http://www.ck12.org Chapter 4. Newton’s Three Laws
“whiplash injury.” Suppose a moving car rear-ends another car that is at rest. Imagine that the driver’s seat of the
“front” car does not have a head rest. In that case, the car, and the driver’s body, lurch forward, but the driver’s head
tends to stay in place due to its inertia, straining the neck. The resulting damage to the neck is called a whiplash
injury. This bit of physics often creeps into the courtroom.
http://demonstrations.wolfram.com/Inertia/
It’s not uncommon in science fiction shows to show a space ship “towing” another disabled ship. As long as the
towing ship is moving with constant velocity, there is no reason to keep towing. The disabled ship’s inertia will
maintain the same velocity since there is negligible friction in deep space. In reality, we can send unmanned probes
into deep space with only enough fuel to break out of Earth’s orbit. Once the probe is moving fast enough to escape
the Earth’s gravity, its thrusters are turned off for months or years depending upon how far the journey.
FIGURE 4.2
The New Horizons space probe will spend
most of its year journey traveling nearly
60,000 km/h without a need of fuel as a
result of its inertia. It will arrive at Pluto
July 14, 2015.We use the symbolFto mean force in physics and we use the Greek letter sigma,∑, read as “the sum of” to express
the condition under which Newton’s First Law holds, that is:
∑F=0; the sum of all forces acting on an object is zero (in other words, the net force,Fneton the object is zero,
∑F=Fnet=0). Under this condition the object may be at rest or have a constant velocity.
Check Your Understanding
A student holds a physics text book out the window of a helicopter ascending with a speed of 10 m/s. When the
helicopter reaches a height of 100 m, she releases the text. The highest position above the ground that the book
achieves is:
a. 100 m
b. Greater than 100 m
Answer:The correct answer is B. Due to the book’s inertia it continues moving upward after being released. The
unbalanced force of gravity slows the book down until it reaches its highest position above the ground. (The book’s
highest position is approximately 105 m.)