Transcript press the brake to apply a force in the opposite direction, so that the

Physics
(14 - 16)
Newton’s Laws
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Force
A force is a pulling or pushing effect.
Force is a vector quantity, so it has magnitude and direction.
Force
A force will cause a free body to change velocity. Press the
accelerator to apply the driving force, so that the velocity increases;
press the brake to apply a force in the opposite direction, so that the
velocity decreases...
Force
A force will cause a free body to change velocity. Press the
accelerator to apply the driving force, so that the velocity increases;
press the brake to apply a force in the opposite direction, so that the
velocity decreases...
Resultant Force
When two or more forces are applied to a body, they combine to
produce a single resultant force. The resultant force is the overall
effect of all the forces acting together on a body.
Resultant Force
When two or more forces are applied to a body, they combine to
produce a single resultant force. The resultant force is the overall
effect of all the forces acting together on a body.
Use the diagram to see what happens to the resultant force when...
1. Two forces
have the
same
direction?
2. Two forces
have the
opposite
direction?
3. Two forces
are applied
at an angle?
Resultant Force
When two forces act in the same direction on a body, the resultant
force is in the same direction as the individual forces, and the
magnitude is the sum of the two individual forces.
Force from 1st tug
Force from 2nd tug
Resultant force
When two forces act in the opposite direction, the resultant force is
in the direction of the largest force, and the magnitude is the
difference between the two individual forces.
Force from 1st tug
Force from 2nd tug
Resultant force
Resultant Force
When two forces act at different angles on a body, the resultant
force is calculated using the parallelogram of forces...
Force from tug 1
Resultant Force
When the forces acting on a body produce a zero resultant, we say
that the forces are balanced. Draw three lines representing three
different forces. Alter the forces to see the effect on the resultant.
Resultant Force
When the forces acting on a body produce a zero resultant, we say
that the forces are balanced. Draw three lines representing three
different forces. Alter the forces to see the effect on the resultant.
Isaac Newton
Isaac Newton was a great mathematician and
scientist. His discoveries made major advances
to our knowledge of the physical laws that
govern the Universe.
Isaac Newton
1643 - 1727
He invented a new branch of mathematics
called calculus, that enabled mathematicians
to solve many problems.
He researched into gravity, light, rotating bodies, planetary orbits,
and tidal motion. He had amazing powers of concentration,
sometimes studying and solving problems continuously for 19
hours a day.
In 1687, Newton published his most important discoveries in a
famous book called “Principia Mathematica”.
Newton’s Laws of Motion
Newton stated three laws concerning force and motion.
Newton’s First Law
Every object travelling at constant speed
in a straight line (or at rest) will continue
at this same speed, unless a resultant
force acts on it.
Examples:
 If the resultant force acting on a stationary body is zero, the
body will remain stationary.
 If the resultant force acting on a moving body is zero, the body
will continue to move at the same speed and in the same
direction (i.e. with the same velocity).
Newton’s First Law
When a vehicle travels at a constant speed, the frictional forces
exactly balance the driving force, so the resultant force is zero.
Newton’s First Law
When a vehicle travels at a constant speed, the frictional forces
exactly balance the driving force, so the resultant force is zero.
An object in space will continue at the same speed forever, unless it
is acted upon by the gravity of a celestial body.
Newton’s Second Law
The acceleration of a body is
proportional to the resultant force and
in the same direction as the force.
Newton’s second law is represented by the equation:
F =
Example:
Force
m x a
mass
acceleration
 If the resultant force acting on a stationary body is not zero,
the body will accelerate in the direction of the resultant force.
 A car accelerates when the driving force is greater than the
frictional forces. A greater driving force will produce a greater
acceleration.
Demonstrating Newton’s Second Law
Using F = ma
F = mxa
m = F/a
a = F/m
Using F = ma
F = ma connects the resultant force to the mass of the object and
its acceleration:
Force = mass x acceleration
N
kg
m/s2
Force is measured in Newtons (N). One Newton is the force
required to accelerate a mass of 1kg by 1m/s2.
