I think next semester I will stay away from physics, as I lack the ability to th
ID: 1478684 • Letter: I
Question
I think next semester I will stay away from physics, as I lack the ability to think the way physicists do. :shrug:
True/False:
a. Water flows through a pipe of varying widths. More water flows per second through the wide section that through the narrow section because there is more room for it to flow.
b. An object's velocity will change if a net force acts on the object.
c. In order not to slow down, an object moving at a constant velocity needs a small net force applied.
d. The net force which acts on an object which maintains a constant velocity is zero.
(True? F_net = ma. If constant velocity means a = zero, then F_net = ma = zero.)
e. The net force which acts on an object which remains at rest is zero.
(True? Same explanation as above.)
f. If a force always acts perpendicular to an objects direction of motion, that force cannot change the objects kinetic energy.
g. The linear speed of objects in circular orbits is independent of the orbiting radius.
Explanation / Answer
a. Water flows through a pipe of varying widths. More water flows per second through the wide section that through the narrow section because there is more room for it to flow.
ANSWER: False
b. An object's velocity will change if a net force acts on the object.
ANSWER: TRUE because a net force on an object implies that there is a net acceleration on the object and, hence, a change in velocity.
c. In order not to slow down, an object moving at a constant velocity needs a small net force applied.
ANSWER: TRUE, A truck moving on a road in air needs a small net force to counter road and air friction (this is why you must constantly use fuel despite driving in a straight line at constant speed). One moving in empty space in a vacuum would not.
d. The net force which acts on an object which maintains a constant velocity is zero.
ANSWER: TRUE, as F = ma = 0 when a = 0. However, keeping in mind air friction and other sources of energy loss, it does not happen so.
f. If a force always acts perpendicular to an objects direction of motion, that force cannot change the objects kinetic energy.
ANSWER:
A good example of a force acting perpendicular to an object's motion is a planet in a circular orbit around a star at constant speed.
A any instant, the direction of velocity is along a tangent. But the force is the inwards pull of gravity - this is always perpendicular to the velocity.
So the KE is not changed. KE is only changed if speed changes. This can only happen if there is a component of force in the direction of velocity (because only then is work done by the force).
However momentum is a vector. It changes if its direction changes. But the direction continually change during circular motion.
So the statement is FALSE.
The force cannot change the object's kinetic energy, but it CAN change its momentum (direction). That's why the planet goes in a circular orbit without changing speed.
Not sure about (g) so skipping the part.
Hope this helps. :)
And don't be so afraid of Physics, it is amongst the easiest subjects. I used to think the same, but now I love this subject the most. Try giving it some more time.