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Physics and Astrophysics (100)

PHYA10H3 (30)

Brian Wilson (10)

Lecture 9

Department

Physics and AstrophysicsCourse Code

PHYA10H3Professor

Brian WilsonLecture

9This

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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

meter stick. Estimate your error for both masses. Show your TA your results

(including error estimates) before using the digital scale to conﬁrm your result.

Is your measurement within your error estimate of the scale’s measurement?

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