1. Applied Physics Chap 3 Forces
Chapter 3 Forces and Motion
Applied Physics Chap 3 Forces

2. Applied Physics Chap 3 Forces
Chapter 3: Forces and Motion
Force: a push or a pull that changes an objects motion or its shape
Newton: The SI unit of force and its symbol is “N” written as a capital letter because its named after Isaac Newton:
For example, a force of 12 N.
Friction: a special force that always opposes motion as moving surfaces rub against each other.
Gravity is a special force that always pulls downward
Applied Physics Chap 3 Forces

3. Applied Physics Chap 3 Forces
Video: Forces and Motion
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Video: constant motion
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5. Applied Physics Chap 3 Forces
Isaac Newton
Newton was born in 1642 the year Galileo died.
Newton was dissatisfied with the classical science (Aristotle) he studied at Cambridge after he read Galileo’s writings about motion.
School closed for 2 years during the great plague, so Newton fled to his families countryside home where he now had plenty of time to think about motion, force and inertia.
Newton thought up his three laws of motion and the mathematics to support it (calculus, the mathematics of change).
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Video: Newton’s 3 Laws of Motion
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Newton’s 1st Law of motion: (often called the “law of inertia”)
An object at rest tends to stay at rest, an object in motion tends to stay in motion, unless acted upon by a net external force.
Newton’s law of inertia how the first two conditions of motion that occur without forces:
Ø     no motion where v = 0 (an object at rest)
Ø     constant speed motion where vi = vf (an object in motion)
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8. Applied Physics Chap 3 Forces
Newton’s First Law of Motion
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Net Force: the algebraic sum of all forces acting on an object.
–5 N
12 N
7 N
Net force = 12 N + – 5 N = 7N
–15 N
6 N
–9 N
Net Force = – 15N N = – 9N
Net forces can also act vertically:
Upward forces = positive and downward = negative.
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Equilibrium—When Net Force Equals Zero.
Equilibrium: a situation where equal and opposite forces act on an object.
If an object is in Equilibrium, then no net forces act on it and neither its speed or direction will change.
Equilibrium does NOT mean No forces it means that forces are balanced.
Example: A wooden block weighing –5 N is hanging motionless from a spring which pulls upward with a force of +5 N.
Net force = +5 N –5 N = 0 N
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Balanced and Unbalanced forces.
Balanced Forces: forces that are equal and opposite
Balanced forces are in equilibrium and motion can be either no motion, or constant speed motion
Unbalanced forces are not in equilibrium and there is a net force acting on the object.
An object subject to unbalanced forces will accelerate in the direction of the net force.
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Newton’s Second Law The law of constant acceleration
Net force causes acceleration
Small Force = small Acceleration
Large Force = large Acceleration
CONCLUSION: acceleration is directly proportional to the applied force: a  F
and always acts in the same direction as the net force.
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Mass resists Acceleration
Light mass = large Acceleration
Heavy mass = small Acceleration
Equal Forces
An objects acceleration is inversely proportional to the objects mass.
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Newton’s Second Law
Newton’s second law: The acceleration produced by a net force is directly proportional to the size of the net force and inversely proportional to the object’s mass.
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Video: Newton’s 2nd Law or Motion
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Weight and The Force of Gravity
Gravity A force of attraction between two massive objects that is directly related to the product of the two masses and indirectly proportional to the square of the distance between the objects.
Ø     Gravity gets stronger if the distance between two masses decreases.
Ø     Gravity gets stronger if one of the masses is very large (like a planet)
Ø Gravity gets weaker the further away the masses get.
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Video: Gravitation
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Free Fall: A body that is continually falling toward the Earth, pulled by only gravity.
Newton pictured a cannon ball shot from a very high mountain.
The more horizontal velocity the cannon gives, the further out the cannon ball will fly.
Give it enough and it will fly around the Earth.
The only difference between the apple and the moon is horizontal velocity. The apple has none so it falls straight down.
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Newton’s universal law of gravity
The force of gravity (weight) is:
 Directly proportional to the product of the Earth’s mass mE and a falling objects mass mO.
 Inversely proportional to the square of the distance between the centers of the Earth and the falling object.
Where G is a coefficient 6.67 x m3/(kg s2)
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Acceleration of Gravity: is the acceleration an object experiences when freely falling toward the ground.
Since Fg = mog then
Near the surface of the Earth, objects have an acceleration of m/s2 in falling. This value is called called “g” and has the same units as any other type of acceleration.
Weight: is a measure of the force that gravity pulls down on an object.
Specifically, if you have 1 kilogram mass of matter the Earth’s gravity pulls down on it with a force of (9.8 m/s2)(1 kg) = 9.8 Newtons.
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Your weight is different on different planets, since every planet has a different amount of mass.
Ø    The moon is about 1/6th the Earth’s mass so, you weigh about 1/6th what you do on Earth.
