CH 4.5 - 4.8 Textbook Notes

Weight

force of gravity, acts at all times and must be counteracted to keep an object from falling

Normal Force (N)

if a force supporting a load is perpendicular to the surface of contact between the load and its support

Do all objects regardless of weight deform when force is applied to them?

yes whether deformation is visible or not

When will an object stop exerting restoring force?

when it is deformed beyond its limit

Increase deformation means…

increase restoring force

When restoring force equals the weight of the load, then…

the net external force on the load equals 0

When can normal force be less than an object’s weight?

when the object is on an incline

When do you divide the force of gravity acting on an object into two components?

when the object on the incline makes an angle theta with the horizontal

What are the two components you divide force into?

force acting perpendicular to plane (𝑤⊥) and force acting parallel to plane (𝑤∥)

Describe the direction and magnitude of 𝑤⊥

equal in magnitude and opposite in direction to normal force N

Describe the acceleration of 𝑤∥

causes acceleration down the incline

Describe the direction of force

it opposes an object’s motion so it acts upward

What is the equation for 𝑤∥?

𝑤∥ = wsin(theta) = mgsin(theta)

Describe the equation for 𝑤⊥

𝑤⊥ = wcos(theta) = mgcos(theta)

Tension

force along the length of a medium

What is the only direction tension can act in?

only in directions parallel to its length

What are some equations solving for T?

T = mg (same as weight w), T = w/2sin(theta) = 𝐹⊥/2sin(theta)

How can you easily exert a large amount of tension?

exert a force perpendicular to the flexible connector

Real Forces

have a physical origin (e.g. gravitational pull)

Fictitious Forces

those that arise because an observer is in an accelerating frame of reference (e.g. one that rotates) or undergoes linear acceleration (e.g. car slowing down)

Inertial Frame of Reference

one in which all the forces are real and one where Newton’s laws apply

How can you determine if a frame of reference is inertial?

based off of if it rotates relative to a known inertial frame

Free-Body Diagram

diagram showing the system of interest and all the external forces, not acceleration of velocity

Give an example of inertial frame of reference and perspective of the observer

an observer in the northern hemisphere is looking at a satellite that is traveling due north above, the observer sees the satellite appearing to experience a force to the west with no physical origin

If the system is accelerating horizontally but not vertically, what can you assume?

F_netx = ma and F_nety = 0

Give an example of how forces produce accelerations

a person on a scale in an elevator going up will show an increase in scale reading because of acceleration, if going down with a negative acceleration the scale reading will be less than the person’s weight until constant downward velocity is reached, if going down in free-fall the person will appear weightless

What are the four distinct forces that account for all known phenomena?

gravitational, electromagnetic, weak nuclear, strong nuclear

In general, how do the four distinct forces act?

they act through the exchange of microscopic carrier particles and characteristics of basic forces determined by the types of particles exchanged

Force Field

explains the existence of action at a distance rather than “physical contact”

Describe weak and strong nuclear forces

both are short range, we don’t experience them directly

determine which nuclei are stable, which decay, and are the basis of release of energy in certain nuclear reactions

determine relative abundance of elements in nature

Describe gravitational and electromagnetic forces

both have infinite ranges

gravitational somewhat weak because it is always active so we barely notice it

electromagnetic nearly cancels for macroscopic objects, but would overwhelm gravitational if they did (all forces except gravity are these)