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# Forces - PowerPoint PPT Presentation

Forces. Equations in Chapter 3: Forces. a= F net /m Acceleration F=G[( m 1 m 2 )/ d 2 ] Gravitational Force F=mg Gravitational Acceleration W=mg Weight p= mv Momentum F=( mv f -mv i )/t Net Force & Momentum. Section 1 Newton’s Second Law.

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### Forces

• a= Fnet/m Acceleration

• F=G[(m1m2)/d2] Gravitational Force

• F=mg Gravitational Acceleration

• W=mg Weight

• p=mv Momentum

• F=(mvf-mvi)/t Net Force & Momentum

Section 1Newton’s Second Law

Force & motion are connected

Objects have greater acceleration if greater force is applied

Mass of object & force applied affect acceleration

Friction depends on kinds of surfaces and force pressing surfaces together.

Areas where surface bumpers stick together

The source of friction

Friction - force that opposes

sliding motion of 2

touching surfaces.

Rolling friction is force between

a rolling object and the

surface it rolls on.

Static friction is the force preventing two surfaces from sliding past each other. No motion occurs.

Applied Force Friction

Sliding friction is force that opposes the motion of two surfaces sliding past each other.

This force overcomes friction and motion occurs.

Air resistance is the force that opposesmotion of objects that move through the air.

Acts in opposite direction to gravity.

Air resistance depends on speed, size and shape.

Gravity is downward force

Air resistance is upward force

• The faster you fall the greater the air resistance

• Eventually these 2 forces will become equal.

net force on object = 0

acceleration (changing speed) = 0

• Object falling at constant speed = terminal velocity

Terminal Velocity is different for different objects

Depends on size, shape and mass

The acceleration of an object is in the samedirection as the net force on the object.

Equation

a= Fnet/m

a=acceleration (m/s2)

Fnet=net force (newtons)

m=mass (kilograms)

a= Fnet/m Use the triangle to manipulate!

• If the mass of a helicopter is 4,500 kg & net force on it is 18,000 N. what is the helicopters acceleration?

• What is the net force on a dragster with a mass of 900 kg if its acceleration is 32.0 m/s2?

• A car is being pulled by a tow truck. What is the car’s mass if net force on car is 3,000 N and it has an acceleration of 2.0 m/s2?

Section 2Gravity

One of the four basicforces.

This long-range force gives

the universe structure.

Due to inertia, all objects fall with

same acceleration regardless of

mass (g is constant)

Gravity is an attractive force between any 2 objects.

Depends on masses of objects & distance apart.

Force will increase as

distance decreases.

Force will increase as

mass increases

Equation

F=G[(m1m2)/d2]

F= gravitational force

G=universal gravitational constant

m=mass of objects

d=distance between the objects

Gravitational force between 2 massesdecreases as distanceincreases.

N

Gravitational force

can never = zero.

Equation

F=mg

F=force of gravity (Newtons)

m=mass (kilograms)

g=acceleration of gravity (m/s2)

g= 9.8 m/s2 on Earth

g= 1.6 m/s2on the Moon

Objects in space shuttle float because they have noforcesupporting them.

As you get farther away

from Earth the

gravitation force

acting on you

decreases

Weight is the gravitational force exerted on an object.

(F=mg )

Equation

W=mg

W=weight (Newton’s)

m=mass (kilograms)

g=acceleration of gravity (m/s2)

Mass is a measure of the amount of matter an object contains.

Weight = Force (changes as g changes)

Mass = Measurement

Weight

Mass

F=mg Force of gravity

g= 9.8 m/s2 (Earth) g= 1.6 m/s2(Moon)

W=mg Weight

1. What is the force of gravity on a sky diver with a mass of 60 kg on Earth?

2. What is the force of gravity on a sky diver with a mass of 60 kg on the Moon?

Free fall is simply the lack of a downwardforce upon an object thus noupwardforce exists. (all forces except gravity are ignored)

Example from book—standing on a scale in an elevator, in freefall you weight zero.

Projectile is anything thrown through the air.

Projectile motion is the curvedpath the object takes due to Earth’s gravity.

Have horizontal and vertical velocities

Two identical items hit

the ground at same time

due to identical downward

accelerations even if one is

dropped and other is

thrown horizontally.

Catapult Lab&Projectile Motion

You are going to use everyday objects to build a catapult.

Catapults were often used as weapons of war during the Middle Ages.

You can design your catapult however you please, but you may use only the materials that have been provided and nothing extra.

Centripetal acceleration is the accelerationtoward the center of a curved or circular path.

Centripetal force is the net force exerted toward the center of a curved path.

Draw a circle, trace the line with

a block or some other object.

Notice you have to twist the object towards the center to stay on the path of the circle.

This is centripetal—towards the center.

Section 3Newton’s Third Law of Motion

This is the law that rocket propulsion is based on.

action-reaction

When one object exerts a force on a second object, the second one exerts a force that is equal in strength and opposite in direction.

For every action there

is an equal and

opposite reaction.

The forces are equal but not balanced.

They act on different objects.

Action Reaction

Direction seen as a result is forward

Momentum is amount of force needed to changemotion of a movingobject.

Equation

p=mv

p=momentum (kg m/s)

m=mass (kilograms)

v=velocity (m/s)

Newton’s 2nd and Momentum

Equation

F=(mvf-mvi)/t

F=Net force exerted

mvf-mvi=change in momentum

t=time

Momentum has a direction.

Momentum changes if mass, velocity or both change.

Momentum can be transferred.

Total amount of momentum is conserved.

No loss & No gain!

2 objects moving toward each other p=0

p=mv Calculations Page 86

1. What is the momentum of a car with a mass of 1,300 kg traveling at 28 m/s?

2. If a baseball has a mass of .15 kg and is thrown with a momentum of 6.0 kg*m/s what is speed?

3. What is mass of person walking .8 m/s if momentum is 52.0 kg*m/s?

• a= Fnet/m Acceleration

• F=G[(m1m2)/d2] Gravitational Force

• F=mg Gravitational Acceleration

• W=mg Weight

• p=mv Momentum

• F=(mvf-mvi)/t Net Force & Momentum