(L7) Mechanics 3 - Types of force

Question 1

A large object consists, of course,
of many parts. Gravity will act
downwards on each part. Must
we treat these all separately?

Answer 1

NO! Every object has a point through which its entire weight,
mg, appears to act.

Question 2

What is that point which an objects entire weight,
mg, appears to act.

Answer 2

Centre of gravity, centre of mass.

Question 3

How to find centre of mass?

Answer 3

Hang object from different parts. Find the coss section centre.

Question 4

What is friction?

Answer 4

Force between 2 surfaces, parallel to surface. Friction opposes relative motion.
At rest/constant velocity. f=Fpull

Question 5

What is max friction force equation between 2 surfaces?

Answer 5

f,max = cN
N = normal reaaction, often N=mg but not always
µ is coefficent of friction

Question 6

What is µ? How does it relate to animal joints?

Answer 6

Normally µ < 1, often µ «1, especially for animal joints

Question 7

What does friction depend on?

Answer 7

N - normal force
µ - friction coefficent (nature of surfaces)

Question 8

What is Atmospheric Friction (‘Air Resistance’):

Answer 8

Onjects moving rapdily through the air experience a backwards force (a drag) due to having to push air out of the way.

Question 9

What is terminal velocity?

Answer 9

When an object is dropped it accelerates at g=9.81.
As velocity increases, atmospheric friction increases.

When atmospheric friction = mg, net force is 0 and object will go no faster. As terminal velocity has been reached.

Question 10

What’s another name for Torque?

Answer 10

Moment of the force / Torque
t= F * d

Question 11

What is torque/ the moment?

Answer 11

Measure of tendency to rotate

Question 12

What is the Law of Lever (Principle of moments)?

Answer 12

At balance, in equilibrium:
effort x effort arm = load x load arm

Question 13

What is equation for law of lever (principle of moments)? What’s this equations saying in comparison to torque?

Answer 13

torque = F*d.
so when in equilibrium, torque of one side = torque of other side

Question 14

How does law of lever (principle of moments) relate to clockwise?

Answer 14

At equilibrium, the sum of clockwise moments = the sum of anticlockwise moments

Question 15

What is lever class 1?

Answer 15

fulcrum is between load and effort
load - fulcum - effort

Question 16

What is lever class 2?

Answer 16

effort – load – fulcrum (ankle)

Question 17

What is lever class 3?

Answer 17

load – effort – fulcrum (elbow)