Newtons Laws Forces cause changes in motion Review

Newton’s Laws Forces cause changes in motion!

Review � Force is: Push or pull � Vectors vs. scalars � Vector diagrams � Adding vectors � Balanced and unbalanced forces � Speed and Acceleration � Newton’s 1 st Law � SI Units: mass and force

Newton’s 1 st Law � All objects remain at rest OR continue in a straight line path at constant speed � UNLESS acted on by a net force � The law of inertia � Its all about equilibrium (balanced forces)

Inertia � “inertia” = an object’s tendency to keep doing what it was already doing � inertia = mass › Inertia is measured by an object’s mass › Mass is the amount of matter contained in an object � More mass = more resistance to changes in motion = more inertia

Mass is not Weight � Mass = amount of matter contained within an object › SI unit: kilogram (kg) � Weight = amount of gravitational pull on an object › SI unit: Newton (N) › Due to amount of mass: proportional but not equivalent › 1 N = (1 kg)*(1 m/s²) Derived from Acceleration due to gravity: � w = mg Newton’s 2 nd 10 m/s² Law weight mass

Example: mass to weight � How much does a 0. 02 kg fish weigh on Earth? �w = mg � w = (0. 02 kg)*(10 m/s²) � w = 0. 2 N � 1 N = 1 kg*m/s²

Example: weight to mass � How much mass is contained in a 2 N apple? � w = mg

Newton’s 2 nd Law: WHY things accelerate Rate at which an object speeds up or slows down � Its Resultant of all forces acting on the object Mass of the object all about net force (unbalanced forces)

Units

Free-Body Diagrams AKA vector diagrams � Used to figure out what the net force on an object is � It’s just a diagram of the forces acting on the object � Draw the forces: diagram the vectors › Each force = an arrow › Length of arrow = magnitude of the force › Direction of arrow = direction of the force

Free Body Example Imagine you are pushing a huge box across the floor But the box is too heavy, so your friend comes to help You push with a force of 25 N, and your friend pushes with a force of 15 N. The crate weighs 100 N.

The crate is a dot � Start identifying forces: � Your push: 25 N � Your friend’s push: 15 N � The weight of the crate: 100 N � The normal force on the crate: 100 N � New vocabulary: Normal force = support force

Example: Newton’s 2 nd � You push a crate of oranges across a frictionless floor with a force of 25 N. � If the crate has a mass of 5 kg, what will be its resulting acceleration (while you push it)?

1 st step: free body diagram to determine net force 2 nd step: Newton’s 2 nd Law equation Support force: 50 N 3 rd step: plug what you know into the equation Your push: 25 N Weight of crate: w = mg = 50 N 4 th step: evaluate

Example 2: Newton’s 2 nd Law and net force Imagine you are pushing a huge box across the floor But the box is too heavy, so your friend comes to help You push with a force of 25 N, and your friend pushes with a force of 15 N. The crate has a mass of 10 kg. What is the acceleration of the crate?

1 st step: free body diagram to determine net force Your push: 25 N Your friend’s push: 15 N 2 nd step: Newton’s 2 nd Law equation Support force: 100 N 3 rd step: plug what you know into the equation Weight of crate: w = mg = 100 N 4 th step: evaluate

Extend your understanding � You and your friend push that box from rest for 4 seconds. How fast is it going at the end of your 4 second push? v = (0 m/s) + (4 m/s²)(4 s) � v = 16 m/s � How far has it gone during the 4 second push? � x = (0 m/s)*(4 s) + ½(4 m/s²)*(4 s)² � x = 32 m �

Solids: Friction Is a force: just like any other � Acts on materials that are in contact with each other � Always acts in a direction to oppose motion � › Actual motion › Intent to move Caused by irregularities between surfaces and weak chemical bonds � Amount of friction depends on 2 factors: � › Type of materials › Amount of force holding the 2 surfaces together: Normal force!

Fluids: Friction Is a force: just like any other Acts on materials that are passing through a fluid � Always acts in a direction to oppose motion � Amount of friction depends on many factors: � � › › Roughness of the object Surface area of the object, perpendicular to motion Speed of the object Density and viscosity of the fluid

Example: Newton’s 2 nd law and friction � You are skydiving, and you weigh 600 N. At one moment in the dive, the force on you due to air resistance is 50 N. At that moment, what is your acceleration? Air resistance: 50 N Net force on you: 550 N (down) Your weight: 600 N

Imagine you are pushing a huge box across the floor But the box is too heavy, so your friend comes to help AND this is real life, and there is friction You push with a force of 25 N, and your friend pushes with a force of 15 N. Friction between the floor and the crate is 5 N The crate has a mass of 10 kg. What is the acceleration of the crate?

1 st step: free body diagram to determine net force Your push: 25 N 2 nd step: Newton’s 2 nd Law equation Support force: 100 N 3 rd step: plug what you know into the equation Friction: 5 N Your friend’s push: 15 N Weight of crate: w = mg = 100 N 4 th step: evaluate

Newton’s rd 3 Law � “For every action force there is an equal and opposite reaction force. ” › Forces always occur in pairs! › If object A pushes on object B, object B pushes back on object A with an equal force but in the opposite direction › This happens immediately and every time any object exerts any type of force › Action force acts on object B; reaction force acts on object A. the action/reaction forces never both act on object A (or both on object B)

Seems Easy… � Action: › Object A pushes on object B � Reaction: › Object B pushes back on object A › Switch the nouns! Example: A baseball player hits a ball with a bat. What is the reaction? � Action: the bat pushes on the ball � Reaction: the ball pushes back on the bat �

More examples Its not so easy to see the reaction when the objects aren’t every-day things… use the formula! � Action: your weight � Whaaat? � Your weight = the Earth’s gravitational pull on you � Action: The Earth’s pull (downward) on you � Reaction: You pull (upward) on the Earth �

Why it is confusing � Force versus effect of the force (acceleration) � Example: A 5 kg bat pushes with 5 N on a 0. 2 kg ball � Action: Bat pushes on ball with 5 N force � Effect: � a = F/m � = (5 N)/(0. 2 kg) � = 25 m/s² � The 0. 2 kg ball accelerates at 25 m/s²

� (Action: Bat pushes on ball with 5 N force) � Reaction: Ball pushes back on bat with 5 N force � Effect: � a = F/m � = (5 N)/(5 kg) � = 1 m/s² � The 5 kg bat accelerates (if you immediately dropped it) at 1 m/s²

The idea Newton’s 2 nd Law Same force but big mass = tiny accel. O R Same force but tiny mass = big accel.

� An apple weighs 1 N � Action: the Earth pulls down on the apple with a force of 1 N � Effect: � a = F/m � = (1 N)/(0. 1 kg) � = 10 m/s² � The 0. 1 kg apple accelerates downward at 10 m/s²

� Action: the Earth pulls down on the apple with a force of 1 N � Reaction: the apple pulls up on the Earth with a force of 1 N � Effect: � a = F/m � = (1 N)/(6, 000, 000, 000 kg) � = 0. 00000000000017 m/s² � the 6, 000, 000, 000 kg Earth accelerates upward at 0. 00000000000017 m/s²