## Posts in category Physics Homework Help

# Relativity student exam time

Spacecraft I, containing students taking a physics exam, approaches the Earth with a speed of 0.7c (relative to the Earth), while spacecraft II, containing professors proctoring the exam, moves at 0.2c (relative to the Earth) directly toward the students. The professors stop the exam after 88.0 min have passed [...]

# Space ship landing craft

Questions
A spaceship traveling at 0.540c toward the Earth launches a landing craft. The landing craft travels in the same direction with a speed of 0.880c relative to the mother ship. As measured on the Earth, the spaceship is 0.280 ly from the Earth when the landing craft is launched.
(a) What spee [...]

# Siphon Phyiscs Problem

A siphon is used to drain water from a tank with a steady frictionless flow.
See the diagram for the set up.
We can use Bernoulli equation to solve for the v. Here we can assume that the cross section area of the tube is much smaller than that of the beaker so we can treat the velocity that the water level drops in the beaker to b [...]

# Physics of Hydraulic Brake System

See more at LeiAcademy.org.
Consider a hydraulic brake system as shown below. The area of the piston in the master cylinder connected to the foot paddel is 1.8 cm2. The area of the piston connected to the brake shoe is 6.4 cm2. The kinetic frictional coefficient between shoe and wheel drum is 0.50. If the wheel's [...]

# U-tube Pressure and Bernoulli Equatio...

More at LeiAcademy.org
Question: A U-tube open at both ends is partially filled with water. Oil having a density of 840 kg/m3 is then poured into the right arm and forms a column L = 4.68 cm high (Figure b).
(a) Determine the difference h in the heights of the two liquid surfaces.
Answer: To show more clearly how to set up the locations for c [...]

# Answered: Bikes, angular acceleration...

See answers to these related questions.
http://tutor.leiacademy.org/qa/index.php/91/901-slide-show-slide-13-15
Need to calculate the torque and then use |||tau=I\alpha||. to find ||\alpha||.

# Bikes, angular accelerations, and for...

Two 1 kg wheels with fixed hubs starts from rest and equal magnitude 1 N forces are applied to each. The hub and spokes are virtually massless. In order to impart identical angular accelerations, at which angle (theta) must F1 be applied with respect to horizontal?
This is from the lecture slides. I’m not really sure how to solve this o [...]

# Answered: Parallel Axis Theorem

For any object rotating around its center of mass, you will need to use ||I_{com}||. If the rotational axis is away from the center of mass, you will need to use the parallel Axis theorem to find the momentum of inertia in that situation. In applying the theorem, the M would be the total mass of the object and h would be the distance between [...]

# Answered: Angular speed of two points...

The angular speed describes how much rotational angle (usually in radians) are cover per unit time. Being on the same rotational object, if one point goes around certain angle, e.g., ||2\pi|| for a complete circle, any other points on the same object would also experience an rotation of ||2\pi||. Therefore the angular displacement/velocity/ac [...]

# Parallel Axis Theorem

Can someone give me a good explanation of the Parallel Axis Theorem and when to use it? I’m slightly confused about when to apply this equation. Thanks!

# Angular speed of two points on a circ...

A disk is rotating CCW with an angular speed omega.
Two stickers A and B are fixed to different locations on the disk as shown.
Compare the angular speed of the two stickers A and B.
This problem is from one of the lecture slides. Can someone give me a good explanation of why the angular speeds of A and B are the same, but why the linear spee [...]

# Answered: the questions in PPT

This is all depends on the angular momentum of the block before it is dropped on the disk. In the first two cases, the angular momentum of the block is 0. The path of the block goes through the rotational axis, therefore the momentum would be zero — because ||R_\perp =0||.
So in the first two cases, before and after collision, the total [...]

# Answered: conservation Energy and mom...

(a) the two cars had an inelastic collision (stick together), so the total mechanical energy before and after the collision doesn’t conserve.
(b) momentum is always conserved in a collision as long as there is no external forces applied on the system.
(c) without frictional forces and external forces, the total mechanical energy is co [...]

# Answered: Defining a Coordinate Syste...

You can choose positive to be any direction you want. As long as it is consistently maintained in the problem, it should be fine. So pick the one that is more intuitive to you and be consistent with it.

# Answered: Is hw #10 due for a grade?

It is a review homework, but this one does count for grades.

# Answered: 901 slide show slide 13-15

These are problems concerning calculation of torque. You can find first the component of force that is perpendicular to the R. Then multiply that force component with R to get torque ||\tau=F_\perp R||.
Once have torque, you can use ||\tau_{net}=I\alpha|| to get the angular acceleration.
Here, the moment of inertia of the wheels will be appr [...]

# conservation Energy and momentum?

A boxcar at a rail yard is set into motion at the top of a hump. The car rolls down quietly and without friction onto a straight, level track where it couples with a flatcar of smaller mass, originally at rest, so that the two cars then roll together without friction. Consider the two cars as a system from the moment of release of the boxcar [...]

# Defining a Coordinate System in Torqu...

When choosing which direction of the system will be positive and which will be negative, do we always have to choose the counterclockwise direction to be positive in free body diagrams involving torque(s) applied on pulleys with mass? Will points be taken off on exams if we define the positive direction to be clockwise, if that is w [...]