# Homework Assignment #3

Due: 19 February

Chapters 5 and 6.

### QUESTIONS

Chapter 5 - 6, 8, 9, 10, 17, 18, 19, 21.
Chapter 6 - 8, 12, 13.

### PROBLEMS

Chapter 5 - 1, 18, 21, 26, 28, 32, 38, 39, 51, 65.
Chapter 6 - 1, 5, 6, 15, 27, 38A, 38, 51, 52.

I suggest that you try several unassigned problems, especially the ones with corresponding figures, for practice.

These are the ANSWERS only, not the SOLUTIONS. It is not sufficient to copy these and turn them in as homework. You must show your work.

Q 5-6) The scale measures the tension T in the cord attaching the fish to the scale. This force is NOT equal to the force called the weight W of the fish. In an inertial reference frame (a non-accelerating frame), the magnitudes of these two distinct forces are equal. But the elevator is accelerating; it is therefore not an inertial reference frame, and the two forces T and W will not have the same magnitude.

Q 5-8)
1. Remember that action-reaction paired forces NEVER act on the same object. Gravity acts on the ball; this is the force of the Earth pulling down on the ball. The reaction to this force is the gravitational attraction of the ball pulling up on the Earth. Both forces in this pair have the same magnitude and opposite direction.

The normal force due to contact with the hand also acts on the ball. This normal force prevents gravity from accelerating the ball. The reaction to this force is the force of the ball on the hand; this is the force that you feel, the way you know that a ball is in your hand.

2. Only the force of gravity due to the Earth acts on the ball as it falls. The reaction force is the same as in part (a): the gravitational attraction due to the ball acting on the Earth. The Earth is so massive that the Earth's acceleration aE=F/mE is hardly noticeable.

Q 5-9) No resultant (= net = total) force.
Q 5-10)
1. Static friction between the crate and the truck bed.
2. If the acceleration (or deceleration) of the truck is larger than the largest possible force of static friction, then the crate will slide. If the truck driver brakes, the crate may slide forward.

Q 5-17) Action-reaction paired forces NEVER act on the same object.
Q 5-18) 200 N (no, it is NOT 400 N!)
Q 5-19) The coefficient of static friction is larger than the coefficient of kinetic friction.
Q 5-21) The 20-N rock is twice as massive, or in other words, twice as hard to accelerate.
Q 6-8) The water will not spill as long as the pail accelerates toward its open end faster than the water does. The acceleration of the pail can be horizontal, vertical (up or down), or toward the center of a circle.
Q 6-12) Driving in a circle at constant speed.
Q 6-13) There was a typo: Q6-23 should be Q6-13
Not possible.
P 5-1)
1. 1/3
2. 0.75 m/s2

P 5-18)
1. (4 i + 3 j) m/s2 or 5 m/s2 at 36.9o
2. (5.5 i + 2.6 j) m/s2 or 6.08 m/s2 at 25.3o

P 5-21)
1. (2.5 i + 5 j) N
2. 5.59 N

P 5-26)
1. T1=31.5 N
T2=37.5 N
T3=49 N
2. T1=113 N
T2=56.6 N
T3=98 N

P 5-28)
1. 648 N each (The old answer 575.8 was an error.)
2. No, an infinite force is required.

P 5-32)
1. 51.06o
2. 217.8 N

P 5-38) a = F/(m1 + m2)
T = F m1/(m1 + m2)
P 5-39) The bug has a greater chance of surviving case (a).
P 5-51) 0.077 (no units)
P 5-65)
1. T1=Mg/2
T2=Mg/2
T3=Mg/2
T4=3Mg/2
T5=Mg
2. F=Mg/2

P 6-1)
1. 8.0 m/s
2. 3.02 N

P 6-5) 0 < v < 8.08 m/s
P 6-6)
1. 5.6 x 103 m/s
2. 240 min = 14400 s
3. 735 N

P 6-15) 3.13 m/s
P 6-27)
1. 491 N
2. 50.1 kg
3. 2 m/s2

P 6-38A)
1. T=m2 g
2. Tension T
3. v = (m2 g r / m1 )1/2

P 6-38)
1. 9.8 N
2. 9.8 N
3. 6.3 m/s

P 6-51)
1. 1.58 m/s2
2. 455 N
3. 329 N
4. 397 N at 9.15o tilted from the vertical toward the center of the wheel

P 6-52)
1. 5.19 m/s
2. 555 N

Please report any corrections to Professor Scalise.