Ø     Jupiter has 2 ½ times the Earth’s gravity
Ø     The sun has 27 times more gravity than the Earth
Ø     Mass does not change wherever you are, your weight changes depending on the strength of gravity which depends on the mass of the planet you are on.
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Video: mass and weight
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Following Newton’s second law: F = ma Replace the F with a W for weight and the acceleration a with a “g” to represent the acceleration of gravity
To calculate an object’s weight.
Multiply mass in kilograms by ‘g’
W = mg
To calculate an object’s mass
Divide the weight in Newton’s by ‘g’
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Newton’s Second Law and Acceleration:
Region A: Acceleration
Time
Velocity A FP Ff
The positive slope of the line indicates that the box is accelerating (velocity increases with time) as the pulling force is greater than the friction force between the box and the table. FP > Ff
The Net Horizontal Force FN = FP – Ff
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Region B: Motion at constant speed
time v A B FP Ff
In this region, the Pulling force is equal to but in the opposite direction from the force of friction FP = Ff and FN = FP – Ff = 0 The box is in equilibrium moving at constant speed as shown by the horizontal graph
The Net Force = 0 Therefore acceleration is zero and velocity is constant and to the right.
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Region C: Deceleration
time v A B C FP Ff
In this region, the slope is negative since the pulling force is either reduced or zero so FP < Ff . This makes the net force negative, and acting in the opposite direction from the velocity.
If the Net Force is negative then the acceleration will also be negative but the velocity remains positive, to the right
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·      If the net force is in the same direction as the velocity, the object will increase speed in that direction.
·     If the net force is zero, the object is in equilibrium, either motionless or its inertia will keep it moving at constant speed in the same direction.
·     If the net force is in the opposite direction from the velocity, the object will decelerate or slow down while it continues moving in the same direction.
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Friction a force that resists motion
FRICTION: a force that resists motion of two objects that are in physical contact.
Friction always acts opposite to the direction of motion.
Friction (F)
Pull Force (P) and motion
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Causes of Friction:
Metal surfaces may look smooth to the eye but are actually composed of microscopic hills and valleys. As the hills of two objects interact, they momentarily make and break electrical forces, attraction between positive and negative charges in atoms which results in the force we know of as friction
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The strength of Friction depends on:
 The amount of force pressing two surfaces together
The type and roughness of the surfaces in contact with each other
 The amount of surface area in contact between the two surfaces.
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Ways of reducing friction
lubricant
Lubrication smooths out surface roughness and separates surfaces from rubbing directly together. Forces the hills apart.
 Reducing the weight pressing the surfaces together
Reducing the contact area. Like for example using ball bearings which have a very small contact area.
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Coefficient of friction  is the ratio of the force of friction and the weight pressing the surfaces together.
F is the force of friction in Newtons
W is the weight of the upper object, in Newtons.
Use F =  W to find the frictional force between two surfaces.
Look up values of  in a table.
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Fluid Resistance: a force that resists the motion of a solid object moving through a liquid like water, or a gas like air.
caused by particles of air or water hitting a moving object. The faster the object is moving, the more force the particles hit it with.
The shape of the object moving through the fluid. Flat objects moving through a fluid impact the particles directly. Streamlined objects let the particles flow around them.
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Streamlining is one way to reduce resistance by smoothing out the shape of an object
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Turbulent flow: high velocity flow where eddy’s and whirlpools form in the flow causing the fluid resistance
Laminar Flow: low velocity flow where fluid resistance is uniformly low. Fluid flows smoothly over the object. F O R C E
Fluid speed
laminar
turbulent
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Air resistance: an upward force, caused by friction between air molecules and a falling object. Air resistance force increases as the downward velocity increases.
Terminal velocity: an equilibrium condition when the downward force of gravity is exactly equal but opposite to the upward air resistance force.
Net Force = R –W = 0 W R
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Video: Newton’s 3rd Law of motion
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Newton’s Third Law – Action and Reaction
Force occurs as part of an INTERACTION between two or more objects.
My push force
walls’ resisting force
Since no motion occurs when I push on the wall, the force I apply and the opposite force the wall applies must be equal and opposite:
“Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.”
“For every action (force), there is an equal and opposite reaction (force)
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Action and Reaction forces
Action reaction pairs never act on the same object – they always act on different objects. BUT the action and reaction forces are equal and opposite
BUT the action and reaction forces are equal and opposite
The bullets mass is smaller than the rifles but it exits the rifle at a very large acceleration a while the rifle moves backward with a smaller acceleration. So the while the action and reaction forces are equal and opposite, their effects (accelerations) are not.
for the bullet F = m  a small mass times large acceleration equals Action force
For the Rifle F = m  a large mass times small acceleration) equals reaction force
Applied Physics Chap 3 Forces