Mechanics 2 and 3

Question 1

Is centripetal force a physical force?

Answer 1

No it is not, it is the result of the net physical force. it does not change speed and acts inward. Does not change speed

Question 2

What are the two changes in speed:

Answer 2

Ac=(v^2)/r

Fc=(mv^2)/r

Question 3

If a car is going around a circular turn of 12m , and the frictional coefff. is 0.3, wha is the maximum speed at which the car can take the turn?

Answer 3

What force is keeping you in the circle? 
Static Friction (remember rolling is static) 

So what is the static friction in this problem?

(Us)(mg)=(mv^2)/r
cancel mass out
v=6

Question 4

Centripetal force is another term for

Answer 4

net force that takes a “circular form”

Question 5

In whole blood fractionation, in which blood is centrifuged to separate it into it’s component parts, how does distance d from the axis of rotation compare to centripetal force felt by an equal mass of plasma at distance d/2?

Answer 5

Ac=v^2/R

Fc=M(v^2)/R

make sure you account for change of radius on the change on the velocity.

After solving, you realize Fc is linearly related to distance from center. So 2 times greater radius=2 times greater force.

Question 6

What is required to unscrew tight bolt?

Answer 6

Pull at at end of the lever,

proper angle

Question 7

What is the formula for torque?

Answer 7

torque=rFsin (theta)

Question 8

Is torque a vector?

Answer 8

Yes, but it’s easy to think of torque as rotation.

Question 9

Torque is maximized when theta in sin(theta) is

Answer 9

90 degrees

Question 10

If nothing is accelerating, net torque is:

Answer 10

zero

Question 11

Two people are pushing on a 1m door, one is trying to open it, another is trying to close it. Guy opening is at 0.5 m, guy trying to close the door is at 1m. The door is motionless, so what force is the person trying to open door pushing?

Answer 11

you also have Force of wall against the guy opening the door in order to not have the door rip apart.

but since it is at the pivot, so the torque force is zero.

torque=r sin(theta) (Force)

rF (sin theta)=rF(sin theta)
(1)(50N)(1)=(0.5)(F)
F=100N

Question 12

What is an alternate to the torque formula?

Answer 12

Torque=lF
l= shortest distance from the pivot to the line of action of force.

l=rsin(theta)

Question 13

How do you keep track of two or more torques, how do you keep track of direction?

Answer 13

1) Locate the pivot

2) Then compare torques that at clockwise to those that act counterclockwise.

Question 14

Ex) Suppose you go down a Sui Slide and your friend goes down a meandering slide, who gets to the bottom first?

Who is going fastest at the bottom?

Assume same initial height, final height, same initial speed.

Answer 14

Who is going fastest at the bottom?

Question 15

What is the formula for Work?

Answer 15

work: Fdcos (theta)

Force must be constant and specified

units are newtons times meters =joules

Question 16

Describe the sign of work:

Answer 16

Your force has a component which goes in the same direction as displacement

Question 17

Describe the work done by:

Answer 17

kinetic Friction:
negative

Centripetal Force:
No work

Normal Force:

No work (usually)

Question 18

Gravity is a ——- force

Answer 18

conservative force

Question 19

What does conservative force mean?

Answer 19

It means it only is concerned with the initial and final heights of the object, not the path the object follows.

Question 20

What is the formula for Work done by gravity?

Answer 20

Wg=mgh

Question 21

A tension force in a rope is 200N directed at 45 degrees above horizontal pulls a 50 kg crate to the right for 5 m. If Uk=0.2, find the work done by each of the forces acting on the object and the net work done on the object.

Answer 21

The overall forces on this block are Force of friction, and, tension force, Force of Gravity, and normal force.

Gravity and normal force presumably cancel each other out, as the block is neither crashing through the table or floating.

So network=work of the rope +work from Kinetic Friction+ work normal+work gravity

=340 overall.

Question 22

What does power mean?

Answer 22

Power is the rate at which work is done

Question 23

What are units of power?

Answer 23

Watts/ (kg m^2)/s^3

Question 24

For a constant force parallel to a constant velocity:

Answer 24

P=(W/t)=(Fd/t)=F(d/t)=F(v)

Question 25

What is kinetic energy?

is it scalar or vector?

Answer 25

It is the energy of motion

It is scalar

Question 26

What are the formula for kinetic energy?

Answer 26

It would be (0.5)(mv^2)

Question 27

How are work and kinetic energy related?

Answer 27

Work can be seen as the change in kinetic energy (final-initial)

Question 28

How much work do you need to stop a 1000kg car moving at 20m/s?

Answer 28

We know final kinetic energy is 0, so we have to solve for initial kinetic energy, and subtract form zero

Question 29

When calculating work done by gravity, what is important to keep in mind?

Answer 29

it is important to keep in mind gravity is a conservative force.