I have my first physics exam coming up in January. To study I've been doing the past exams from the previous years, but there were are a few questions that I wasn't 100% sure about. I'd be greatful if anyone could tell me if my answers are correct or not(adsbygoogle = window.adsbygoogle || []).push({});

1)Which of the following forms of Newton’s Second Law may be used when considering a body of constant mass?

i) [itex]F=m\vec{a}[/itex]

ii) [itex]F=m\frac{d\vec{v}}{dt}[/itex]

iii) [itex]F=\frac{d\vec{p}}{dt}[/itex]

A. (i)

B. (ii)

C. (iii)

D. (i), (ii) and (iii)

E. None apply

Now, I know that if the mass is constant then equation iii) can be used. That makes me think the answer isC. But then again, if the mass is constant I could technically use i) and ii) if I wanted to aswell? So the answer isD.?

2)A woman pulls a car 50 m along the ground using a tow-rope attached to

the front of the car. If she does 1000 J of work, exerting a force of 40 N, at

what angle is the rope to the ground?

A. 30°.

B. 45°.

C. 60°.

D. 75°.

E. 90°.

Since she's doing half the work she 'should' be doing, the y-component must be taking half the force, or 20N. So is this really as simple as sinθ = 20/40, meaning the answer isA.30°?

3)The potential energy for a two-dimensional force is given by the formula

U= 6x^{2}y + 6y^{4}. What is the force acting at the point (x, y)?

A. F = (12x)i + (6y^{3})j N.

B. F = (6xy + 6y^{4})i + (6x^{2}+ 18y^{3})j N.

C. F = 6x^{3}y^{2}+ 6xy^{5}N.

D. F = 12xy + 24y^{3}N.

E. F = (-12xy)i + (-6x^{2}- 24y^{3})j N.

Differentiating with respect to X gives12xy, and with respect to Y gives6x. The only answer with these values is answer E, but E gives negative forces. Is this because gravitational potential energy (U) is negative? Or is my answer just wrong?^{2}+ 24y^{3}

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Are my answers correct? (newton's laws etc)

**Physics Forums | Science Articles, Homework Help, Discussion**