Physical Science Chapter 12 Forces and Newton’s Laws of Motion

The Nature of Force By definition, a Force is a push or a pull. Just like Velocity & Acceleration Forces have both magnitude and direction components

Forces A force causes an object to move, accelerate, change speed, or direction Forces are represented by: Arrows Direction Strength represented by the length of the arrow

Balanced & Unbalanced Forces Balanced forces – opposite and equal forces acting on the same object result in NO motion of the object Unbalanced forces – two or more forces of unequal strength or direction acting upon on an object results in motion of the object

Balanced or Unbalanced Forces? Unbalanced but WHY? Balanced but Why?

Force/Free body diagrams (to show forces) used to show the relative magnitude and direction of all forces acting upon an object in a given situation. a special example of vector diagram size of the arrow represents amount of force

Force / Free-body Diagrams direction of the arrow shows the direction which the force is acting each force arrow is labeled The object is usually represented by a box. Force arrows are drawn from the center of the box outward in the direction which the force is acting.

Combining Forces Net force: All forces are added or subtracted to create one total force If net force is Zero then there is no change in motion. If there is a net force there is an acceleration

Newton’s 1st Law of Motion(cont.) If you roll a ball across the floor, why does it eventually come to a stop. Is there a force acting against it? If so, what is it? Friction—force that opposes motion. Always acts in the direction opposite of motion. Anything slowing this Coke as it slides across the bar?

Newton’s 2nd Law of Motion called the “Law of Acceleration”. Relates force, mass, and acceleration. Force and acceleration directly related. Mass and acceleration inversely related.

Newton’s 2nd Law of Motion(cont.) Uses this Formula: F = m a Where: F = force in Newton’s(N) m = mass in kilograms(kg) a = acceleration in m/s2

Ex. What force is required to accelerate a 1500 kg car by a rate of 1 Ex. What force is required to accelerate a 1500 kg car by a rate of 1.5 m/s/s? Given: m = 1500 kg a = 1.5 m/s/s F = ? F = m a F = 1500kg(1.5 m/s/s) F = 2250 N

Ex. A force of 290 N is applied to a 20 kg rock Ex. A force of 290 N is applied to a 20 kg rock. At what rate will the rock accelerate? F = m a a = 14.5 m/s2 Given: m = 20 kg a = ? F = 290 N

Ex. A force of 140 N is applied to a mover’s dolly to accelerate a load of Cokes across the grocery store at 3 m/s2. What is the mass of the load of Cokes? Given: F = 140 N a = 3 m/s2 m = ?

A force is applied to accelerate a 50 kg mass at a rate of 2. 5 m/s2 A force is applied to accelerate a 50 kg mass at a rate of 2.5 m/s2. What force is required? 20 N 25 N 120 N 125 N

D, F = 125 N

A 250 N force is applied to a 12 kg mass A 250 N force is applied to a 12 kg mass. At what rate will the object accelerate? 3000 𝑚 𝑠 2 240 𝑚 𝑠 2 20.8 𝑚 𝑠 2 .05 𝑚 𝑠 2

C, 𝒂 = 20.8 𝑚 𝑠 2

Newton’s 3rd Law For every action, there is an equal but opposite reaction. Forces always occur in pairs!!

Which of the following scenarios does not represent Newton’s 3rd Law? A rocket being launched into space. A book resting on a desk. A person paddling a canoe. A large force pushing on a small mass, causing it to accelerate.

Choice D is Newton’s 2nd law.

Force Problem What is the net force on an object being pulled toward the west with a force of 30N and another force pulling the object toward the east with a force of 75N?

Types of Friction a force that opposes the motion of objects that touch as they move past each other acts at the surface where objects are in contact All moving objects encounter friction Without it most motion would be impossible 4 types, static, sliding, rolling, fluid

Static Friction The force that keeps an object from moving It is the largest frictional force Always opposite direction of the applied force Pushing a Car Walking

Sliding Friction Once the object is in motion it experiences sliding friction Opposite direction from applied force Less than static friction so less force is needed to keep it in motion

Rolling Friction As something rolls, the object and floor bend slightly This bend causes rolling friction It is a much smaller force than static friction As much as 1000 times smaller Allows you to move heavy objects Ball bearings reduce friction

Fluid Friction It opposes the motion in the liquid Like swimming, it is hard to move If you are in the air, fluid friction is called air resistance At higher speeds it is very noticeable

Force of Gravity a force that acts between two objects an attractive force, pulls objects together causes objects to accelerate downward Falling objects, gravity pulls down, What pushes up? air resistance acts in opposite direction Terminal Velocity- when Gravity and air resistance cancel each other out (velocity of falling body becomes constant)

Terminal Velocity

Projectile Motion When an object not only goes up and down but out. It is a curved path Air resistance and gravity are the only forces acting on a projectile What falls faster, an object that falls straight down, or one that has a horizontal velocity?

Balanced and Unbalanced Forces 1.Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge? 2.Draw arrows showing the individual and combined forces of the tugboats in #1.

3.Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge? 4.Draw arrows showing the individual and combined forces of the tugboats in #3.

5.Could there ever be a case when Tugboat A and Tugboat B are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.

Newton’s Laws of Motion Begins Here!!

Great Scientists Aristotle- Incorrectly said force was required to keep an object in constant motion Galileo- with no resistance objects would move indefinitely Newton- based on Galileo’s findings he came up with his law’s of motion

Newton’s 1st Law of Motion AKA The Law of Inertia Inertia- an objects tendency to resist changes in motion an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity until acted on by another force. Remember: The greater the mass of an object the greater the inertia

You need to remember this about Inertia!!! Mass is the measure of inertia of an object!!! Remember: The greater the mass of an object the greater the inertia So, which would have more inertia? Bowling ball 0.1 kg baseball traveling at 20 m/s 5 kg bowling ball traveling at 3 m/s 10 kg sled traveling at 0 m/s 0.001 kg bumblebee traveling at 2 m/s Which would have the least?

