A pair of protons in an atomic nucleus repel each other, but they are also attracted to each other. Why?
A) The pair repel each other by nuclear force but attract one another by an electrical force.
B) The pair repel each other since they are both positively charges but they are held together by much stronger forces known as quarks.
C) The pair attract each other by nuclear force but also repel one another by a weaker electrical force.
D) Although it is known that protons repel each other at short distances because of like charge, it is not known what actually holds protons together in the nucleus because we have never seen the nucleus of an atom.
Answer: C
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If gravity alone acts to retard the expansion and nothing acts to accelerate it, the Universe is said to be closed if I. gravity is strong enough to stop the expansion in a finite time. II. the density of the Universe is less than the critical density. III. the Universe is finite
a. I & II b. I & III c. II & III d. I, I, & III e. None of the other choices are correct.
A proton in a certain particle accelerator has a kinetic energy that is equal to its rest energy. What is the momentum of the proton as measured by a physicist working with the accelerator? (c = 3.00 × 108 m/s, mproton = 1.67 × 10-27 kg)
A) 2.51 × 10-19 kg ? m/s B) 2.89 × 10-19 kg ? m/s C) 4.34 × 10-19 kg ? m/s D) 5.01 × 10-19 kg ? m/s E) 8.68 × 10-19 kg ? m/s
. 
Reference: Ref 5-1
The substance is entirely a solid between points ______________ and ______________.
A 150-kg crate is placed on an adjustable inclined plane. If the crate slides down the incline with an acceleration of 0.70 m/s2 when the incline angle is 20°, then what should the incline angle be for the crate to slide down the plane at constant speed? (g = 9.8 m/s2)
a. 21° b. 16° c. 11° d. 26° e. 6°