Download 2. Laws of Motion

22 May, 2017
NEWTON’S
ND
2
LAW
Objectives
To understand why different objects accelerate at different
rates and be able to use the equation for Newton’s 2nd Law.
HSW: AF1 – Thinking scientifically
Used before in: lesson 1 – resultant forces
Will use again in:
PLTS: Independent enquirers - support conclusions, using reasoned arguments and
evidence.
Used before in: Science A
Will use again in:
Keywords
Force, Mass, Acceleration, N, Kg, m/s/s,
proportional, inverse.
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OUTCOMES
All students should be able to:
• Describe that resultant forces make objects accelerate and
the more force you give something the more it will
accelerate.
Most students should be able to:
• Calculate the force required to produce a given
acceleration of an object of known mass.
• State that objects of larger mass require greater forces to
cause large acceleration.
• Determine the direction of the acceleration on an object.
Some students should be able to:
• Rearrange and use the equation:
force = mass x acceleration.
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Introducing unbalanced forces
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What is Newton’s second law?
If the resultant force acting on an object is not zero, all the
forces are said to be unbalanced.
This forms the basis of Newton’s second law of motion,
which states:
If the forces on an object are unbalanced, two
things about the object can change:
 the speed of the object may change – it may either
increase or decrease
 the direction of motion may change.
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Demos
Qualitative description
Classic demo
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How is movement calculated from force?
The resultant force acting on an object is related to the
object’s mass and acceleration. These three factors are
linked by the following equation:
force = mass x acceleration
 Resultant force is measured in newtons (N).
 Mass is measured in kilograms (kg).
 Acceleration is measured in metres per second per
second (m/s2).
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How do we use Newton’s second law?
A car has a mass of
1,000 kg. What force
must the car’s engine
supply to cause an
acceleration of 2 m/s2?
force = mass x acceleration
= 1,000 x 2
= 2,000 N
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Using a formula triangle
A formula triangle helps you to rearrange a formula. The
formula triangle for force (f), mass (m) and acceleration (a)
is shown below.
Cover the quantity that you are trying to work out, which
gives the rearranged formula needed for the calculation.
So to find force (f),
cover up f…


…which gives
the formula…
f = mxa
x
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How do we use Newton’s second law?
A lorry has a mass of 12,000 kg. What acceleration is caused
by a force of 10,000 N?
force = mass x acceleration
acceleration = force
mass
= 10,000
12,000
= 0.83 m/s2
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Questions. – use sheet if necessary
1
Complete the table below showing the resultant force, mass and acceleration of objects in
different situations.
Resultant force
(in newtons)
Mass
(in kilograms)
Acceleration
(in m/s2)
70
8.0
a) Athlete accelerating at start of 100 m
race
b) Car accelerating
c) Lorry braking
3000
16 000
d) Plane taking off
2,
0.8
8000
5.0
A vehicle of mass 1500 kg braked to a standstill from a velocity of 24 m/s in 12 s.
a)
Show that the deceleration of the vehicle was 2.0 m/s2.
b)
Calculate the resultant force on the vehicle.
3.
1200
a)
A cyclist accelerated along a flat road from a standstill to a velocity of 12 m/s in 60 seconds. The mass of the cyclist and the bicycle
was 80 kg.
i)
Show that the acceleration of the cyclist was 0.2 m/s2.
ii)
Calculate the resultant force on the cyclist and the bicycle.
b)
On reaching a velocity of 12 m/s, the cyclist in a) stopped pedalling and slowed down to a velocity of 8 m/s in 10 s, when she started
pedalling again.
Calculate:
i)
The deceleration of the cyclist when she slowed down.
ii)
The size and direction of the resultant force on the cyclist when she slowed down.
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Answers
1 a)
560 N,
b)
2.5 m/s2,
c)
20 000 kg,
d)
40 000 N.
b)
3000 N.
3 a)
ii) 16 N,
2
b)
i) 0.4 m/s2,
ii) 32 N in the opposite direction to
her direction of motion.
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Plenary Game
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