Newtonian Mechanics

Question 1

What are the SI units for Force?

Answer 1

Newton (kg*m)/s²

Question 2

What is the difference between mass and weight?

Answer 2

mass= kg (scalar and indep. of gravity)

weight= F_g=mg= N (vector)

Question 3

Newton’s 1st Law

Answer 3

A body in motion will stay in motion and a body at rest will stay at rest unless acted upon by an outside force

Question 4

Newton’s 2nd Law

Answer 4

F=ma

Question 5

Newton’s 3rd Law

Answer 5

For every action there is an equal and opposite reaction

F_{BA (on A from B)} = - F_{AB (on B from A)}

(ie. rockets)

Question 6

Gravity

Answer 6

F=(Gm₁m₂)/r²

• weakest of 4 forces
• always attractive

Question 7

Torque

Answer 7

Torque=rFsinØ

Torque=r_†F

• application of force at some distance (lever arm) to fulcrum
• Torque is measured in N*m not J

Question 8

Centripital Force/ accel

Answer 8

F_c=(mv²)/r

a_c=v²/r

• F_c is force necessary to keept an object of mass, m, in orbit with radius, r
• when F_c no longer working on an obj, objcect will assume tangential pathway
• F_c always directed towards center of circle

Question 9

frictional force

Answer 9

• always opposes an objects motion

Question 10

static friction

Answer 10

F_s=F_applied

F_{s, max}=μ_sF_n

Question 11

kinetic friction

Answer 11

F_k=μ_kF_n

• kinetic friction is a constant value, whereas static friction is not

Question 12

which is always greater μ_s or μ_k?

Answer 12

μ_s

• therefore, F_{s, max} will always be greater than F_k

Question 13

Translational equilib

Answer 13

• vector sum of all forces in x and y direction acting on an obj is zero
• therefore no accel, constant motion (could be non-zero velocity just constant)

Question 14

Rotational Equilib

Answer 14

• vector sum of all torques acting on an object is zero
• “clocks are negative” CW= - CCW= +
• choose pivot point in order to eliminate a torque

Question 15

Do you weight more or less when going up in an elevator?

What about when the elevator is going down?

Answer 15

• Elevator going up: You weigh more on an elevator accelerating up because F=mg + ma, where a is the acceleration of the elevator (same as elevator decelarating on way down)
• You weigh less on an elevator accelerating down because F=mg - ma, where a is the acceleration of the elevator (same as elevator decelarating on way up)

Question 16

What is the component of gravity going down an inclined plane?

Answer 16

“sin slide”

In a non-moving object on an inclined plane: normal component of gravity = normal force; parallel component of gravity = static friction

Question 17

In an obj going down an inclined plane at constant velocity, is the parallel component of gracvity equal to, less than, or greater than the kinetic friction?

• What about when obj begins to slip down plane?
• What about when obj accel. down plane?
• When push an obj up a plane what forces are you overcoming?

Answer 17

Parallel component of gravity = kinetic friction

(b/c constant velocity = no acceleration = no net force)

• object that begins to slip on the inclined plane: parallel component of gravity > static friction
• object that accelerates down the inclined plane: parallel component of gravity > kinetic friction
• When you push an object up an inclined plane, you need to overcome both the parallel component of gravity and friction.

Question 18

What are the four forces in order of strength?

Answer 18

1. The strong force (nuclear force) is the strongest force, but only acts at subatomic distances-binds nucleons
2. Electromagnetic force: acts at observable distances. Binds atoms together. Allows magnets to stick to your refrigerators. Responsible for the fact that you’re not falling through chair right now
3. Weak force: Responsible for radioactive decay
4. Gravity: Responsible for weight (not mass!) and planet orbits

Question 19

How does the F_s change as Ø increases (for a box on an incline)?

Answer 19

F_s=F_applied=mgsinØ

therefore as Ø increases, F_app increases so F_s increases

(easier to start moving on a steep slope, harder to start moving on a shallower slope)

MISTAKE is that F_s,max=mgcosØ so then as Ø increases, F_s,max decreases

Question 20

g = ?

Answer 20

g= Gm_e/r_e²

F_g=Gm_em/r_e²

• two masses exert an attractive force invserly proportional to square of dist b/w them

Question 21

Uniform cicular motion

frequency

Answer 21

Question 22

What is the relationship b/w KE and frequency in uniform cicular motion

Answer 22