Unit 3 - Dynamics

How Forces Affect Motion

Unit 3 Part 1 – Newton’s Laws of

MotionPhysics Book Chapter 4

Conceptual Physics Book Chapters 4-6

Motion Theories The old way looked at the what The new way looked and looks at the why

4th century BC - Aristotle Did not experiment Used only logic and observation Two states of motion natural and violent Earth at rest everything else moves in complete

circles

Motion Theories1600 Galileo

Used experiments with rampsDiscussed friction came up with the idea of

inertia: property of all matter

1670 NewtonBuilt off of Galileo’s idea

1st Law of Motion: The Law of Inertia

Newton said, “An object at rest wants to stay at rest unless acted on by a net (unbalanced) force, and an object in motion wants to stay in motion in a straight line at a constant speed unless acted on by an net (unbalanced) force.

Forces do not cause motion they change motion

Inertia is a property of all matter

1st Law of Motion: The Law of Inertia

Inertia is related to the mass of an objectMass is the measure of inertia or how

easy or hard it is to change an objects positionMass can also be how much stuff makes up

an object.

Units of mass kilograms or slugs1 slug = 14.59 kg

1st Law of Motion: The Law of Inertia

Which one has more inertia?

1st Law of Motion: The Law of Inertia

http://science360.gov/obj/video/70fadaa8-c3d4-4132-ba1f-c98be5caeb14/science-nfl-football-newtons-first-law-motion

Forces and Free Body DiagramsA force is defined as any

push or pullTo display forces free

body diagrams are used.Include only the forces

acting on the object.Force is a vector! Use an

arrow to indicate a force.

Forces and Free Body DiagramsFit in two main categories

Contact forces between 2 objectsShoot a basketballPush a shopping cartFriction

Non-contact or at a distanceGRAVITYElectricityMagnetism

Forces and Free Body DiagramsEquilibrium

All forces are equal up = down , lefts = rightsIf any one force is not equal then there is a

non-zero net force on the object

Forces and Free Body DiagramsEquilibrium involving gravity

Proportions

Directly ProportionalMeans two quantities are on opposite sides of the equal sign, either both in the numerator or both in the denominator

In the equation , s and d are directly proportional

What happens to s when d increases?When d decreases?

Proportions

Indirectly ProportionalMeans two quantities are on opposite sides of the equal sign, one in the numerator and the other in the denominator

In the equation , s and t are indirectly proportional

What happens to s when t increases?When t decreases?

Newton’s 2nd Law: Law of AccelerationNewton came up with is in 1680Law of acceleration is a proportion

Deals with the sum of the forces, acceleration and mass

is the equation rearranged.

Newton’s 2nd Law: Law of Acceleration“When a net external force acts on an object of

mass m, there is an acceleration that results.”The acceleration is directly proportional to the

force applied and inversely proportional to the mass.

Newton’s 2nd Law: Law of Acceleration

Units of force are Newtons and pounds

A Newton is a kg m/s2

A pound is a slug ft/s2

Newton’s 2nd Law: Law of Acceleration

https://www.youtube.com/watch?v=iwP4heWDhvw

Example Problem

Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other person applies a force of 395 N in the same direction. A third force of 560 N also acts on the car, but in the opposite direction. (This force is due to the friction of the tires on the pavement.) Find the acceleration of the car.

Newton’s 3rd Law of Action/Reaction

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 there is an equal and opposite reaction”Hammer hits nail and nail stops hammerSwimming: you push the water backward

and the water pushes you forward

Newton’s 3rd Law of Action/Reaction

If this is true, then why doesn’t the cannon move as far as the cannonball?

Remember that and that a and m are indirectly proportional.The cannon has larger m, smaller a.The cannonball has smaller m, larger a.

Weight and Normal ForceWeight is the force of gravity on an

object.It depends on where an object is. (On the

moon, in space, at Earth’s sea level, at high altitude…)

It is directly proportional to mass, but not the same thing!

Weight and Normal ForceMore About Weight

(g=9.8 on Earth) This is really still the equation!The weight vector always points DOWN in a free-body diagram.

The SI unit for weight is the Newton.

Weight and Normal ForceThe normal force is the perpendicular

component of the force that a surface exerts on an object with which it is in contact.Does NOT necessarily point directly upwards!Equal and opposite of the weight IF no other

forces are acting and the object is on a horizontal surface.

Think of “atomic springs” in the surface compressing to produce the normal force.

Newton’s 3rd Law of Action/Reaction

https://www.youtube.com/watch?v=91QYouih4bQ

Friction Friction is the parallel component of the force

that a surface exerts on an object with which it is in contact.

Two types: If the object IS NOT moving, then the force is

called static friction. If the object IS moving, then the force is called

kinetic friction.

Friction The FUN equation to calculate the frictional force:

µ is the coefficient of friction and depends on the materials involved and whether it is static or kinetic friction. Rubber on Dry Concrete: , Teflon on Teflon: , The higher the coefficient, the greater the friction.

Friction Static friction: calculates the maximum static

frictional force. If a force exerted is greater than this, the object will start

moving! If the force exerted is less than this, the object remains

stationary and the frictional force is equal to the force exerted.

Kinetic friction:

True Weight vs. Apparent Weight

The apparent weight is the force that an object exerts on a scale.

If the scale and the object are either at rest or moving at a constant speed (a=0), then True=Apparent.

If the scale and object are accelerating, then apparent weight will be different!

True Weight vs. Apparent Weight If the elevator is accelerating upwards, the apparent weight will be more than the true weight. (You get a heavier feeling as the elevator starts to move up.)

If the elevator is accelerating downwards, the apparent weight is less than the true weight. (You feel lighter as the elevator starts to move down.)

If the elevator is in free fall ( then apparent weight is 0. (weightlessness!)