1 Chapter 10-Forces All forces that affect motion

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Chapter 10-Forces

All forces that affect motion.

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Nature of Force

A force is a push or a pull. Forces have a strength and

direction. The net force is all the

forces that are added together.

Unbalanced forces acting on an object will change the object’s motion.

Balanced forces acting on an object will not change the object’s motion.

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Newton’s Laws of Motion

Newton’s First Law of Motion states that an object at rest will remain at rest and an object that is moving at constant velocity will continue moving at constant velocity unless acted upon by an unbalanced force. Inertia is the tendency of an

object to resist change in its motion.

The amount of inertia of an object depends on its mass.

Law of Inertia

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Newton’s Second Law of Motion The net force on an object is

equal to the product of its acceleration and its mass.

Force = mass x acceleration

Force is measured in units called Newtons. One newton equals the force

required to accelerate one kilogram mass at 1 meter per second per second.

Acceleration = Force / Mass

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Newton’s Second Law(continued) Forces are unbalanced

This law is dependent on two variables: Net forces Mass of the object

As the force applied to an object increases, the acceleration increases.

As the mass of an object increases, the acceleration decreases.

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Understanding the law.

Newton's laws declare loudly that a net force (an unbalanced force) causes an acceleration; the acceleration is in the same direction as the net force.

Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a

given situation. (diagram on right)

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Magnitude & Direction of Forces Vectors are quantities which are

fully described by both a

magnitude and a direction.

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Friction

Friction is the force that one surface exerts on another when the two rub against each other.

The strength of the force of friction depends on two factors:

Types of surfaces involved.

How hard the surfaces push together.

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Types of Friction

Types of friction:

Sliding friction-solid surfaces slide over each other.

Rolling friction-object rolls over the surface.

Fluid friction-object moves through a fluid.

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Gravity

Gravity is the force that pulls objects towards Earth.

Objects free fall when the only force acting on a falling object is gravity.

The law of universal gravitation states that the force of gravity acts between all objects in the universe.

Weight is a measure of the force of gravity on an object, and mass is a measure of the amount of matter in that object.

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Projectile Motion

An object that is thrown is a projectile.

Projectile motion is the motion of an object that is thrown horizontally.

Gravity pulls down on an object that is in projectile motion.

An object thrown will hit the ground at the same time that an object is dropped.

Gravitational acceleration is 9.8 m/s2.

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Air Resistance

Air resistance is a type of fluid friction that an object falling through air experience.

The greater the surface area of an object, the greater the air resistance.

Terminal velocity is the greatest velocity that a falling object can reach.

Terminal velocity is reached when the air resistance equals the force of gravity.

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Newton’s Third Law of Motion If one object exerts a force on

another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

Otherwise, for every force there is an equal and opposite reaction force.

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Momentum

Momentum is the product of the mass and its velocity.

Momentum = Mass x Velocity Law of Conservation of

Momentum states that the total momentum of the objects that interact does not change.

The total momentum of any group of objects remains the same unless outside forces act on the objects.

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Placing Satellites in Space

A rocket can rise into the air because the gases it expels with a downward force exert an equal but opposite force on the rocket.

Satellites orbit the earth in circular paths because of centripetal force.

Satellites in orbit around Earth continually fall toward Earth, but because Earth is curved they travel around it.

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Chapter 10 Review Questions

Which of Newton’s Laws states that an object at rest will remain at rest and an object in motion at a constant velocity will continue in motion at a constant velocity unless acted on by an outside force?

(First Law) What are the two factors the friction depends on? (type of surface & how hard the surfaces push

together) Which law states that the force of gravity acts

between all objects in the universe? (The Law of Universal Gravitation)

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Which of Newton’s Laws states that force is equal to mass times acceleration?

(Second Law) What are the three types of friction? (sliding, rolling & fluid) Which law states that the total momentum of the objects that

interact does not change? (The Law of Conservation of Momentum) Which type of friction has solid surfaces slide over each other? (sliding) What is the formula for calculating acceleration? (acceleration = force/mass)

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Which of Newton’s Laws states that for every force there is an equal and opposite force?

(Third Law) Which type of friction is illustrated by traveling on a sled down a

hill? (sliding) Which force causes an object to move in a circle? (centripetal force) Which type of friction is when an object rolls over a surface? (rolling) Which of Newton’s Laws of Motion allows a rocket to be

launched into space? (Third Law)

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What do you call a push or a pull? (force) Which type of friction is illustrated by a plane flying? (fluid) What is the formula for momentum? (momentum = mass x velocity) Which type of friction is when an object moves

through a fluid? (fluid)

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What do we call the tendency of an object to resist change in its motion?

(inertia) Which type of friction is illustrated by a bowling ball

traveling down a bowling alley? (rolling) What is do we call the greatest velocity of a free

falling object? (terminal velocity) How does unbalanced forces affect motion? (start motion or change speed)

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What metric unit is equal to the force required to accelerate one kilogram of mass at 1 meter per second per second?

(Newton) What is the value of gravitational acceleration? (9.8 m/s2) What is the motion called when a horizontally

thrown object is pulled down? (projectile motion) How does balanced forces affect motion? (doesn’t change motion)

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What do we call the force that one surface exerts on another when the two rub against each other?

(friction) What is the formula for calculating weight? (weight=mass x acceleration due to gravity) Which object will hit the ground first if dropped from

the same height at the same time, a crumpled sheet of paper or an uncrumpled sheet of paper?

(crumpled sheet of paper) What is the formula for calculating force? (force = mass x acceleration)

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1 Chapter 10-Forces All forces that affect motion