“The acceleration of an object is
directly proportional to the net force and inversely proportional
to its mass.” This can be expressed in equation form:
Force = (Mass) x (Acceleration)
This choice is not the right
one for the example.
However, it still applies to rocket flight! When
a net (or excess) force is applied
to an object, it will accelerate in the direction of that force.
In physics we say that the acceleration is directly proportional
to the force.
Of course, the more massive the object, the slower
it will accelerate. For this case we say that the acceleration is
inversely proportional to the mass (as mass increases, the acceleration
decreases, and vice-versa).
We write the equation as:
Force (thrust) = Mass x Acceleration
Or we can write it as:
Acceleration = Force ÷ Mass
Example: The force is 25.0 lbs
of thrust. The rocket has a mass of 1.400 kilograms. Calculate its
acceleration during HORIZONTAL movement on some frictionless surface.
(Let's not fight gravity yet.)
Solution: First, let's change
pounds of thrust into the metric equivalent of pounds…a unit
called Newtons. Multiply 4.45 Newtons per pound times the number
of pounds of force. So…25.0 lbs x 4.45 Newtons/lb = 111.3
Newtons of metric force.
= (111.3 Newtons) ÷ (1.4 kilograms)
= 79.5 Newtons per kilogram
= 79.5 meters per second each second
That means that the rocket’s
speed increases by 79.5 meters per second every second.
If you fire the rocket vertically, its acceleration will be a bit
less since it will struggle against gravity. That calculation is:
Acceleration = Force ÷
If a rocket moves vertically, there are TWO forces:
(1) the upward thrust of 25 pounds, or 111.3 Newtons, and (2) the
downward pull of gravity, which is the rocket’s weight.
First, what does the rocket weigh? The rocket has a mass of 1.40 kilograms. Any object’s
weight is found by multiplying its mass times the force of gravity.
On the surface of the Earth, the force of gravity is 9.80 Newtons
for every kilogram of mass (written as 9.8 N/kg).