Download File - Thomas Tallis Science

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Balanced forces
Force is a vector. The vector sum of all the forces on an
object gives a net or resultant force.
Look at the forces on this
object:
4N
To see the vector sum of the
forces, add the vectors together
nose-to-tail.
3N
4N
3N
3N
4N
3N
There is no ‘net’ or resultant
force on the mass: the forces
are balanced.
4N
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Forces and motion
If there is no resultant force on an object, can it be moving?
Aristotle thought that all objects tend to
move towards their natural place in the
universe, and that their velocity was
determined by the strength of this force.
For instance, heavier objects would
move towards the ground more
quickly than lighter ones.
Galileo later realised that the acceleration of an object, not
its velocity, is proportional to the size of the total force
acting on it, and in 1687, Sir Isaac Newton published his
first law of motion, which confirmed this.
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Newton’s first law
A moving object has kinetic energy. This energy will not be
lost unless a force acts to slow the object down.
Objects moving in free space with no forces acting
on them will continue to move in a straight line
at a constant velocity until a force causes them
to change speed or direction.
This is Newton’s first law, also known as the law of inertia.
For instance, a space shuttle requires large forces from its
engines during launch, but can move through space with
little or no propulsion if far enough away from gravitational
influences.
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Applying Newton’s first law
In everyday life, most things don’t move unless a force is
applied. Why is this?
What forces act on this car as it travels at a steady speed?
Air resistance acts against
the direction of motion.
Friction from the road
pushes the car forward.
This force is a reaction to
the backwards push of the
wheels on the road.
The car is travelling at a steady speed, so these two forces
must be equal.
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Relative motion
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Newton’s first law: true or false?
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Free body diagrams
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Equilibrium on a slope
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Unbalanced forces
The forces on this mass are unbalanced:
What is the resultant force? Add the
vectors together again, nose-to-tail.
8N
3N
4N
3N
8N
F
θ
4N
3N
F = √32 + 42 = 5 N
4N
θ = tan-1(4/3) = 53°
There is a resultant force of 5 N at a direction of 53° from
the horizontal.
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What is Newton’s second law?
If there is a resultant force on an object, it accelerates in the
direction of that force. Its acceleration is directly proportional
to the force, and inversely proportional to the object’s mass.
This can be stated in a vector equation:
F = ma
For example, the downward force on an object due to gravity
(weight) at the Earth’s surface is:
weight = mg
Where g = 9.81 ms-2 (standard acceleration due to gravity).
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Newton’s second law example
A car and caravan are accelerating at 0.5 ms-2.
1. Find the driving force produced by the engine.
?
3000 kg
5000 kg
The engine must accelerate both the car and the caravan:
F = ma = (3000 kg + 5000 kg) × 0.5 ms-2 = 4000 N
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Newton’s second law example
A car and caravan are accelerating at 0.5 ms-2.
2. Find the tension in the tow-bar.
?
4000 N
3000 kg
5000 kg
The tow-bar accelerates the caravan only:
F = ma = 5000 kg × 0.5 ms-2 = 2500 N
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Newton’s second law example
A car and caravan are accelerating at 0.5 ms-2.
3. Find the resultant force on the car.
2500 N
4000 N
3000 kg
5000 kg
resultant force = driving force – tension
= 4000 – 2500 = 1500 N
or F = ma = 3000 kg × 0.5 ms-2 = 1500 N
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Skydiving
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Using Newton’s second law
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Action and reaction
A football rests on flat ground. There are four key forces
acting between the ball and the Earth. What are they?
 The ball presses down
on the ground.
 The ground reacts to
the ball.
 Earth’s gravity acts on
the ball.
 The ball’s gravity acts
on the Earth.
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Newton’s third law
For every action there is an equal and opposite reaction.
Forces always come in pairs.
List all the horizontal forces acting here, between the car,
the caravan and the road:




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The car pushes the road backwards.
The road pushes the car forwards.
The car pulls the caravan forwards.
The caravan pulls the car backwards.
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Action and reaction
“If every action has an equal and opposite reaction,
how do unbalanced forces ever occur?”
Newton’s third law applies only to forces between objects.
Pairs of action and reaction forces are of the same kind.
This diagram shows a pair
of frictional forces between
a box and the ground:
friction of the box on the
ground, and friction of the
ground on the box.
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Balanced forces
When considering the forces that cause acceleration, they
must all act on the same object. These forces can be
considered individually and can be of different kinds.
For instance the pulling
force on this box is not
equal to the frictional
force acting against it,
so the box will start to
move to the right.
Free body diagrams only include the forces that act on one
object. This makes it easy to work out whether the forces on it
are balanced or not, and whether the object will accelerate.
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Balanced forces: true or false?
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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