Forces and Newton's Laws of Motion

Kinematics

-study of an object’s motion

Kinematics terms

-time

-displacemnet

-position

-speed

-velocity

0acceleration

Dynamics

study of cause of object’s motion

Dynamics term

force

time

mass

acceleration

Force

push or pull on an object

The unit of force is

Newton (N) which is equal to (kg)(m/s²)

Exmaples of everyday situations-Force

-pulling on a rope

-pushing a cart

-using braces to straighten teeth

System

-refers to the object or object’s under consideration

• sometimes multiple objects are moving together

• Ex: a dog carrying their favorite toy on a walk

External force

force acting on system caused by something outside of the system

Ex- gravity or applied force (like someone pushing something)

Net force

-sum of all external froce

• also referred to as resultant force

Newton’s first law

when no net force acts on an object, the acceleration of the object is zero (rest or constant velocity)

Intertia

refers to the property of an object to continue the same motion or remain at rest

newton’s second law

-we found a way to quantify net acceleration and force

• aceleration caused by forces is prpoportional to force and inverselyproportional to mass

• a=Fnet/m

Newton’s third law

we found a symmetry among forces between two objects

An object experiences no acceleration. What is true of the object

-no forces act on the object

-forces act on the object, but the forces cancel

If a fly collides with the windshield of a fast moving bus, which experiences an impact force with a larger magnitude?

the same force is experienced by both

Common forces

a result of newton’s 3rd law

Restoring forces

an object always returns to its original position

resistive forces

resists the motion of an object

Restoring force: Gravitational force

when objects leave the earth’s surface, gravity causes objects to return to the surface

Mass (m)

-the amount of stuff (matter) that makes up an object

-scalar

-does not depend on position

-unit-kg

weight

-gravitational force

-vector

-depends on position

-unit- newton or pound

Two common forces as a result of newton’s 3rd law

-normal force

-tension

Normal force

reaction force caused by surface to support system

-perpendicular to the supporting surface opposite in direction of weight or applied force

-Fn or n

Tension

reaction force caused by pulling on a string-like

-acts along medium opposite in direction of weight or applied force

-FT or T

A banana is suspended from the ceiling

if you create a free body diagram of the banana, which forces are present?

tension and weight

The string attached to the banana was cut by a bird’s beak allowing the banana to fall on the floor

if we were to create a free body diagram of the banana laying on the floor, which forces are present?

normal force and weight

Hooke’s law

Fs=kx

Fs

spring force

k

spring constant

x

spring stretch or compression

When the block is displaced to the right of equilibrium

the force exertered by the spring acts to the left

When the block is at its equuilibrium position,

the force exerted by the spring is zero

When the block is displaced to the left of equilibrium

the force exerted by the spring acts to the right

Examples of other objects that compress and stretch

-bones

-metal rod

-wood column

-boat rope

-string instruments

in the spring and hanging mass exampl,e what would happen to the value of the spring constant if we doubled the mass of the hanging mass but we stretched length stays the same?

spring constant would increase

In the spring and hanging mass example, what would happen to the value of the spring constant if the streyched length doubled but the mass of the hanging mass stays the same?

spring constant would decrease

Stress

ratio of force to cross-sectional area

Strain

ratio of change in length to original length

If an object’s ratio of stress vs. strain stays in the Elastic region

the object will return to its original shape

If an object’s ratio of stress vs strain stays in the plastic region,

the object will be permanently deformed but will not fail to support the load

Most metals are ductile which means

they will deform and still be able to support a load

Bones are brittle which means

a bone will fracture is stress vs strain is outside of elastic region

Friction

-opposes motion of system due to surfaces in contact

-proportional to normal force of system

-depends on the surface material

Kinetic friction

during motion

Static friction

from rest

Resistance force casued by surfaces in contact

kinetic friction

static friction

Drag force

-resistance force of object or system moving in a fluid

-proportional to velocity of object

magnitude of drag force depends on

shape of object

size of object

velocity of object

fluid objects is in

Terminal velocity

-for an object or system in falling due to the force of gravity, at a certain point during the motion, the object will reach a max velocity

-this occurs when the drag force is equal in magnitude to the weight (gravitational force) of the object/system

-why does this happen?

• drag force depends on velocity of object

• If velocity increases since the object is falling (with g) the drag force will increase