The formula, F = ma can be rearranged using a formula triangle...
Using F = ma
If you know any two of these values, you can use the formula
triangle to find the remaining value...
m = F
a = F
F = mxa
a
m
Using F = ma
Example 1
A lorry with a mass of 3500kg decelerates steadily at 3m/s2.
What is the braking force applied?
F = m x a
F = 3500 x 3
F = 10500N
The braking force applied is 10500N
Using F = ma
Example 2
A force of 250N acts on an object with a mass of 25kg.
What is the acceleration produced?
a = F
m
a = 250
25
a = 10m/s2
The acceleration produced is 10m/s2
Using F = ma
Example 3
A force of 80N acts on an object, causing an acceleration of
3.2m/s2. What is the mass of the object?
m = F
a
m = 80
3.2
m = 25kg
The mass of the object is 25kg
Mass & Weight
Mass is a measure of the amount of matter in an object. It depends
on the number and type of atoms present. Mass is a scalar quantity
because it has only magnitude.
Your mass on the Earth is exactly the
same as your mass on the Moon, or
anywhere else.
Weight is the gravitational force acting on a mass.
Weight is a vector quantity, because it has
magnitude and direction (assumed to be acting
towards the centre of the Earth).
On the Moon objects weigh roughly 1/6th their weight on the Earth.
This is because the mass of the Moon is about 1/6th the mass of the
Earth and therefore its gravity is also 1/6th that of the Earth.
Mass & Weight
The acceleration due to gravity, g is 9.8m/s2, although we often use
10m/s2 in calculations.
From Newton’s second law...
Mass & Weight
The acceleration due to gravity, g is 9.8m/s2, although we often use
10m/s2 in calculations.
From Newton’s second law...
F = mxa
Weight = mg
Sometimes, we say “I weigh 50kg”, when scientifically we should
either say “my mass is 50kg”, or “I weigh 500N”.
Mass & Weight
On the Earth
In Space
On the Moon
Mass = 90 Kilograms
Mass = 90 Kilograms
Mass = 90 Kilograms
Weight = 900 Newtons
Weight = 0 Newtons
Weight = 150 Newtons
When an astronaut has zero weight (weightless) in space it does
not mean that that astronaut has ceased to exist!
Newton’s Third Law
For every action, there is an equal
and opposite reaction.
Examples:
 If you lean on a wall, the wall pushes you back with the same
amount of force in the opposite direction.
 A book resting on a table applies a downward force on the table
equal to the weight of the book. The table applies an equal and
opposite force on the book. The book remains at rest because
the forces acting on the book (the weight acting downwards
and the upward force from the table) add up to zero.
Newton’s Third Law
Fire the cannon, then describe how a cannon works, using
Newton’s third law on Motion.
Newton’s Third Law
For every action, there is an equal
and opposite reaction.
 When the cannon fires, the chemical energy of the gunpowder
is converted into the kinetic energy of the cannonball.
 The cannon applies a force to the cannonball, causing a sudden
increase in velocity.
 The cannonball applies an equal and opposite force to the
cannon, causing a recoil effect (a sudden but brief increase in
velocity of the cannon in the opposite direction).
Newton’s Third Law
Click on the image, then use ‘Z’ for left, ‘X’ for right, and ‘M’ to fire
the jetpack. Describe how a jetpack works, using Newton’s 3rd law.
Newton’s Third Law
For every action, there is an equal
and opposite reaction.
 When the jetpack fires, the chemical energy of the fuel is
converted into the kinetic energy of the exhaust gases.
 The jetpack pushes the exhaust gas downwards or sideways.
 The gas pushes the jetpack with an equal and opposite force.
The man is attached to the jetpack, so he moves with it.
Reaction Force
A mass m, resting on a floor exerts a force equal to its weight on the
floor.
The floor also exerts an opposite
force on the mass. This force is
called the reaction force (or just
‘reaction’).
As there is no change in velocity
of the mass, from Newton’s first
law, the resultant force must be
zero, so the weight and reaction
force must be equal in magnitude
and opposite in direction...
R = mg
End of Show
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