Turning Forces

Question 1

when do thing rotate

Answer 1

when a force is applied to an object that is not a point mass

Question 2

why do frisbees spin when you throw them

Answer 2

because the force is applied at he edge of the disc

Question 3

what is the turning effect of a force called

Answer 3

its moment

Question 4

what are the two variables the moment of the force depends on

Answer 4

• the size of the force

- the perpendicular distance of its line of action from the axis of rotation

Question 5

what is the equation for the moment and its units

Answer 5

• moment = force x distance

- Nm

Question 6

the equation for work done is force x distance which is also Nm. what differentiates moments from this

Answer 6

• moments are a type of vector whereas work done is a scalar (for the most part)
• as moments can cause a clockwise or anti clockwise rotation

Question 7

what condition needs to be met in order for an object to be in rotational equilibrium

Answer 7

the sum of the clockwise moments equals the sum of the anticlockwise moments

Question 8

what is this principle called

Answer 8

the principle of moments

Question 9

what is the method used for finding the centre of gravity of an irregular object

Answer 9

• cut three small holes into the object at positions close to the edges
• pivot the object on a rod through one of the holes
• tie a piece of string with weight on the rod and allow it to come to rest
• draw a line down the object which is aligned with the string
• repeat with the other holes

Question 10

why does this method work

Answer 10

• it uses the principle of moments
• and the concept that weight acting on an object can be thought of as the centre of gravity
• so if the object is moved one side there will be an unbalanced anti / clockwise moment acting upon it
• causing it to rotate and be in equilibrium for that point

Question 11

where should the centre of gravity lie

Answer 11

the point where all three lines intersect

Question 12

how would you test that the point of intersection of the lines is actually the centre of gravity

Answer 12

• try balancing the object on your finger at that point

- if it remains stable and doesnt fall, it probably is the centre of gravity

Question 13

what is different about how you apply forces on a tap when turning it on or off

Answer 13

you usually apply a pair of equal forces in opposite directions at each end of the tap

Question 14

what is this resultant moment called

Answer 14

a couple

Question 15

what would the general equation for this resultant moment be

Answer 15

couple = Fs x Fs = 2Fs

Question 16

what would the worded equation be

Answer 16

couple = the magnitude of each force x perpendicular separation of their lines of action

Question 17

what is the turning effect due to two or more forces called in engineering and what are its units

Answer 17

• a torque

- Nm