Basic Physics Forces and motion Physics is Physics

Basic Physics Forces and motion

Physics is… § Physics = study of how force and energy affect matter. § Basic Physics = the study of forces and motion.

Part One Position

Two Dimensional Position-Time Graphs § Show far an object has moved from a reference point (Position Zero). § An object can move away from position zero in two directions. In front of it or behind it. § One direction is designated as positive and one is negative.

Two Dimensional Position-Time Graphs § In the previous graph the object started 3 meters in front of reference point zero. § It moved passed reference point zero and kept going until it was 3 meters behind point zero (-3 meters). § The object then changed direction and started to move back toward point zero.

Two Dimensional Position-Time Graphs § Position-Time graphs can also be used to calculate Velocity. § The Slope of a position time graph equals Velocity. § Every time the slope changes, velocity also changes. § When the slope is steady (a straight slope) it means that the velocity is steady.

Velocity is constant Velocity is changing

Two Dimensional Position-Time Graphs § From a Position – Time graph you should be able to: § Calculate Velocity. § Determine where the object is not moving. § Determine where the object is changing direction. § Determine how far an object has moved in a given amount of time.

What kind of motion would create these position-time graphs?

Calculating Slope § Since the slope of a Distance vs. Time graph equals velocity, it is helpful to be able to calculate slope. § If you can find two points on the graph (x, y) you can calculate slope. § Slope = (y 2 – y 1) / (x 2 – x 1)

Slope = (0 – 60) / (30 – 15) Slope = -4 Velocity = -4 m/s (15, 60) Point A (30, 0) Point B

Part Two Velocity

Velocity § Velocity describes how far an object moves in a certain amount of time. § Velocity = Distance divided by Time § V = d/t § Velocity is different from speed because it specifies direction.

Velocity § Example: § 5 meters per second (m/s) is a speed. § 5 meters per second (m/s) north is a velocity.

Two Dimensional Velocity -Time Graphs § Show when an object is changing speed. § One direction is designated as positive and one is negative. § When an object’s velocity positive it is moving in one direction. § When an object’s velocity is negative it is moving in the opposite direction.

Two Dimensional Velocity -Time Graphs § In the previous graph the object Doesn’t move § § for 2 seconds (Velocity = Zero). After 2 seconds the object accelerated to 2 m/s in the negative direction. At 5 seconds the object begins to slow down until it hits a velocity of zero. At 7 seconds the object has stopped and is changing directions. Note that as the velocity approaches zero the object must be slowing down.

Not Moving Accelerating Constant Speed Stopping & Changing Direction

Two Dimensional Velocity -Time Graphs § Velocity -Time graphs can also be used to calculate Acceleration. § The Slope of a Velocity-Time graph equals Acceleration. § Every time the slope changes, the object accelerates. § When the slope is steady (a strait line) it means that the object is accelerating at a steady rate.

Velocity is Changing (Acceleration is occurring) Constant Velocity

Two Dimensional Velocity - Time Graphs § From an Velocity – Time graph you should be able to: § Calculate Acceleration. § Determine where the object is not moving. § Determine where the object is changing direction. § Determine when an object is slowing down.

Part Three Acceleration

Acceleration § Acceleration = the rate at which velocity changes. § When something is accelerating its velocity is constantly changing. § Measured in m/s 2 (meters per second).

Example: § If a car is accelerating at 4 meters per second § § § per second (4 m/s 2) from rest, how fast is it going after 1 second? 4 m/s How fast is it going after two seconds? 8 m/s After 3 seconds? 12 m/s Its velocity increases by 4 m/s every second.

Two Dimensional Acceleration -Time Graphs § Show fast velocity is changing. § One direction is designated as positive and one is negative. § When an object’s acceleration is acting in the same direction as it’s motion, it speeds up. § When an object’s acceleration is acting in the opposite direction as its motion it slows down.

Direction of Motion With a positive Acceleration the car speeds up With a negative Acceleration the car slows down

Two Dimensional Acceleration -Time Graphs § When acceleration is at zero it means that velocity is not changing. IT DOESN’T NECESSARILY MEAN THAT THE OBJECT IS NOT MOVING!!

