Originally posted by PBE6 (A) The tension on the suspending cord does no work on the weight, because the force it exerts is always tangential to the movement of the weight.
(B) The air resistance always does negative work on the weight, because air resistance opposes the motion of the pendulum and therefore always acts opposite to the direction of movement of the pendulum.
(C) The ...[text shortened]... ding on whether the pendulum is on an upswing (negative work) or on a downswing (positive work).
Perfect and well said, 100% correct.
Next question to follow.
6. An open box slides across a frictionless, icy surface of a frozen lake. What happens to the speed of the box as water from a rain shower collect in it, assuming that the rain falls vertically into the box? Explain.
[b]6. An open box slides across a frictionless, icy surface of a frozen lake. What happens to the speed of the box as water from a rain shower collect in it, assuming that the rain falls vertically into the box? Explain.[/b]
The rain will add mass to the box without adding horizontal momentum, so the box will slow down.
One might think of the slowing as being caused by the moving rear wall of the box impacting with the horizontally stationary raindrops. Kinetic energy is transferred from the box to the raindrops to get them to move with the box, slowing it down.
Originally posted by AThousandYoung The rain will add mass to the box without adding horizontal momentum, so the box will slow down.
One might think of the slowing as being caused by the moving rear wall of the box impacting with the horizontally stationary raindrops. Kinetic energy is transferred from the box to the raindrops to get them to move with the box, slowing it down.
7. If the mass of the earth were doubled at the same time as its radius were doubled, the free fall acceleration would....
A. Stay the Same. B. Decrease. C. Increase.
Explain.
[b]7. If the mass of the earth were doubled at the same time as its radius were doubled, the free fall acceleration would....
A. Stay the Same. B. Decrease. C. Increase.
Explain.[/b]
The force of gravitational attraction F acting between two objects m1 and m2 is given by:
F = G*m1*m2/(r^2)
The free-fall acceleration A acting on m1 is given by:
8. In some motorcycle races, the riders drive over small hills so the motorcycle is airborne for some time. If the motorcycle racer keeps the throttle open while leaving the hill & going into the air, the mororcycle's nose tends to rise upwards. Why does this happen?
[b]8. In some motorcycle races, the riders drive over small hills so the motorcycle is airborne for some time. If the motorcycle racer keeps the throttle open while leaving the hill & going into the air, the mororcycle's nose tends to rise upwards. Why does this happen?[/b]
Conservation of angular momentum. The wheel is spinning one way but is not in contact with the Earth which would "spin" the other way (very, very slowly due to the difference in mass). Thus the motorcycle itself must spin about the axis of the rear wheel.
Originally posted by AThousandYoung Conservation of angular momentum. The wheel is spinning one way but is not in contact with the Earth which would "spin" the other way (very, very slowly due to the difference in mass). Thus the motorcycle itself must spin about the axis of the rear wheel.
It is a conservation of angular momentum issue. Restated: Since the rear wheel is no longer in contact with the ground, it will accelerate if the throttle remains at the same position, as it has much less force countering it tangential to its surface. Viewed from the rider's left side, the force to the left that the earth exerts on the wheel is removed. This force was creating a clockwise moment on the wheel. The wheel thus gains counterclockwise momentum (wheel spins forward faster) when the bike is in the air. The total moment of the system (the bike and rider) is conserved when the nose rotates upward (clockwise moment when viewed from the left).
Originally posted by HolyT It is a conservation of angular momentum issue. Restated: Since the rear wheel is no longer in contact with the ground, it will accelerate if the throttle remains at the same position, as it has much less force countering it tangential to its surface. Viewed from the rider's left side, the force to the left that the earth exerts on the wheel is removed. Thi ...[text shortened]... rider) is conserved when the nose rotates upward (clockwise moment when viewed from the left).
Good, well described at that. Next one to follow - may take a bit to come up with.
BTW after each set of 10, I will produce a very challenging problem based on the conceptuals for each 10.
9.Tornadoes and Hurricanes often lift the roofs of houses. (A) Why does this happen? Explain; use the Bernoulli Effect to explain why. (B) Why should you keep your windows open under these conditions?
[b]9.Tornadoes and Hurricanes often lift the roofs of houses. (A) Why does this happen? Explain; use the Bernoulli Effect to explain why. (B) Why should you keep your windows open under these conditions?[/b]
Thanks!
The immediate physical effects of hurricanes (high straight-line winds) are far different from those of tornados (high-speed rotating winds). Forgetting the effects that I'm sure you weren't referring to, such as storm surge, hail, lightning, and rain, the common ingredient is the wind, and its effects are different for both. Can you narrow the problem down to either hurricanes or tornadoes? Or are you talking about high straight-line winds regardless of the source? I don't think you're talking about low pressure, since the pressure differences by themselves don't lift roofs.
[b]9.Tornadoes and Hurricanes often lift the roofs of houses. (A) Why does this happen? Explain; use the Bernoulli Effect to explain why. (B) Why should you keep your windows open under these conditions?[/b]
In a simplistic model the outside moving air causes a lower pressure (Bernoulli) than the inside motionless air. The pressure difference causes the building to 'explode'.
Keeping the windows open will obviously dramatically reduce the effect.
Anyone a Master of Physics?
19 Sep '07 19:44
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