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Lecture 9

PHYA10 Lecture 9: PHYA10 Practical-week9

Department
Physics and Astrophysics
Course Code
PHYA10H3
Professor
Brian Wilson
Lecture
9

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PHYA10 Practical Week 9
1. Three balls are the same height habove the ground and are ﬁred with the same initial
speeds v0. Ball A is ﬁred straight up, ball B is ﬁred horizontally, and ball C is ﬁred
straight down. Air resistance is negligible.
(a) Rank the speeds of the three balls when they hit the ground. Explain.
(b) Rank the time it takes the three balls to hit the ground. Explain.
2. Two blocks are on a frictionless surface as shown below. A hand exerts a force of
magnitude Fon block A, which pushes it to the right for a distance d, then the hand
lets go of the block. Another hand exerts a force of the same magnitude Fon block
B, which pushes it to the left for a distance d, and then the hand lets go of the block.
The above diagram illustrates the situation.
(a) Is the work down on block A positive, negative, or zero? Explain.
(b) Is the work down on block B positive, negative, or zero? Explain.
(c) Is the net external work done on the system (both blocks) positive, negative, or
zero? Explain.
(d) What if there was a spring attached between the two blocks? Now is the net
external work done on the system positive, negative, or zero? Explain.
3. (a) A 40 kg girl is riding in an elevator. The elevator is traveling up at 3 m/s. With
what power does gravity aﬀect the girl? What about the normal force?
(b) The same girl is now in a diﬀerent elevator. It is moving up faster, but begins to
slow down as it approaches her ﬂoor. At the moment that the elevator is traveling
at exactly 3 m/s and slowing down, ﬁnd the power with which gravity aﬀects the
girl. Similarly, ﬁnd the power which with the normal force aﬀects the girl.
4. (a) You have a meter stick. Place your index ﬁngers at either end so that you are
holding the stick up with just two ﬁngers. Move your ﬁngers slowly together.
Where do they end up? Describe the motion of your ﬁngers. Explain in detail
why this happens.
(b) Along with the meter stick you also have a wooden triangle and two masses;
one mass is known, the other is not. Use this equipment however you like to
determine what is the value of the unknown mass. Explain what you did and
why it works. While you are at it, use the setup to determine the mass of the