Forces in NatureForces in Nature

Science 7

Darren Nigh

Lets Review the 3

laws of motion

Law of InertiaLaw of InertiaAn object at rest will remain at rest, and an object in motion will remain in motion with constant velocity unless acted upon by an external force.

InertiaInertia

Inertia is the resistance of an object to a change in the speed

or the direction of its motion

The acceleration of an object increases with increased force and decreases with increased mass

Law of Law of AccelerationAcceleration

Newton’s Second Law at work in a Grocery Store

The acceleration of a shopping cart depends on two things. Name these two things.

1. The size of the force applied to the shopping cart

2. The mass of the shopping cart

Force = the size of person

Mass= the increasing amount of groceries to be purchased

NewtonNewton’’s Second Law and Falling s Second Law and Falling ObjectsObjects

Heavy objects and light objects dropped from the same place hit the ground at the same time.

They both accelerate at the same rateAcceleration = force / massBut, falling objects don’t accelerate through theirentire fall.

Falling objects accelerate at

the same rate, but don’t

accelerate through the entire fall.

Eventually, the force of air

resistance pushing up equals the

force of gravity pulling down.

The object stops accelerating. It has reached its final speed called Terminal Speed.

An elephant versus a feather

As a skydiver falls, he accelerates downwards, gaining speed with each second. The increase in speed is accompanied by an increase in air resistance (as observed in the animation below). This force of air resistance counters the force of gravity. As the skydiver falls faster and faster, the amount of air resistance increases more and more until it approaches the magnitude of the force of gravity. Once the force of air resistance is as large as the force of gravity, a balance of forces is attained and the skydiver no longer accelerates. The skydiver is said to have reached a terminal velocity.

Law of InteractionLaw of Interaction

• For every action there is an equal but opposite reaction

• Put on a pair of roller skates and stand facing a wall. Push against the wall. You move backward. Why?

• Your push is an action force. The wall pushes back, hence the third law.

PressurePressure is the measure of how much force is

acting on a certain area. Pressure = Force/Area P= F/A

MotionMotion is the change of position over time.

If you missed the motion of this long jump, you would still know that motion had occurred because of the distance between the beginning and ending position

Speed• Speed is a measure of how fast

something moves or the distance it moves in a given amount of time.

• The greater the speed, the faster it changes position

• Speed = distance/time s=d/t

Velocity

• Velocity is a speed in a specific direction

Velocity = distance/time in a certain direction

• If you say you are walking east at 5 miles per hour, you are describing velocity

• A person walking north at 5 miles per hour would have the same speed, but not the same velocity.

AccelerationAcceleration• Acceleration is the measure of how

quickly velocity is changing.• If velocity does not change, there is

no acceleration• If you know the starting velocity of an object,

the final velocity, and the time interval in which the object changed velocity, you can calculate acceleration

*Acceleration= final velocity- initial velocity

time

ForceForce• A force is a push or pull on an object

Force = mass x acceleration F=ma

• Objects exert forces on each other (sit on a chair)

• Forces can speed up or slow something down

• A force can affect an object without making it move (sit on an inner tube and the sides bulge)

Force is measured in newtons

Yes, the term is named after Isaac Newton

If the same force is applied to two objects of different mass, which object will have the greater acceleration?

The object with the less Mass

Types of

Forces End of Part 1

Forces of FluidsForces of Fluids

Buoyancy is the upward force on objects in any fluid also called buoyant force. Because of buoyant force, objects in water seem lighter. It is easier to move a large boulder in water, than on land.

As long as the object is less dense than the fluid, it will float.

• The greater the volume of an object in a fluid, the greater the buoyant force on it.

• Archimedes’ principle states that the buoyant force on an object is equal to the weight of the fluid that the object displaces.

Archimedes’ Principle

If the weight of an object is greater than the buoyant force acting on it, the object sinks.

The boat on the left floats because it is filled with air instead of water.

BernoulliBernoulli’’s Principles PrincipleBernoulli’s Principle says that an increase in the speed of

the motion of a fluid (or air ) decreases the pressure within that fluid (or air).

Thus there is less pressure over a rock in a whitewater stream, than a rock in a pond

Bernoulli’s Principle

PascalPascal’’s Principles PrinciplePascal’s Principle states that when outside

pressure is applied (at any point) to a fluid in a container, the pressure is transmitted throughout the liquid at equal strength.

