http://www.ck12.org Chapter 4. Motion and Forces
A: Tony exerts a backward force against the ground, as you can see in theFigure4.46, first with one skate and then
with the other. This force pushes him forward. Although friction partly counters the forward motion of the skates, it
is weaker than the force Tony exerts. Therefore, there is a net forward force on the skates.
FIGURE 4.46
Direct and Inverse Relationships
Newton’s second law shows that there is a direct relationship between force and acceleration. The greater the force
that is applied to an object of a given mass, the more the object will accelerate. For example, doubling the force on
the object doubles its acceleration.
The relationship between mass and acceleration is different. It is an inverse relationship. In an inverse relationship,
when one variable increases, the other variable decreases. The greater the mass of an object, the less it will accelerate
when a given force is applied. For example, doubling the mass of an object results in only half as much acceleration
for the same amount of force.
Q: Tony has greater mass than the other two boys he is racing (pictured in the opening image). How will this affect
his acceleration around the track?
A: Tony’s greater mass will result in less acceleration for the same amount of force.
Summary
- Newton’s second law of motion states that the acceleration of an object equals the net force acting on the
object divided by the object’s mass. - According to the second law, there is a direct relationship between force and acceleration and an inverse
relationship between mass and acceleration.
Vocabulary
- Newton’s second law of motion: Law stating that the acceleration of an object equals the net force acting on
the object divided by the object’s mass.
Explore More
At the following URL, use the simulator to experiment with force, mass, and acceleration. First keep force constant
at 1 N, and vary mass from 1–5 kg. Next keep mass constant at 1 kg, and vary force from 1–5 N. In each simulation,
record the values you tested and the resulting acceleration. Finally, make two line graphs to plot your results. On
one graph, show acceleration when force is constant and mass changes. On the other graph, show acceleration when
mass is constant and force changes. Describe in words what the two graphs show.