Transcript Motion graphs – Velocity/time - Science

Topic – Physics 2a
Momentum
Topic – Physics 2a
What is momentum?
• Think about a foam ball
and a cricket ball hitting a
window.
• Both are about the same
size so why would only
one break the window?
Topic – Physics 2a
What is momentum?
• Momentum is the mass of an object times the
velocity of that object
•  = mv
• p= momentum in kgms-1 ; m = mass in kg ;
= velocity in ms-1
v
Topic – Physics 2a
Link to equation
• The cricket ball can have a mass of about
160 g while the foam ball has a mass of
about 20 g. If both are thrown at 15 ms-1
you can quickly see that there is a large
different in momentum and the cricket ball
has a much larger effect on the window.
 cricket ball = mv
= 0.160 x 15
= 2.4 kgms-1
 foam ball = mv
= 0.020 x 15
= 0.3kgms-1
Topic – Physics 2a
Change in Momentum
• You are leisurely riding home on your bike when
it starts to rain. Quickly you increase your
velocity to limit the time you get wet. Here you
have experienced a change in momentum. The
change in anything is the final minus the initial,
• so
• change in momentum = final momentum - initial
momentum or
•  = mv - mu
Topic – Physics 2a
Example
• Example:
• While running in the park you are initially
travelling at 3.50 ms-1 east. A dog runs out and
starts to chase you so you increase your velocity
to 6.00 ms-1 east. Assuming that your mass is
60.0 kg, what was your change in momentum?
• m = 60.0 kg
• u = 3.50 ms-1
• v = 6.00 ms-1
•
 = mv - mu
= m (v – u) = 60.0 (6.00 – 3.50)
= 60.0 x 2.5
 = 1.50 x 102 kgms-1 east
Topic – Physics 2a
A change in momentum can mean
a change in direction
• Example:
• The 20.0 g foam ball hits the window at 15.0 ms-1 north and
rebounds at 15.0 ms-1 south. What is the change in momentum?
• m = 0.020 kg
•
•
•
•
•
•
u = 15.0 ms-1 north
v = 15.0 ms-1 south
V = (v – u)
= (15.0 ms-1 south – 15 ms-1 north
= (15.0 ms-1 south + 15 ms-1 south)
= 30 ms-1 south
 = m (v – u) = m V
= 0.02 x 30
 = 0.600 kgms-1 south
Remember you cannot
subtract a vector – it must be
added with the direction
reversed!
Topic – Physics 2a
Impulse
• If you are unlucky enough to have a car accident, you
will experience a force on you that could cause injury or
death. Car designers try to make cars as safe as
possible, so to do this they try to decrease the force
acting on you. In a car crash your velocity changes so
you undergo a change in momentum, this can be rewritten as
•
Ft = mv - mu
F∆t=m∆v
• Ft is known as impulse (units Ns (Newton seconds)
and is equal to the change in momentum:
• Impulse = Ft
Topic – Physics 2a
Increase impact time
• While the impulse (also
known as impulsive force)
can’t be changed, the time of
the crash can.
• If the time over which the
crash takes place is
increased, the force must be
decreased (simple maths).
• To increase the time of the
crash, car designers now
design cars that crumple – this
increases the time for the car
to stop in a crash, thus
decreases the force and
possibly saving you from injury
or death
m( v  u)
F
t
Topic – Physics 2a
Brick wall or hay stack
F
m( v  u)
t
This can be re-written as: Ft = mv - mu
F∆t=m∆v
Ft is known as impulse (units Ns) and is equal to the change in momentum:
Impulse = Ft
Topic – Physics 2a
Question????
F
m( v  u)
t
How does hitting the haystack instead of the wall reduce
the force acting on you?
Topic – Physics 2a
answer
• By hitting the haystack you extend the time of
impact – you extend the time during which your
momentum is brought to zero. Your change in
momentum will be the same (impulsive force is
the same), but the longer the time over which
the change in momentum occurs, the less the
force on you. If you extend the time of impact
100 times, you reduce the force of impact by
100.
• Now complete activity 1,2,3
Topic – Physics 2a
Answers to activity
1)People often put ‘roo bars’ on the front of their cars. What is the purpose of this
?
– To protect the front of the car from animals.
– How does this affect the safety aspect of the car in a crash?
– Unless roo bar crushes if you hit another car, then the roo bar
prevents the front of the car from crumpling. Therefore the change of
momentum happens over a very short time so the force on the people
in the car will be much greater than if the front crumpled. Ft =  =
m(v – u).
2)Why do you move your hand away from a high-speed ball upon contact instead
of towards the ball?
–  = m(v – u) = Ft. The change of momentum is a constant (can’t
change mass, initial and final velocity). By moving your hand
backwards, the time of impact is increased so the force on your hand
is decreased.
3)Why do they have sand under playground slides instead of concrete?
– As above, the sand increases the time over which the change of
momentum occurs and as a result decreases the force on the child
when they hit the sand.
Topic – Physics 2a
Topic – Physics 2a
Answers to activity
Topic – Physics 2a
Formulas to remember
• Force = mass x acceleration (F = ma)
• Force = mass (v-u)
∆t
• F∆t=m∆v (IMPULSE)
•
•
•
•
Momentum = Mass x change in velocity
 = mv
 = mv - mu
 = m (v – u) = m V