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Principles of Physics
 Forces
cause acceleration
 Acceleration is a change in
velocity with respect to time
 Change in velocity results in a
change in momentum
Newton’s 2nd Law
Fnet = ma
Fnet = m(vf – vi)
Fnet t = m(vf – vi)
change in
(kg m/s)
1Ns = 1 kg m/s
(vf – vi) =
net force (N)
mass (kg)
change in velocity (m/s)
time (s)
Car crashes
By increasing the time of the collision the force felt by the
passengers is reduced.
Airbags cushion the impact (allow impact to take longer)
Ft = m(vf – vi)
Why would it be better to hit a hay stack instead of a brick
What do catchers allow their arms to move backwards
when they catch?
A hockey player takes a slap shot, exerting a
force of 30 N on a hockey puck for 0.16 s. If
the puck had a mass of 0.115 kg and started
from rest, what speed does it have as it heads
towards the net?
F =30 N
t = 0.16 s
m = 0.115 kg
vi = 0
Ft = m(vf – vi)
(30)(0.16 ) = 0.115(vf – 0)
vf = 41.8 m/s
Small rockets are used to make small
adjustments in speed of satellites. One such
rocket has a thrust of 35 N. If it is fired to
change velocity of a 72,000 kg spacecraft by
0.63 m/s, how long should it be fired?
F =35 N
m = 72,000 kg
(vf – vi) = 0.63 m/s
Ft = m(vf – vi)
(35)(t) = 72,000(0.63)
t = 1,296 s
= 21.6 min