Forces and Motion Physical Science, Physics in the Workplace Anne Rettig, Instructor

Part 1: Forces

Forces affect Motion  A force is a push or a pull that changes an object’s motion.  This means that forces make objects speed up, slow down, or change direction.  Forces also transfer energy from one object to another object like when you hit a ball with a bat.  The SI unit used to measure force is the newton. (N).  One newton is the amount of force needed to accelerate a 1 kg mass by 1 m/sec 2.

Gravity and Friction are Forces  The force of gravity on an object is called weight.  Weight is different than mass.  Mass is the amount of matter in an object and weight measures the force of gravity on that mass.  Mass does not change on different planets, but weight does, depending on how strong the force of gravity is!  The force of friction acts to slow objects that are already moving or make them stop.  Friction is caused when objects rub together.  There are three kinds of friction: sliding, rolling, and fluid friction.

Types of Friction  Sliding friction - when two objects slide over one another; determined by object’s weight and types of surfaces  Fluid friction - when an object moves through a fluid; the smallest force of the three types of friction for similar objects  Rolling friction -when an object rolls over a surface; a smaller force than sliding friction for similar objects

Balanced and Unbalanced Forces  According to Newton’s 3rd Law, forces always act in pairs.  Forces may be balanced (equal in amount and opposite in direction).  Balanced forces cause no change in an object’s motion.  Forces can also be unbalanced (opposite in direction but unequal in amount).  Unbalanced forces do change an object’s motion (The racquet’s force makes the ball reverse direction!

Net Force  Net force is the sum of all the forces acting on an object.  If the forces are acting in the same direction, add them together to find the net force. Which way will the boats be pulled? (Hint: Don’t forget direction.)  If the forces are acting in the opposite direction, subtract to find the net force. Net force on B = _____ balanced or unbalanced? Net force on C = _____ balanced or unbalanced? 15 N10 N 16N 4 N 12 N C B Net force on A = _____ balanced or unbalanced? N S EW 5 N 15 N AA

Universal Forces  There are four forces that work together to control the universe as we know it. Some of these forces work over very large distances and some can only be detected at very small distances. The four forces are: 3.Strong Force - holds the nucleus of an atom together by binding the protons and the neutrons. 2.Electromagnetic Force -may attract or repel. These are much stronger than gravity. 1.Gravitational Force - is the weakest of the universal forces. It depends on the mass of the objects and the distance between them. 4.Protons, neutrons, and electrons are made of smaller particles called “quarks.” Weak force - glues the quarks together. Quarks stuck together to make a proton

Part 2: Measuring Motion

Speed, Velocity, and Acceleration  Since forces cause an object’s motion to change, we need to be able to measure those changes.  There are three important measurements you must be able to make to understand how forces affect motion. These measurements are speed, velocity, and acceleration.

Speed  Speed measures how fast something moves.  Speed is measured in units like meters/sec or miles/hour (always a unit for distance over a unit for time).  Speed = distance / time D ts If you know any two of these values, you can solve for the third one: D = s  t s = D / t t = D / s

Velocity  Velocity measures both speed and direction.  Velocity is also equal to distance / time, but you must tell the direction of the movement too.  These are correct ways to describe velocity: “The car traveled at 70 miles/hour east.” “The rocket’s velocity was 34,000 m/s straight up.”  Velocity has the same units as speed. D tv If you know any two of these values, you can solve for the third one: D = v  t v = D / t t = D / v Compare these formulas to the ones on the previous slide! What do you see?

Acceleration  Acceleration measures how fast you change velocity.  Acceleration happens whenever you speed up, slow down, or change direction.  When you speed up, acceleration is a positive number.  When you slow down, acceleration is a negative number, and is sometimes called “deceleration.”  Acceleration is measured by change in velocity / time. The unit for acceleration is meters/sec 2. Δ v ta If you know any two of these values, you can solve for the third one: Δ v = a  t a = Δv / t t = Δv / a

Falling Objects  If no other force acts on an object, the force of gravity will make the object fall towards the center of the earth.  As the object falls, it will accelerate (speed up) by 9.8 m/sec 2 every second it falls until it hits something.  If the object falls from a very high place, like a tall building or an airplane, it may reach terminal velocity before it hits the ground.  At terminal velocity, the upward force of friction and the downward force of gravity are equal and opposite (balanced), so the object’s speed stays the same until it hits.

Projectile Motion  A projectile is anything that is thrown or shot in a roughly horizontal direction, like a baseball or a bullet.  Projectile motion is always a curved path followed by the moving object that was thrown or shot.  This curved path of projectile motion is caused by the combination of two forces: the horizontal push from the gun or the hand and the downward force of gravity. Horizontal Force of cannon firing Downward Pull of gravity

Part 3: Newton’s Laws of Motion

Newton’s Laws of Motion  1st Law: An object at rest stays at rest, and an object in motion stays in motion, unless acted on by an outside unbalanced force.  2nd Law: Force = (mass)(acceleration)  3rd Law: For every force or action, there is an equal and opposite force or reaction. F = m  a

Newton’s First Law  First Law: “An object at rest stays at rest, and an object in motion stays in motion, unless acted on by an outside unbalanced force.”  This means that an object will stay in place until you exert a force on it (push or pull it). This heavy statue does not move unless it is pushed or pulled! That rocket travels in a straight line at constant speed unless something makes it speed up, slow down, or turn.  If the object is already moving, then it will keep going in the same direction and at the same speed until something pushes or pulls on it.

Newton’s Second Law  Second Law: “Force = mass x acceleration”  This means that it takes more force to make a heavy object move than a light one.  It also means that it takes more force to make an object change speed fast than to change speed slowly. F am If you know any two of these values, you can solve for the third one: F = m  a m = F / a a = F / m

Newton’s Third Law  3rd Law: “For every force or action, there is an equal and opposite force or reaction.”  This means that forces always act in pairs. If a wrecking ball pushes on a wall, the wall also pushes back on the wrecking ball with equal and opposite force. When you apply force to a tennis ball with a racquet, the ball also applies an equal and opposite force to the racquet.

Circular Motion  Centripetal force always points toward the center of the circle.  An object moving in a circle has a special kind of acceleration called centripetal acceleration.  Newton’s Laws of Motion explain circular motion just like other kinds of motion.  When objects move along a circular path, they are accelerating, even if they don’t change speed—Why? (Review the definition of acceleration.)  What things move in a circular path? Think of at least three examples.  If an object is accelerating, then a force must be acting on the object.  Centripetal force is the force needed to make an object move in a circle.