Physical Physical ScienceScience

Unit:Unit:ForcesForces

FORCE

A PUSH OR A PULL

A FORCE MAY GIVE ENRGY TO AN OBJECT AND CAUSE IT TO START MOVING OR CHANGE ITS MOTION

The force gives energy to an object

The energy can set the object in motion, stop its motion or change the speed and direction of its motion

The Nature of ForceThe Nature of Force By definition, a By definition, a ForceForce is a is a pushpush or a or a pullpull..

A Push

Or

A Pull

Just like Velocity & Acceleration Forces have

both magnitude and direction

components

Just like Velocity & Acceleration Forces have

both magnitude and direction

components

Force

Force is a vector quantity. Described by magnitude and

direction. An arrow represents the

direction and strength of a force and the longer the arrow, the greater the force.

The SI unit for force is a Newton.

VectorsVectors Vectors are a method used to visually show forcesVectors are a method used to visually show forces A A vectorvector is a quantity which has both magnitude (size) and direction. is a quantity which has both magnitude (size) and direction.

The The lengthlength of the arrow shows the of the arrow shows the magnitudemagnitude of the vector. of the vector. The The angleangle of the arrow shows the vector's of the arrow shows the vector's directiondirection..

Just like numbers, we can add two or more vectors together Just like numbers, we can add two or more vectors together and get a and get a net forcenet force called the called the resultant resultant

Adding 2 or More Adding 2 or More VectorsVectors

Add vectors A and B to get the Resultant C Add vectors A and B to get the Resultant C A + B = CA + B = C

Fig 1 - shows the magnitude & direction of the 2 vectors we are addingFig 1 - shows the magnitude & direction of the 2 vectors we are adding Fig 2 – we move the beginning of vector B to the end of Vector A, making sure to keep the magnitude & direction exactly Fig 2 – we move the beginning of vector B to the end of Vector A, making sure to keep the magnitude & direction exactly

the samethe same Fig 3 – Connect the beginning of Vector A to the end of Vector B, this is your “Resultant” C.Fig 3 – Connect the beginning of Vector A to the end of Vector B, this is your “Resultant” C.

Fig 1

Fig 2

Fig 3

Combining Forces

Often, more than a single force acts on an object at one time.

Net Force is the combination of all forces acting on an object.

Combined forces that are balanced are always equal to zero

Balanced Forces

Balanced Forces are equal forces acting on one object in opposite directions

When equal forces are exerted in opposite directions there is no net force.

Balanced forces acting on an object do not change the object's velocity (keeps velocity constant)

Unbalanced forces

Forces that are not opposite and equal

While balanced forces cause no change in motion, unbalanced forces always cause a change in motion.

Unbalanced forces

When 2 unbalanced forces are exerted in opposite directions, the combined force is the difference between the 2 forces

Balanced & Unbalanced Balanced & Unbalanced ForcesForces

With a Balanced force – opposite and equal forces acting on the same object result in NO motion of the object

Unbalanced forces Unbalanced forces – two or more forces of unequal – two or more forces of unequal strength or direction acting upon on an object strength or direction acting upon on an object results results in the motion of the objectin the motion of the object

Friction

Friction The force that opposes the motion of an object, It is a force that two surfaces exert on each other when they rub against each other.

Friction acts in the direction opposite to motion. and is the force that brings an object to rest

Without friction or other unbalanced forces, an object will not stop until it hits another object.

Isaac Newton 17th century

Said that no force is necessary to keep an object in motion at a constant speed, and that objects do not come to rest on their own

Causes of Friction

Smooth surface has less friction than rough surface.

Strength of force of friction depends on types of surfaces involved and how hard the surfaces push together. (i.e. rubbing hands together)

4 Types of Friction

1. Static Friction - Friction that acts on objects that are not moving

Example: moving a piece of furniture across room.

Must use extra force to start friction of stationary objects.

2. Sliding Friction - Friction where two solid surfaces slide

over each other.When an object is pulled at a constant

velocity across a level surface, the pulling force equals the sliding friction.

The amount of sliding friction present depends on 2 factors:

1. weight of object moving

2. type of surface object is sliding across

3. Rolling friction - Friction where an object rolls across

a surface.

Rolling friction is less than sliding friction.

4. Fluid friction - Friction where a solid object moves

through a fluid.

Less than sliding friction. A substance that changes

sliding friction to fluid friction is called a lubricant, ex parts of machines bathed in oil.

Example Problem

. A 5 kg block is pulled across a table by a horizontal force of 40 N with a frictional force of 8 N opposing the motion.  Calculate the acceleration of the object.  

