Transcript Ch 8.1 and 8.2 chap 8.1

Chapter 8
Linear Momentum and
Collisions
Example: Truck Collision
In a head-on collision:
Which truck will experience the greatest impulse?
Which truck will experience the greatest change in
momentum?
Which truck will experience the greatest force?
Which truck will experience the greatest acceleration?
Which truck will experience the greatest change in
velocity?
Which truck would you rather be in during the collision?
• Discussion of the questions
Chapter 8.1 and 8.2
• Linear Momentum
• Momentum and Newton’s Second Law
• Impulse
•Graphical analysis and calculation
8-1 Linear Momentum
A measure of how hard it is to
stop a moving object.
Momentum is a vector; its direction is the
same as the direction of the velocity.
Quantity of motion
Momentum and Newton’s Second Law
Newton’s second law:
is only valid for objects that have constant
mass.
Here is a more general form, also useful
when the mass is changing:
8.2 Impulse
The impulse is the change in momentum
Impulse is a vector, in the same direction
as the average force.
8.1 Impulse
The impulse is equal to the change in
momentum.
Think
A 10 g rubber ball and a 10 g clay bear are
thrown at a wall with equal speeds. The rubber
ball bounces, the clay bear sticks. Which one
exerts a larger impulse on the wall?
demo
Change in Momentum
Change in momentum of
the clay bear:
(a) mv
(b) 2mv
(c) zero
(d) none of these
Impulse
Therefore, the same
change in momentum
may be produced by a
large force acting for a
short time, or by a
smaller force acting for a
longer time.
Let’s talk about traffic law
• Why are bicyclists required to ride in the
same direction as car traffic?
• How does impulse work the safety
features for cars (or bicycle helmets)?
Example: Truck Collision
In a head-on collision:
Which truck will experience the greatest change in
momentum?
Which truck will experience the greatest impulse?
Which truck will experience the greatest force?
Which truck will experience the greatest acceleration?
Which truck will experience the greatest change in
velocity?
Which truck would you rather be in during the collision?
• Discussion of the questions
Answer: Truck Collision
Comparison of the collision variables for the two
trucks:
A greater change in
velocity implies a
greater change in
kinetic energy and
therefore more work
done on the driver.
Ride in the bigger truck! There are good physical reasons!
Answer
Demo
Impulse (J)
F(N)
3000
2000
area under curve
1000
0
0
1
2
3
4
t (ms)