114
8. Some garden shears are like a pair of scissors: one sharp blade slices past
another. In the “anvil” type, however, a sharp blade presses against a flat one
rather than going past it. A gardening book says that for people who are not
very physically strong, the anvil type can make it easier to cut tough
branches, because it concentrates the force on one side. Evaluate this claim
based on Newton’s laws. [Hint: Consider the forces acting on the branch,
and the motion of the branch.]
9. A uranium atom deep in the earth spits out an alpha particle. An alpha
particle is a fragment of an atom. This alpha particle has initial speed v, and
travels a distance d before stopping in the earth. (a) Find the force, F, that
acted on the particle, in terms of v, d, and its mass, m. Don’t plug in any
numbers yet. Assume that the force was constant. (b) Show that your
answer has the right units. (c) Discuss how your answer to part a depends
on all three variables, and show that it makes sense. (d) Evaluate your result
for m=6.7
x
10
–27
kg, v=2.0
x
10
4
km/s, and d=0.71 mm.
10. You are given a large sealed box, and are not allowed to open it. Which
of the following experiments measure its mass, and which measure its
weight. (a) Put it on a frozen lake, throw a rock at it, and see how fast it
moves after being hit. (b) Drop it from a third-floor balcony, and measure
how loud the sound is when it hits the ground. (c) As shown in the figure,
connect it with a spring to the wall, and watch it vibrate. [Hint: Which
experiments would give different results on the moon.]
Problem 10c.