps Theory
When Body A exerts a force N3 is the easiest of Newton's Laws to get wrong (l always do if I
on Body B, Body B exerts an don't apply the steps). At the time of writing even GCSE bitesize
equal and opposite force on page has it wrong (have a look after completing these
Body A. questions). So tread carefully!
Newton's third law applies if- Learn the five criteria-
1. Forces are of the same 1. The most obvious- the forces are the same size
magnitude 2. Opposite is a key word. Not different directions- opposite.
2. Forces act in opposite 3. That means if one force is a contact force, the other has to
directions be a contact force. If one is a gravitational force, the other
3. Forces are of the same is also a gravitational force.
kind 4. Anothertricky one. An object may have two forces acting
4. Forces act on different on it, but as they're acting on the same
body, they are not
bodies a N3 pair.
5. Forces arise in pairs 5. If you have one force there must
be a corresponding one
on a different body.
Application to an exam question- (first read the question
and try and do the steps)
For question one the correct answer is B, for question 2
the answer is D.
For question 3 you already have the definition for
Newton's First Law.
For 3b) looking at the checklist, the two which are the
same are magnitude
which are different are opposite direction, and act and type, and the ones
on different bodies.
For b)iii) think about what needs to change in the
sentence and what
should stay the same. The
The car exert and upward gravitational force
of 1200N on the
Earth,
, apply steps 1 to 5-
Steps Read the question and try to
When Body A exerts a force on Body
B, Body B exerts an equal and
opposite force on Body A.
Newton's third law applies if-
1. Forces are of the same
magnitude
2. Forces act in opposite directions
3. Forces are of the same kind
4. Forces act on different bodies
5. Forces arise in pairs
Application to an exam question- (first
read the question and try and do the
steps)
First we need to state Newton's Third
Law, this is simply- "When Body A
exerts a force on Body B, Body B exerts
an equal and opposite force on Body
The rest is applying the five steps and
seeing whether or not they apply.
1. Both forces are 300N so forces
are of the same magnitude
2. One force is forward, and the
other is backward, so they act
in opposite directions,
3. Both forces are frictional and so
are of the same kind.
4, One force is from the tyre on
the road, and the other is from
the road on the tyre, so their
acting on different bodies
5. And the forces have arisen as a
pair
And then as their all true we should
write a final conclusion
, try to apply steps 1 to 5-
Read the question and
Steps
When Body A exerts a force on Body B,
Body B exerts an equal and opposite force
on Body A.
Newton's third law applies if-
1. Forces are of the same magnitude
2. Forces act in opposite directions
3. Forces are of the same kind
4. Forces act on different bodies
5. Forces arise in pairs
Correct the work of a professional
Countless people have fooled themselves into
thinking they understand N3, including the
author of this article who I'm sure is
otherwise very competent, don't be among
them! Apply the five steps very carefully to
each scenario before reading on, otherwise
you'll fool yourself into thinking you could
have always get to right answer without
reading this.
For the first book, steps 1, 2, 3 and 5 all
(appear) to apply to the situation, but 4 does
not. The forces are acting on the same body.
Can you think of a N3 law pair for this
scenario? If you were pushing the book with
your hand then the N3 pair would be the
contact force exerted by your hand on the
book, and the contact force exerted by the
book on your hand,
For the second book example, steps 1, 2, and
5 all (appear) to apply but 3 and 4 do not.
Weight is a gravitational force, but the
reaction force is a contact force- they are not
of the same kind. And again, they are not
acting on different bodies, Think of an actual
N3 pair for this situation? The earth pulls the
book downwards with a gravitational force,
so the book must pull the earth upwards with
an equal (and opposite) gravitational force.
