Download Lesson 13 Moments – Turning forces

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Can you read the
“impulse” sheet you
stuck in last lesson?
Last lesson
Impulse
Impulse
Ft = mv – mu
The quantity Ft is called the impulse, and of
course mv – mu is the change in
momentum (v = final velocity and u = initial
velocity)
Impulse = Change in momentum
Units
Impulse is measured in N.s (Ft)
or kg.m/s (mv – mu)
5 m/s
Impulse
-3 m/s
Note; For a ball bouncing off a wall, don’t
forget the initial and final velocity are in
different directions, so you will have to
make one of them negative.
In this case mv – mu = 5m - -3m = 8m
Example
• Dylan punches Henry in the face. If
Henry’s head (mass 10 kg) was initially at
rest and moves away from Dylans fist at 3
m/s, and the fist was in contact with the
face for 0.2 seconds, what was the force
of the punch?
Example
• Dylan punches Henry in the face. If Henry’s head (mass 10
kg) was initially at rest and moves away from Dylans fist at
3 m/s, and the fist was in contact with the face for 0.2
seconds, what was the force of the punch?
• m = 10kg, t = 0.2, u = 0, v = 3
Example
• Dylan punches Henry in the face. If Henry’s head (mass 10
kg) was initially at rest and moves away from Dylans fist at
3 m/s, and the fist was in contact with the face for 0.2
seconds, what was the force of the punch?
• m = 10kg, t = 0.2, u = 0, v = 3
• Ft = mv - mu
Example
• Dylan punches Henry in the face. If Henry’s head (mass 10
kg) was initially at rest and moves away from Dylans fist at
3 m/s, and the fist was in contact with the face for 0.2
seconds, what was the force of the punch?
• m = 10kg, t = 0.2, u = 0, v = 3
• Ft = mv – mu
• 0.2F = 10x3 – 10x0
Example
• Dylan punches Henry in the face. If Henry’s head (mass 10
kg) was initially at rest and moves away from Dylans fist at
3 m/s, and the fist was in contact with the face for 0.2
seconds, what was the force of the punch?
• m = 10kg, t = 0.2, u = 0, v = 3
• Ft = mv – mu
• 0.2F = 10x3 – 10x0
• 0.2F = 30
Example
• Dylan punches Henry in the face. If Henry’s head (mass 10
kg) was initially at rest and moves away from Dylans fist at
3 m/s, and the fist was in contact with the face for 0.2
seconds, what was the force of the punch?
• m = 10kg, t = 0.2, u = 0, v = 3
• Ft = mv – mu
• 0.2F = 10x3 – 10x0
• 0.2F = 30
• F = 30/0.2 = 150N
Today’s lesson
• Know what the turning effect of a force is
• Know that the moment of a force depends
on force and the distance from the pivot
• Know the principal of moments
What happens when you try to
open a door with one finger?
Where do you push?
Hi, my
name is
Mr Stick
hinges
Do you push near the hinges?
I can’t do
it!
Do you push far from the hinges?
That’s
easier!
Moments – Turning forces
The turning effect of a force
depends on two things;
The size of the force
Obviously!
The turning effect of a force
depends on two things;
The distance from the pivot (axis of rotation)
Not quite
so obvious!
Axis of rotation
Turning effect of a force
The turning effect of a force is called the
moment of the force
The moment is calculated by multiplying
the force by the distance from the pivot
Turning effect of a force – moment
of a force
Moment (Nm) = Force (N) x distance from pivot (m)
Note the unit is Nm, not N/m!
A simple example!
50 N
0.15 m
nut
spanner
(wrench)
A simple example!
Moment = Force x distance from pivot
Moment = 50 N x 0.15 m
50 N
Moment = 7.5 Nm
0.15 m
nut
spanner
(wrench)
What do you do if the nut won’t
move and you can’t push harder?!
50 N
0.15 m
nut
spanner
(wrench)
Get a longer spanner!
Moment = Force x distance from pivot
Moment = 50 N x 0.25 m
50 N
Moment = 12.5 Nm
0.25 m
nut
spanner
(wrench)
More than one force
Take an uneven see-saw for an example
Do you think we’ll
be safe in this
power point?
pivot
It’s not
looking
good!
If the see-saw is balanced, what must be the
weight of the dog on the left?
1.2 m
2.2 m
pivot
?N
110 N
The force on the left is trying to turn the seesaw anticlockwise about the pivot
1.2 m
2.2 m
pivot
?N
110 N
The force on the right is trying to turn the
see-saw clockwise about the pivot
1.2 m
2.2 m
pivot
?N
110 N
If the see-saw balances, the turning effect
anticlockwise must equal the turning effect
clockwise
1.2 m
2.2 m
pivot
?N
110 N
Anticlockwise moment = clockwise moment
Anticlockwise moment = clockwise moment
? X 1.2 = 110 x 2.2
? X 1.2 = 242
? = 242/1.2
? = 201.7 N
1.2 m
2.2 m
pivot
?N
110 N
Anticlockwise moment = clockwise moment
Rotational Equilibrium
When there is no
resultant moment on
an object (when
anticlockwise
moments = clockwise
moments) we say the
object is in rotational
equilibrium.
YouTube - Alan Partridge's
Apache office
Whew! We
seemed to have
survived the
power point
pivot
Don’t
speak too
soon!
pivot
BO
pivot
BOMB
pivot
BOMB
pivot
pivot
Nice one!
pivot
You’ll have to
do some
questions
now, or he’ll
kill me too!
pivot
Read pages 54
and 55 and make
notes. Then try
the questions that
Mr Porter is giving
you.
pivot