Solution: F=ma 40N-8N=32N of force

in a direction 32N=5(a) a=6.4 m/s2

Types of Forces:

1. Gravitational Force:

The attraction between any 2 objects that have mass

As mass increases so does the attraction

As distance increases gravitational force decreases

2. Electrical forces

Forces between electrical charges Objects with different charges pull

toward ( attract) each other ex plastic wrap to an object

Objects with the same charge repell

3. Magnetic force

Forces caused by moving electrical charges ex common magnet

4. Weak interactions

Forces believed to cause the nuclei of some atoms to break apart

A mass can be acted upon by one or more of these forces at a time

Forces and Motion

During 1665-1666 Isaac Newton developed his 3 laws of motion.

The 3 laws of motion explain rest, constant motion, accelerated motion, and how balanced/unbalanced forces act.

Newton’s 3 Laws of Newton’s 3 Laws of MotionMotion

Newton’s 1st Law of MotionNewton’s 1st Law of Motion: : AKA AKA The Law of Inertia The Law of Inertia

which states which states an object at rest will remain at rest, and an an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity object in motion will remain in motion at a constant velocity until acted on by another force.until acted on by another force.

Remember: The greater the mass of an object the greater the inertia

Remember:

Newton called the tendency of objects to remain in motion or stay at rest: inertia

Inertia: The property of matter to resist any change in motion ( Latin meaning idle or lazy)

Newton’s Second Law of Motion Newton’s Second Law of Motion aka aka F=maF=ma Force = mass x accelerationForce = mass x acceleration

Can be written as:Can be written as: F=ma ; a= F/m ; m= F/aF=ma ; a= F/m ; m= F/a

What is the basic unit for mass? What is the basic unit for mass? KilogramKilogram What is the basic unit for acceleration? What is the basic unit for acceleration? Meter/sec/secMeter/sec/sec Therefore the basic unit for Force is Therefore the basic unit for Force is

(kilogram)( meter/sec/sec)(kilogram)( meter/sec/sec) An object with a mass of 1 kg accelerating at 1 m/s/s has a An object with a mass of 1 kg accelerating at 1 m/s/s has a

force of 1 Newton force of 1 Newton

Newton’s 3 Laws of Newton’s 3 Laws of MotionMotion

Remember:

Acceleration is the change in speed or direction

According to the 2nd law of motion, an unbalanced force must be present when there is a change in speed or direction

The acceleration is always in the direction of the unbalanced force

F=maF=ma Weight is the force of gravity acting on an object’s mass.Weight is the force of gravity acting on an object’s mass. Therefore weight is a type of ForceTherefore weight is a type of Force The formula for weight: Weight = mass x AThe formula for weight: Weight = mass x Agg

SinceSince A Agg= 9.8 m/s= 9.8 m/s2 2 then then

Weight = mass x 9.8 m/sWeight = mass x 9.8 m/s22

Newton’s 2Newton’s 2ndnd Law & Weight Law & Weight

Remember:1 newton = 0.22 pounds

Example Problem Force

What is the force on an object that goes from 35 m/s to 85 m/s in 20 seconds and has a mass of 148 kg?

Solution: Force = mass x accelerationForce = mass x acceleration 370 N

Example Problem Force

Calculate the force on an object that has a mass of 12 kg and an acceleration of 4 m/s2 .

Solution: Force = mass x accelerationForce = mass x acceleration F = ma = 12 kg ´ 4 m/s2 = 48 kg ´ m/s2 =

48 N

Example Problem Force

What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec2?

Solution: Force = mass x accelerationForce = mass x acceleration f=9,800 n

Example Problem Force

What is the acceleration of a 50 kg object pushed with a force of 500 newtons?

Solution: Force = mass x accelerationForce = mass x acceleration

F= ma 500 = 50(a) a= 10 m/s2

Example Problem Force

The mass of a large car is 1000 kg. How much force would be required to accelerate the car at a rate of 3 m/sec2?

Solution Force = mass x accelerationForce = mass x acceleration

F= ma f= 1000 x 3 f= 3000 N

Your weight Your weight on other on other planetsplanets

& 3 different & 3 different types of types of

starsstars

Newton’s 3Newton’s 3rdrd Law of Motion Law of Motion:: For every action there is an equal & opposite reactionFor every action there is an equal & opposite reaction..

If an object is not in motion, then all forces acting on it are balanced and the net force is zero!If an object is not in motion, then all forces acting on it are balanced and the net force is zero! Friction – the force that one surface exerts on another when the two rub against each other.Friction – the force that one surface exerts on another when the two rub against each other.

Newton’s 3 Laws of Newton’s 3 Laws of MotionMotion

Sliding friction

Rolling friction

Fluid friction

How Do Rockets Lift Off?

Rockets and space shuttles lift into space using Newton’s third law of motion.