1. You use the lever to apply downward force on a piston, which increases force on the liquid.

2. The increase in pressure is transmitted throughout the liquid

3.The pressure increase actson a larger area to produce a Greater force pushing the carUp.

Pressure of Fluids

A fluid applies pressure perpendicular to all sides of an object in contact with the fluid.

Pressure of the atmosphere

The ratio of the weight of all the air above you to your surface area is atmospheric pressure.

On land, atmospheric pressure depends on your elevation.

Under water, the pressure depends on your depth below the water’s surface.

Other ForcesOther Forces

Applied Force

An applied force is a force which is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person

Gravity Force

(also known as Weight) The force of gravity is the force with

which the earth, moon, or other massively large object attracts another object toward itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth. The force of gravity on earth is always equal to the weight of the object.

Air Resistance Force Air resistance force is a special type of

frictional force which acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

Tension Force

The tension force is the force which is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

Spring Force Spring force is the force exerted by a

compressed or stretched spring upon any object which is attached to it. An object which compresses or stretches a spring is always acted upon by a force which restores the object to its rest or equilibrium position. For most springs (specifically, for those which are said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

Knowledge Check True or False1.When air resistance equals gravitational force you

have reached terminal velocity

2.Velocity is the same as speed but only in a specific direction

3.An elephant and a feather have the same acceleration immediately after they jump off a building

4.Spring force is the force which is transmitted through a wire when it is pulled tight by forces acting from both ends

5.The weight of an object is its gravitational pull on a planet surface.

Mass versus Weight

• Mass is a measure of how much matter an object contains

• Weight is the force of gravity on that object

• Mass is a property that an object has everywhere in the universe.

• Weight depends on another force, gravity.

Types of

Forces End of Part 2

GravityGravity

• Gravity is the force that objects exert on each other because of their masses.

• The greater the mass of two objects, the greater the gravity.

• The force of gravity between Earth and an apple is stronger than the force of gravity between you and the apple because Earth has more mass than you do!

Gravity is very strong on the planetGravity is very strong on the planetEarth due to its large massEarth due to its large mass

FrictionFriction

• Friction is a force that opposes motion between two surfaces that are in contact with each other.

• Friction between two solid surfaces sliding against each other is known as sliding sliding friction.friction.

• The amount of friction between two surfaces depends on two things-----------

1.1. Of what material the two Of what material the two surfaces are made.surfaces are made.

2.2. How hard they are How hard they are pressing against each pressing against each other.other.

How would the scenario change if the Box were sitting on ice?

Centripetal ForceCentripetal Force

• Any force that keeps an object moving in a circle is known as Centripetal Centripetal Force.Force.

• The force points toward the center of the circle

• Without Centripetal Force, the object would go flying off in a straight line

When you were younger, you may have played a game in which you and a friend took turns swinging each other in a circle.

If you remember the game, you may also remember that your arms quickly got tired

It took force to change the direction of your friend’s motion.

Without the force, he could not have continued moving in a circle.

Figure skating pairs competition

Suppose the male skater from the previous pair spins his partner faster. Her direction changes even faster than before, so she accelerates more. To get more acceleration he must apply more force.

How does increasing the centripetal force on an

object affect its acceleration?

Its

Acceleration

increases

MomentumMomentum

• The momentum of an object is the product of its mass and velocity. p=mv

• At the same velocity a wrecking ball has more velocity then a tennis ball

• However, if you compare two wrecking balls of the same size, the faster wrecking ball has the most momentum.

WorkWork• In Science, WorkWork is accomplished when

force is applied to an object and the object moves a distance.

• Work is measured in newton-meters, more commonly called joulesjoules.

Work =force x distance, or W = F x d

PowerPower

The raterate at which WorkWork is done is called

PowerPower P = W (work in joules) T (time in seconds)

CollisionCollision• A collision is a situation in which two

objects in close contact exchange energy and momentum

• As another car bumps into the back of yours, the force pushes your car forward.

• Some of the momentum of the car behind you is transferred to your car

• At the same time the car behind you slows because of the reaction force from your car

• You gain momentum and the other car loses momentum

A very bad collision

ConclusionConclusion

• Look over the notes that you have taken the last three class periods

• Be prepared to complete an assessment quiz to show your understanding at the beginning of the next class period.