Transcript Work done (J) - MrSimonPorter

Do now!
Can you read
through the
momentum
Powerpoint
slides?
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-1 (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

A tennis ball (0.3 kg) hits a racquet
at 3 m/s and rebounds in the
opposite direction at 6 m/s. What
impulse is given to the ball?
Example

A tennis ball (0.3 kg) hits a racquet
at 3 m/s and rebounds in the
opposite direction at 6 m/s. What
impulse is given to the ball?
3 m/s
-6 m/s
Example


A tennis ball (0.3 kg) hits a racquet at 3 m/s and
rebounds in the opposite direction at 6 m/s. What
impulse is given to the ball?
Impulse = mv – mu =
= 0.3x-6 – 0.3x3
= -2.7kg.m.s-1
3 m/s
-6 m/s
Do then
tried
some
questions
Today’s lesson
Learn what “work” is!
 Learn how to calculate work
 See who can do the most work!
 If we have time
we will look at power.

A reminder


Energy cannot be made or
destroyed, only transformed
(changed).
Energy is measured in Joules.
That’s me!
Work
In physics, work
has a special
meaning,
different to
“normal” English.
Work
In physics,
work is the
amount of
energy
transformed
(changed)
when a force
moves (in the
direction of
the force)
Work
For example, if Mr Porter pushes a
table, he is doing work against the
friction force of the table against
the floor.
Calculating work
The amount of work done
(measured in Joules) is equal to the
force used (Newtons) multiplied by
the distance the force has moved
Force (N)
(metres).
Distance travelled (m)
Work (J)= Force(N) x distance(m)
W = Fscosθ
Important
The force has to
be in the
direction of
movement.
Carrying the
shopping home
is not work in
physics!
What if the force is at an
angle to the distance moved?
Work = Fscosθ
F
θ
s
Lifting objects
When we lift objects,
we are doing work
because a force is
moving.
Force
Distance
moved
Lifting objects
Our lifting force is equal to the
weight of the object.
Lifting force
weight
Let’s look at
some
examples
Work done (J) = Force (N) x distance (m)
A woman pushes a car with a force of 400
N at an angle of 10° to the horizontal for
a distance of 15m. How much work has
she done?
Work done (J) = Force (N) x distance (m)
A woman pushes a car with a force of 400
N at an angle of 10° to the horizontal for
a distance of 15m. How much work has
she done?
W = Fscosθ = 400x15x0.985
W = 5900 J
Work done (J) = Force (N) x distance (m)
A man lifts a mass of 120 kg to a height
of 2.5m. How much work did he do?
Work done (J) = Force (N) x distance (m)
A man lifts a mass of 120 kg to a height
of 2.5m. How much work did he do?
Force = weight = 1200N
2.5
Work = F x d = 1200 x
Work = 3000 J
How much work can you do?
Can you copy this please?
Name
Mass
(kg)
Force
(N)
Distance
(m)
Work of
one lift
(J)
# of lifts in
1 min
Total work (J)
Arm curls
Force required = weight of object = mass (kg) x 10
distance
Off you go!
Name
Mass
(kg)
Force
(N)
Distance
(m)
Work of
one lift
(J)
# of lifts in
1 min
Total work (J)
Power!
Power!
Power is the amount of energy
transformed (changed) per second.
It is measured in Watts (1 Watt = 1
J/s)
Power = Energy transformed
time
Power
For each of the people in your table,
can you calculate their power?
Work done in stretching a spring
Work done in stretching a spring
Work done in strectching spring = area under graph
F/N
x/m
Let’s do some more work!
Let’s drown
this dog
we’ve
found
Pull!
Coursework!

Hooke’s law and work