They burn fuel and push the exhaust gases downward at a high velocity as they lift off. In turn, the gases push upward on the rocket.

•

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

As long as this upward force, called thrust, is greater than the downward pull of gravity, there is an unbalanced force in the upward direction that causes a change in the rocket's velocity. As a result, the rocket accelerates upward into space.

Newton’s 3 laws of motion can explain all aspects of

an objects motion

Gravity

Gravity is a force that pulls objects toward each other.

Newton realized gravity acts everywhere in universe.

Law of Universal Gravitation states that the force of gravity acts between all objects in the universe.

Any two objects in the universe, without exception, attract each other.

Law of Universal Gravitation

Newton was the 1st scientist to prove that the forces pulling an apple to the ground were the same forces pulling the moon toward the earth

This was the first universal law of force ( applies to all objects in the universe)

The size of the force depends on 2 factors:

1. Mass of object 2. Distance between the objects

The force of gravity increases as objects mass increases

Gravitational force decreases as distance between objects decreases

Free fall Free fall – when the only force acting on an

object is gravity. In free fall, the force of gravity alone causes an

object to accelerate in the downward direction. All objects in free fall accelerate at the same

rate. Since objects fall at the same speed, their

acceleration is the same. All objects accelerate at the rate. Here on Earth

the rate is: Ag=9.8 m/s2 Or Ag=32 ft/s2

air resistance

Any falling object meets air resistance

As the object falls, the air resistance gradually becomes equal to the pull of gravity.

When an object reaches the point that it no longer accelerates, it has reached its terminal velocity ex T.V. of sky diver is 190 km/hr

Air Resistance

Air resistance is a type of fluid friction.

Air resistance is an upward force exerted on falling objects.

Objects with greater surface area experience more air resistance as they fall. (Flat piece of paper vs. crumpled piece of paper)

Air resistance keeps things from falling equally

Projectile Motion

A projectile is an object that is thrown.

A ball thrown horizontally falls at the same rate as ball dropped.

Elastic Forces

Matter is considered elastic if it returns to its original shape after is squeezed or stretched.

Compression is an elastic force that squeezes or pushes matter together.

Tension an elastic force that stretches or pulls matter.

Mass and Distance

Force of gravity between object increases with greater mass and decreases with greater distance.

The further from Earth it is, the less gravitational force.

Mass is the measure of the amount of matter in an object. (Kg)

Gravity & Weight

Weight is gravitational force exerted on a person or object at the surface of a planet.

Stepping on scale (weight) shows the gravitational force Earth exerting on you.

Objects with greater mass have greater weight.

Weight = Mass x Acceleration due to gravity

Weight – Newtons (N). Mass – Kilograms (KG).

Acceleration due to gravity = 9.8 meters per second per second (m/s²) (at Earth's surface)

Weight varies with strength of gravitational force – mass doesn't.

Strength of gravitational force exerted on an object or person by the moon is 1/6 of the force exerted by Earth.

Center of Gravity

The center of gravity of an object is the point at which all an object’s mass appears to be located

An object is balance if its center of gravity is supported ( if not it falls)

Force of gravity and weight both decrease as an object moves away from the Earth’s center of gravity ( mass stays the same)

Formula used to find weight of object

Force = mass of object x constant of 9.8 m/sec2

Example Problem An object of mass 30 kg is in free fall in a

vacuum where there is no air resistance. Determine the acceleration of the object.

  Solution:

Since it is in free fall and is falling due to the force of gravity.....the acceleration will always be 9.8 m/s2

Orbital Motion An object that orbits another object is in

orbital motion. This is the result of the combination of the

objects forward inertia and downward pull of gravity on an object

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

The speed with which an object must be thrown in order to orbit Earth is 7,900 meters per second.

Example Problems A bowling ball rolled with a force of 15 N

accelerates at a rate of 3 m/sec2; a second ball rolled with the same force accelerates 4 m/sec2. What are the masses of the two balls?

Solution: F= ma 15N= (m)3 m=5kg 15N

= (m)4 m=3.75kg

Example ProblemExample Problem

If a 60 kg person on a 15 kg sled is pushed with a force of 300 N, what will be person’s acceleration?

Solution: F=ma 300N=(60+15)(a)

a=4 m/s2

Example Problem

An object of mass 30 kg is falling in air and experiences a force due to air resistance of 50 newtons. 

Determine the net force acting on the object and :

f=ma f=30 x 9.8 f=294N Net force is 294-50 = 244N

calculate the acceleration of the object. F= ma 244N=30 (a)

a=8.13 m/s2

Example Problem

A force of 250 N is applied to an object that accelerates at a rate of 5 m/sec2. What is the mass of the object?

Solution:

F = ma 250N=(m)5 m= 50kg