Chapter 12 forces and motion power point

CHAPTER 12 FORCES AND MOTION

12.1 FORCES

12.1 FORCES

There are 4 distinct forces in our universe: Gravitational, electromagnetic, strong nuclear and weak nuclear forces.

Ex: everyday force – wind

Force – is a push or pull that acts on an object.

A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

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Gusts of wind push you either way.

Weight is a type of force (spring scale) measure force

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Units of Force

Force is measured in Newtons (N)

One Newton is the force that causes a l-kilogram mass to accelerate at a rate of 1 meter per each second (1 m/s2)

1N = 1kg x m/s2

Named after Sir Isaac Newton, English scientist who explained how force, mass and acceleration are related.

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Representing Force

Use arrows to represent the direction and strength, or magnitude, of a force.

Figure 2 page 357

Force arrows represent the weight of items on the scale

The longer the arrow the more weight acting on the scale.

Combining Forces

Net force is the overall force acting on an object after all the forces are combined.

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Balanced Forces (ex: tug of war)

Sometimes the net force acting on an object is zero.

These are called balanced forces.

Equal and Opposite Forces

= 0

When the forces on an object are balanced the net force is zero and there is no change in the object’s motion.

EX: tug of war ~ Where no one wins

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Unbalanced forces

Is a force that results when the net force acting on an object is not zero.

When an unbalanced force acts on an object, the object accelerates.

Forces acting in opposite directions can also combine to produce an unbalanced force.

The net force equals the size of the larger force minus the size of the smaller force.

Accelerate in the direction of the unbalanced force ( larger force)

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Ex: Adding forces

=

Subtracting forces

=

Equal and Opposite Forces

= 0

FRICTION

Friction (4 types)- it is a force that opposes the motion of objects.

It acts on any surface where 2 objects or more are in contact.

Without friction all surfaces would be more slippery than ice.

All objects experience friction.

We need friction to walk. Finger prints helps up pick things up.

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4 main types

1. Static Friction : Is the friction force that acts on the objects that are not moving. It always acts in the opposite direction of the force being applied. Ex: foot and ground allows you to walk. Ground pushing up.

2. Sliding Friction: It is the force that opposes the direction of motion of an object as it slides over a surface. This force is less than static friction. Less force is needed to keep something sliding. Ex: Pushing something along the floor.

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3. Rolling Friction: The friction force that acts on a rolling object

100 to 1000 times less than the force of static friction. Why we use dollies to move heavy objects. Ball bearings are used in machines to reduce friction. Ex: Bikes, cars, inline skates, etc.

4. Fluid Friction: Opposes the motion of an object through a fluid. Can be air or water.

Air = airplane Water = submarine

Fluid Friction increases as the speed of the object moving through the fluid increases.

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Gravity – a force that acts between any two masses

- Attractive force that pulls objects together

- Can act over long distances

- Acts downward toward the center of the Earth

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Falling Objects

- 2 forces act upon this object

- Gravity and Air resistance

- Reach terminal velocity when gravity and air resistance equal each other (forces balance each other out)

Projectile Motion – a curved path of an object in free fall.

Force – initial force forward, force of gravity pulling it down.

12.2 NEWTON’S FIRST AND SECOND LAWS OF MOTION

Aristotle thought force was needed to keep an object moving at constant speed. Not correct and held up the progress in the study of motion for about 2 thousand years.

Galileo found out how gravity produced constant acceleration. Concluded that moving objects that are not subjected to friction or any other force would move indefinitely.

Newton defined mass and force. Wrote his laws of motion

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Newton’s first law of motion- (the state of motion)

• The state of motion of an object does not change as long as the net force acting on the object is zero.

• Basically an object in motion stays in motion with the same speed and direction.

• An object at rest stays at rest unless an unbalanced force acts on it.

• Sometimes called the law of inertia.

• Inertia is the tendency of an object to resist change in its motion.

12.2

Newton’s Second Law of Motion

The acceleration of an object is equal to the net force acting on it divided by the objects mass.

Acceleration of an object depends upon its mass.

Mass is a measure of the inertia of an object and depends on the amount of matter the object contains.

Acceleration is directly proportional to the net force acting on it.

12.2

Formula:

Acceleration = Net Force a = F

Mass m

Units- N/kg and m/s2 are equivalent because N= kg·m/s

Acceleration of an object is always in the same direction as the net force

The 2nd law also applies when a net force acts in the direction opposite to the objects motion.

Force produces a deceleration.

WEIGHT AND MASS

An object’s weight is the product of the object’s mass and acceleration due to gravity.

We have a weight formula which also considered Newton’s second law.

Weight = Mass × Acceleration due to gravity

W = mg (just a different form of a=F/m)

g= 9.8m/s2

SECTION 12.3 NEWTON’S 3RD LAW OF MOTION AND MOMENTUM

Forces never exist alone. Forces all exist in pairs.

Newton’s 3rd Law:

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

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Action and reaction forces

Not all action-reaction force pairs result in motion.

Ex: Swimmer in water- yes

Push on wall-no

Why don’t action-reaction forces cancel out and produce a net force of zero?

It is because reaction and action forces do not act on the same object.

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Ex: Figure 16 P. 373

The action force of the swimmers arms act upon the water.

The reaction force acts upon the swimmer ( the water pushing on the swimmer)

Only equal and opposite forces act on the same object and give a net force of zero.

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Momentum- is the product of an object’s mass and its velocity.

An object with a large momentum is hard to stop.

An object has a large momentum if the product of its mass and velocity is large.

Formula:

Momentum = Mass × velocity

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Law of conservation of momentum

If no net force acts on a system, then the total momentum of the system does not change.

See diagrams page 376

SECTION 12.4 UNIVERSAL FORCES

Electromagnetic forces

• Associated with charged particles

• Electric force and magnetic force are the only forces that can attract and repel.

Electric forces- between charged objects or particles such as electrons and protons.

• Same charge – repel

• Opposite charge - attract

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Magnetic forces – act on certain metals, on the poles of magnets, and on moving charges.

• 2 poles- N and S, attract each other

• 2 like poles (N,N or S,S) repel each other

Nuclear forces

2 forces, the strong and weak nuclear forces act within the nucleus to hold it together.

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Strong nuclear force- overcomes the electric force that acts among the protons.

• goes the diameter of a proton (distance it acts on)

• Powerful force of attraction

• Acts only on the neutrons and protons in the nucleus, holding them together.

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Weak nuclear force- involved in certain types of radioactive processes

• Weaker than strong nuclear force

• Also located in the nucleus of an atom

• Acts over a shorter range than the strong nuclear force.

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Gravitational force

• Is the weakest universal force

• Is an attractive force that acts between any two masses

• Keeps the moon in its orbit

• Pulls the moon towards the Earth.

• Newton’s law of universal gravitation states that every object in the universe attracts every other object.

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• Gravitational force between 2 objects is proportional to their masses.

• It decreases as the distance between the masses increase.

• Greater the mass of the objects, the greater the gravitational force.

• Most effective universal force over long distances.

Why do we 2 high tides and 2 low tides.

Gravitational pull from the moon produces 2 bulges in the Earth’s oceans.

One on the side of the Earth closest to the moon.

One on the side of the Earth farthest from the moon.

This is also why satellites stay in the sky.

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