Velocity is changing at a rate of 1 m/s No acceleration means that velocity is constant

Two Dimensional Acceleration - Time Graphs § From an Acceleration – Time graph you should be able to: § Determine when an object’s velocity is changing and when it is constant.

Calculating Acceleration § Acceleration can be calculated by determining the slope of a velocity vs. time graph, but it can also be calculated using the following formula. § Acceleration = (Vf – Vi)/t (measured in m/s 2) § Vf = Final Velocity § Vi = Starting Velocity § t = Time

Practice Calculating Acceleration § A car is traveling at 28 meters per second north; 10 seconds later the car is traveling 15 meters per second north. What is the car’s acceleration? § A car with a mass of 2150 kg is traveling at west 70 m/s, two minutes later the car is traveling west at 100 m/s. What is the car’s acceleration?

Part Four Vectors

A Vector is a quantity that has magnitude and direction. Examples: § +5 m/s (magnitude is 5, Direction is positive) § 50 mi/hr North (magnitude is 50, Direction is North)

Examples of Vectors § Force § Velocity § Acceleration

Drawing Vectors § Vectors are represented with arrows. § The arrow’s size reflects the magnitude of the vector. § The longer the arrow, the larger the magnitude.

Example: Velocity vector arrows reflect magnitude and direction. The larger the magnitude, the larger the arrow. 90 mph 65 mph

Vector Addition § To use vector addition all vectors must be the same type of vector (Example: Force, Velocity, Acceleration) § Vector arrows can be added together if they are acting in the same direction. § Vector arrows can be subtracted if they act in opposite directions.

Example: § § § These two snowmobiles are trying to move this shack. Both pull with different amounts of force. Their effort adds up for a more powerful force.

Example: Buff Santa runs with a positive velocity. The treadmill moves in the opposite direction at the same speed. Net Velocity = 0 m/s Buff Santa runs at 4 m/s The treadmill moves at - 4 m/s

Advanced Vector Addition § If vectors are not acting in the same direction or opposite directions than simple addition and subtraction won’t work. § Take the example on the following slide…

These vectors are acting at right angles. The canoe is attempting to go straight and the river is attempting to carry it at a right angle. Which direction will the boat actually go and how can we calculate its velocity?

Adding vectors acting at right angles. Velocity of Boat 12 km/h Velocity of river current 5 km/h What is the resultant velocity of the boat?

To add vectors acting at right angles, use the Pythagorean theorem. § a 2 + b 2 = c 2 C A (5 km/h) B (12 km/h) § 122 + 52 = C 2 144 + 25 = C 2 169 = C 2 C = 13 km/h

If the vectors are acting at any angle other than 90°, then trigonometry will be required to find the resultant vector.

Newton’s Three Laws

Newton’s Three Laws § These laws are about FORCE and MOTION § When forces act on objects their motion is affected.

Basics on Force § Before discussing Newton’s laws you need to know some basic information about force. § Force is a vector and can be represented with a vector diagram (or free body diagram). § The unit used when measuring force is the Newton (N).

A classic vector diagram shows the forces acting on an object.

A classic vector diagram shows the forces acting on an object. § The shape of a plane wing § § § (called an air foil) causes air to push up on the plane wing (lift). Gravity pulls the plane down (weight). The engines move the plane forward (thrust). Friction from the air slows the plane down (drag).

Force is measured in Newtons (N) § One Newton (N) is the force needed to accelerate one kilogram (kg) at a rate of one meter per second (m/s 2) § 1 N = 1 kg • 1 m/s 2 § Force (N) = mass (kg) • acceleration (m/s 2)

Calculating Net Force on a box being pushed across the floor. § Applied Forces Force from friction = 5 N Force from push = 8 N § Net Force = 3 N

Typical Free Body Diagram of an object being pushed across a surface. § Fg = Gravity § Fn = Normal force (the force of the ground pushing back against gravity) § Fapp = Applied force (a push or a pull) § Ffrict = Friction

What is the Net Force in these two situations?

Newton’s first law § Every object in a state of uniform motion tends to remain in that state of motion unless an unbalanced force is applied to it. § Inertia – The tendency of an object to resist change in its motion.

