impressed.” The second law explains the units that make up a newton (kg·m/s^2 ); they
are the result of multiplying mass by acceleration.
In the illustrations, you see an example of forces and the acceleration caused by the net
force. The woman who stars in these illustrations lifts a suitcase. The weight of the
suitcase opposes this motion. This force points down. Since the force supplied by the
woman is greater than the weight, there is a net force up, which causes the suitcase to
accelerate upward.
In Example 1, the woman lifts the suitcase with a force of 158 N upward. The weight of
the suitcase opposes the motion with a downward force of 147 newtons. The two forces
act along a line, so we use the convention that up is positive and down is negative, and
subtract to find the net force. (If both forces were not acting along a line, you would
have to use trigonometry to calculate their components.)
The net force is 11 N, upward. The mass of the suitcase is 15 kg. Newton’s second law
can be used to determine the acceleration: It equals the net force divided by the mass.
The suitcase accelerates at 0.73 m/s^2 in the direction of the net force, upward.ȈF = ma
ȈF = net force
m = mass
a = acceleration
Units of force: newtons (N, kg·m/s^2 )
What is the suitcase's
acceleration?
ȈF = ma
F + (ímg) = ma
a = (Fímg)/m
a = (158 N í147 N)/(15 kg)
a = 0.73 m/s^2 (upward)
5.6 - Sample problem: Rocket Guy
Above you see “Rocket Guy,” a superhero who wears a jet pack. The jet pack provides an upward force on him, while Rocket Guy’s weight
points downward.Rocket Guy weighs 905 N and his jet
pack provides 1250 N of thrust,
straight up. What is his acceleration?
(^92) Copyright 2007 Kinetic Books Co. Chapter 05