Newton’s 2nd Law The acceleration of an object is equal to the net force acting on it divided by the objects mass 𝑎 = F/m so F=ma Acceleration is always in the same direction of the net force An object will have greater acceleration if a greater force is applied

Newton’s Second Law of Motion F=ma Force = mass x acceleration F=ma ; a= F/m ; m= F/a What is the basic unit for mass? Kilogram What is the basic unit for acceleration? Meter/sec/sec Therefore the basic unit for Force is (kilogram)( meter/sec/sec) An object with a mass of 1 kg accelerating at 1 m/s/s has a force of 1 Newton

Newton’s 2nd Law & Force of Gravity Have you heard of the FORCE of gravity? Gravity: the force that pulls objects towards each other Since gravity is a force it also obeys Newton’s second law F=ma 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 With this experiment, Galileo proved Aristotle wrong Air resistance keeps things from falling equally With this experiment, Apollo 15 astronauts proved Galileo right. (link to You Tube)

Weight and Mass Weight is the force of gravity acting on an object Weight (N) = mass (g) x acceleration of gravity (m/s2) Note: just a version of F=ma, F = mass x gravity FYI: 1 pound = 4.448 Newtons so 1/4 lb is slightly more that 1 N so a 1/4 lb burger is a "Newton" burger

Newton’s 2nd Law & Weight F=ma So, weight is a type of Force The formula for weight: Weight = mass x Ag Since Ag= 9.8 m/s2 then Weight = mass x 9.8 m/s2 Remember: 1 newton = 0.22 pounds

Your weight on other planets & 3 different types of stars

Newton’s 2nd Law

F= ma Math Practice A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?

F= m a formula problems… An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair?

During a test crash, an air bag inflates to stop a dummy’s forward motion. The dummy’s mass is 75 kg. If the net force on the dummy is 825 N toward the rear of the car, what is the dummy’s accceleration?

A bicycle takes 8.0 seconds to accelerate at a constant rate from rest to a speed of 4.0 m/s. If the mass of the bicycle and rider together is 85 kg, what is the net force acting on the bicycle? (Hint: First calculate the acceleration.) a=(vf-vi)/t = (4.0 m/s) / 8.0 s = 0.50 m/s2 F=ma = 85 kg x 0.50 m/s2 = 43 N

Newton’s 3rd Law of Motion: For every action there is an equal & opposite reaction. This means every time a force is applied in one direction an equal force is applied in the opposite direction. If an object is not in motion, then all forces acting on it are balanced and the net force is zero!

Action-Reaction Forces If you push against a wall it pushes back These forces can cause motion, like a swimmer They do not cancel each other out, because they act on different objects. 2 ice skaters

Momentum is sm p. p = mv The product of an object’s mass x velocity Influences how easily an object can be stopped High velocity or high mass cause a high momentum For some reason, maybe because mass is designated as “m” in formulas, momentum is designated as “p”. Therefore: p = mv The unit for mass is kg, the unit for velocity is meter/second, therefore the unit for momentum is kg m/sec

Which has more momentum? p = mv a 0.046 kg golf ball with a speed of 60.0 m/s a 7.0 kg bowling ball with a speed of 6.0 m/s .046 kg x 60.0 m/s = 2.8 kg m/s 7.0 kg x 6.0 = 42 kg m/s

Conservation of Momentum: When two or more objects interact (collide) the total momentum before the collision is equal to the total momentum after the collision If no net force acts on a system, then the total momentum of the system does not change In a closed system, loss of momentum of one object equals the gain in momentum of another object

Momentum – 2 moving objects During this collision the speed of both box cars changes. The total momentum remains constant before & after the collision. The masses of both cars is the same so the velocity of the red car is transferred to the blue car.

Momentum – 1 moving object During this collision the speed red car is transferred to the blue car. The total momentum remains constant before & after the collision. The masses of both cars is the same so the velocity of the red car is transferred to the blue car.

Momentum – 2 connected objects After this collision, the coupled cars make one object w/ a total mass of 60,000 kg. Since the momentum after the collision must equal the momentum before, the velocity must change. In this case the velocity is reduced from 10 m/sec. to 5 m/sec.

Forces that can be found anywhere in the universe Universal Forces Forces that can be found anywhere in the universe

Electromagnetic Force Electric and Magnetic Forces are the only forces that can both attract and repel Electric forces- between charged particle, positive attracted to negative Clothes in a dryer Magnetic forces- N and S poles, opposites attract, likes repel

Nuclear Forces Two forces- a strong and a weak force hold an atom together The strong nuclear force overcomes the protons repulsion force This force is over 100x stronger than electric forces The weak force is involved in radioactive processes

Gravitational Force Attractive force between any two masses This force is very small compared to all the rest Newton came up with the Law of Universal Gravitation, which explains how we can find the gravitational force of any object A large mass is required for gravity to be felt

Gravity is relational to distance The further apart two objects are the less the gravitational force Gravity still has pull over millions and millions of miles apart As distance doubles the force gets quartered

The Earth Moon System The moon stays in orbit because of Earth’s gravity It works very similar to a centripetal force It pulls in on an object So as the moon follows this circular path, it’s gravitational force has an affect on Earth.

Satellites Satellites stay in orbit with centripetal force of gravity If it were to slow down it would lose altitude