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Forces. Physical Science Chapter 12. Inertia. The tendency of an object to resist any change in its motion. If it is moving, it tends to keep moving at the same velocity unless a force acts on it. If it is not moving, it tends to remain at rest unless a force acts on it. Mass and inertia. - PowerPoint PPT Presentation
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Forces
Physical Science Chapter 12
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Inertia The tendency of an object to resist
any change in its motion. If it is moving, it tends to keep
moving at the same velocity unless a force acts on it.
If it is not moving, it tends to remain at rest unless a force acts on it.
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Mass and inertia The more mass an object has, the
greater its inertia is. Objects with greater inertia take
more force to change their velocity.
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Newton’s first law of motion An object in motion maintains its
velocity unless it experiences a net force. An object at rest stays at rest unless
it experiences a net force. Also called the law of inertia.
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Seat belts When a car stops or turns
suddenly, the passengers’ inertia keeps them moving in the original direction of travel. Slide sideways when turning Slide forwards when stopping
Seatbelts (and friction) stop passengers’ motion.
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Car seats Children are too small for regular
seat belts. Having two straps helps distribute
the force more evenly. Rear-facing car seats distributes
the force over the infant’s entire body, making the force on each individual part smaller.
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Airbags Supplement seatbelts Sensors detect sudden changes in
speed and start a chemical reaction in the air bag.
Nitrogen gas (from reaction) inflates air bag.
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Discuss1. Explain how the law of inertia
relates to seat belt safety.2. How is inertia related to mass?
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Newton’s second law A net force acting on an object
causes the object to accelerate in the direction of the force.
The acceleration is affected by two things size of force mass of object
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Size of Force A larger force will give an object
greater acceleration.
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Mass of object A more massive object will take a
larger force to give it the same acceleration.
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Equation for Newton’s 2nd law
onacceleratimassforce
maF
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Units on force Last chapter we learned that force
has units of Newtons. From Newton’s second law
equation,
2smkgN
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Example How much force is needed to
accelerate a 70-kg rider and the 200-kg motorcycle the rider is on at 4 m/s2?
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You try How much force is needed to
accelerate a 60-kg person and the 500-kg car the person is in at 6 m/s2?
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Rearranging the equation We can rearrange the equation in
the two following ways:
mFa a
Fm
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You try A 63-kg skater pushes off from a
wall with a force of 300 N. What is the skater’s acceleration?
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Gravity Every object in the universe exerts
a force on every other object That force is gravity
Often, the force is too small to notice Example – the force of gravity
between two people
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Gravitational force The amount of gravitational force between
two objects depends on two things Their masses and the distance between them.
The mass of a person is small, so the gravitational force between two people is weak.
The mass of the Earth is large, so the gravitational force between the Earth and a person is strong. 1 2
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mmF Gd
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Gravitational force Gets weaker as the distance
increases
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Weight The measure of the force of gravity
on an object. usually used for the gravitational
force between the Earth and an object near its surface
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Weight Related to mass Objects with greater mass have
greater weight.
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Weight and mass Weight is measured in Newtons
(N). 1 kg of mass at the Earth’s surface
has a weight of 9.8 N
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Weight on Earth vs. Weight on Moon Since the Earth has more mass
than the moon, it exerts a greater gravitational force than the moon.
So, things weigh less on the moon than they do on Earth.
Do they have less mass on the moon?
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Scales Use the principal of balanced
forces to measure how much something weighs.
Scales use springs to balance the force of an object’s weight.
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Weightlessness To be truly weightless, an object
would have to be free from gravity. To feel weightless, something has
to be in free fall along with its surroundings.
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Weightlessness How does a scale measure weight? If it can’t push back, it would read
zero. You would feel weightless.
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Acceleration due to gravity Near Earth’s surface, gravity
causes all falling objects to accelerate at a rate of 9.8 m/s2, regardless of their mass.
Acceleration due to gravity is represented by the letter g.
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Reality check Do all objects really fall with the
same acceleration?
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Air resistance The force air exerts on a moving
object Acts in the opposite direction to
which an object is moving For falling objects, air resistance
pushes up while gravity pulls down
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Air resistance Depends on the speed, size, and
shape of the object The larger the object, the more air
resistance affects it The faster an object is moving, the
more air resistance affects it
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Terminal velocity As on object falls, air resistance
gradually increases until it equals the pull of gravity.
At this point, the object stops accelerating and moves with a constant velocity – called its terminal velocity.
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Projectiles Anything that’s shot or thrown
through the air. Always follow a curved path. Motion can be split into
independent vertical and horizontal parts.
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The horizontal part Once the object is released, there
is no force acting on it horizontally. It maintains a constant horizontal
velocity. (Newton’s 1st)
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The vertical part There is a force acting – gravity
The object is pulled downward with a constant acceleration of 9.8 m/s2. (Newton’s 2nd)
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Discuss1. Explain why your weight would be
less on the moon than on Earth even though your mass would not change.
2. Use examples to expain how changes in mass and changes in distance affect gravitational force.