I have a bunch of questions that I got wrong on a test and I'd appreciate some help on why my answers are incorrect:(adsbygoogle = window.adsbygoogle || []).push({});

1. A projectile is launched with an initial velocity of magnitude V at an angle t to the horizontal. Find the projectile’s maximum vertical displacement. (Ignore effects due to the air).

* KE = PE for the problem, the PE at the top is mgh and KE = 0, and opposite for the bottom, solving for H with KE = 0.5*mv^2 and PE = mgh gives

gh = .5v^2, solving for h gives v^2/(2*g), the sin(t) can be neglected till the end since the vertical copmonent of the velocity is what counts.

3. The coefficient of static friction between a box and a ramp is 0.5. The ramp’s incline angle (with the horizontal is 30°. If the box is placed at rest on the ramp, the box will.

* Friction force = .5mgcos(30)

Gravitation force = mgsin(30)

simplifying to .25g and g*sqrt(3)/2, .25 < sqrt(3)/2 gravitation is larger and the box will accelerate.

An electric dipole consists of a pair of point charges each of magnitude 4.0 nC separated by a distance of 2.0 cm. What is the electric field strength at the point midway between the charges?

=Since the charges are of same magnitude in charge and same polarity (and same distance at the centerpoint) then the field will have the same value from either side. The pull to the left from one charge will be exactly countered by the pull from the charge on the right and by superposition, the field cancels to 0.

=the terminals of a 12 V battery with negligible internal resistance are connected by a wire containing four resistors. Three of the resistors (2 ohms, 3 ohms, and 6 ohms) form a parallel combination, which is in series with the fourth resistor (1 ohm). Find the energy dissipated as heat by the 2-ohm resistor in 4 s.

The equivalent resistance is 1+1/(1/2+1/3+1/6) or which simplifies to 2 ohms, so the current in the circuit is 6 amps. The power over the resistor is then P = I V = 12*6*4s = 288. The multiple choice answers ranged from 8 to 96

An electron (mass = m, charge = – e) is projected with speed v upward, in the plane of page, into a regioncontaining a uniform magnetic field, B, that’s directed into the plane of the page. Describe the electron’s subsequent circular motion.

I guessed on this one trying to use some common sense, the options were arrangements of (mv) and (eB) as a ratio, and the direction of rotation. The rotation should be counter clockwise cuz of the right hand rule, and I figured as B got stronger the particle would deflect more (greatre force) so eB should inversly affect the radius. (Larger pull smaller radius). I guess not.

Traveling at an initial speed of 1.5 x 106 m/s, a proton enters a region of constant magnetic field of magnitude 1.5 T. If the proton’s initial velocity vector makes an angle of 30o with the magnetic field, compute the proton’s speed 4 s after entering the magnetic field.

= I had no idea on this one.

Thanks for the help.

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# Homework Help: Failed physics test

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