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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.