Transcript (F) and the time of impact

Cars
IMPULSE AND
IMPULSIVE FORCE
IMPULSE
Impulse is defined as change in
momentum of an object.
Ft = mv-mu
Change of momentum
F = Impulsive force,
t = time,
m = mass,
v & u = velocity
Impulse
Equations:
The product of impulsive force
(F) and the time of impact (t).
Ft = mv-mu
F=ma
----------
v u
F  m(
)
t
……….?
v u
a
t
Ft = mv-mu
F = Impulsive force, a = acceleration, t = time, m = mass,
v & u = velocity
IMPULSIVE FORCE:
Impulsive Force
is a large force that acts over
a short period of time during collisions or explosions.
Impulsive force is
defined as the rate
of change of momentum of an object in a
collision or explosion.
F  mv t mu
Change of momentum
Time of impact
The shorter the time of impact, the bigger the impulsive force.
F = Impulsive force,
t = time,
m = mass,
v & u = velocity
Example of Impulsive Force:
a.falling down the glass to the floor
b.Hitting a nail using hammer
c.Kicking a football
d.Collision of cars
e.Striking a tennis ball
f.Etc.
How to Decrease the
Impulsive Force…………?
Increase the
time of impact
How to Increase the
Impulsive Force…………?
Decrease the
time of impact
An impulsive force needs to be
reduced
1. In high jump, when the jumper falls on the mattress, the soft
nature of the mattress lengthen the time of collision.
2. When jumping from a high building.
3. Safety Car
4. Parachute
The Benefit of Impulsive Force
1. When a hammer is used to hit a nail into wood.
2. An exponent of karate has the capability to split athick wooden
slab by bringig his hand down hard onto the surface of wood.
3. In construction, a heavy pile hammer is dropped from a high
level onto the top of a pile.
4. When foodstuff such as nuts or onions are pounded using a
mortar and pestle (which are both made from stone), the action
produces large impulsive forces which crush the food.
5. Kicking a football, etc.
Solve Problems
Solving Problems
1. A car with a mass of 1000 kg is moving with a velocity of 25 m s-1.
It then collides with a tree and is stopped in 0.05 seconds. Calculate
the impulsive force acted on the car.
Solution:
Mass of the car = 1000 kg
Initial velocity of the car (u) = 25 m s-1
Final velocity of the car (v) = 0 m s-1
Time taken to stop (t) = 0.05 s
Therefore, the impulsive force:
mv  mu
F
t
(1000 kg x 0 m s 1 )  (1000 kg x 25 m s 1 )
F
, F   500000 N
0.05 s
Group Activity
2. A 60 kg resident jumps from the first
floor of a burning house. His velocity
just before landing on the ground is 5 m
s-1.
a. Calculate the impulse when his legs hit the
ground.
b. What is the impulsive force on the resident’s
legs if he bends upon landing and takes 0.5 s to
stop?
c. What is the impulsive force on the resident’s
leg if he does not bend and stops in 0.1 s?
d. What is the advantage of bending his legs upon
landing?
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DISCUSSION
Summary