Newton’s first law § Moving objects will continue moving while objects at rest will continue to rest. § …Until an unbalanced force changes the object’s behavior. § If the net force is anything but 0 N, the object will accelerate (speed up, slow down, or turn)

What’s the difference between balanced forces and unbalanced forces? § Balanced forces cancel each other. § One finger pushes left and one pushes right. § Because both fingers are pushing with equal force, the car doesn’t move. (it’s motion remains constant)

What’s the difference between balanced forces and unbalanced forces? § Unbalanced forces cause an object’s motion to change. § There is more force pushing to the right than there is to the left. § The forces are not balanced so the car’s motion changes (it starts moving)

A body at rest will remain that way… Until acted upon by a force.

A body in motion will continue until it is acted upon by an outside force.

Notice that in the previous slide the man and the car behaved differently. The wall puts a force on the car causing it’s motion to change. Nothing puts a force on the man (because he isn’t wearing his seat belt) so he continues moving at the same velocity.

Objects in motion stay in motion? § You have probably noticed that objects in motion tend to stop. § If a moving object stops moving, there must be an unbalanced force acting on it.

The box will stop sliding because it is experiencing an unbalanced force.

This cart will continue to move at a constant velocity because it is being pushed with a constant force that balances out the friction force.

Friction § There are many different types of friction but the most common are Static Friction and Kinetic Friction. § Static Friction – Friction between nonmoving surfaces. § Kinetic Friction – Friction between moving surfaces.

There are many types of friction. Forward Velocity (V) Force of Friction (Ff) Rolling friction is a type of Kinetic Friction

Newton’s Second Law § The relationship between an object's mass in kilograms, its acceleration in m/s 2, and the applied force in Newtons is: §F = ma.

Newton’s Second Law § Or… § The heavier something is, the more force is required to accelerate it!!

Force and Acceleration are DIRECTLY CORRELATED! When one goes up, The other goes up.

Force and Acceleration are DIRECTLY CORRELATED!

Mass and Acceleration are INVERSELY CORRELATED! When one goes up, The other goes down.

Mass and Acceleration are INVERSELY CORRELATED!

Newton’s second law and Gravity. § Newton’s second law can be changed to solve for acceleration (a = F/m). § Gravity accelerates EVERYTHING at a rate of 9. 8 m/s 2 § How much force does gravity put on you? (Hint: you need to know your mass in kilograms. 1 kg = 2. 2 lbs)

When surface area is small and mass is large, gravity will accelerate objects at the same rate. To accelerate at them at the same rate gravity must pull the larger mass with more force. Acceleration of gravity 9. 8 m/s 2 A big mass needs a big force

Newton’s second law and Gravity. § Wait a minute… Gravity doesn’t accelerate everything at 9. 8 m/s 2!! § Why do you lie to us Mr. Corbin? !! § What about leaves, and parachutes, and stuff?

Friction from the air slows down falling objects. § As objects fall they accelerate. § The faster they go the more air they must push through per second. § Eventually the air resistance gets so strong that the object stops accelerating. § When a falling object is no longer accelerating it has reached TERMINAL VELOCITY.

Friction from the air slows down falling objects. These parachuters have reached TERMINAL VELOCITY. The force of air friction is equal to the force of gravity.

See how terminal velocity affects the motion of a sky diver http: //www. youtube. com/watch? v= ur 40 O 6 n. QHsw

Newton’s Third Law § Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Action-Reaction Forces § The person pushes the ball causing the ball to fly to the left. § The ball also pushes the person causing her to roll to the right. § NOTE: These forces DO NOT cancel each other out because they are acting on two separate objects.

Newton’s Third Law § These equal and opposite forces are called Action – Reaction forces.

Remember: Action-Reaction Forces act on two separate objects. Bullet puts a force on the rifle Rifle puts a force on the bullet

The planets pull on each other with equal and opposite forces

What is the nature of force? § If you can comprehend this question, you may be a physicist. § Ever wonder what gravity is, or what friction is? § If you are not satisfied with “A force that pulls you down” or “When stuff rubs together, ” then you may care about the next slide.

Scientists have been able to measure and quantify four forces that control everything in the universe.

The Four Universal Forces § Any force acting on an object can be attributed to the following four Universal Forces. § The Gravitational Force § The Electromagnetic Force § The Strong Nuclear Force § The Weak Nuclear Force