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# A physics major is cooking breakfast when he notices that the frictional force between the steel

1.

A physics major is cooking breakfast when he notices that the frictional force between the steel

spatula and the Oiled Steel frying pan is only 0.200 N. Knowing the coefficient of kinetic friction

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between the two materials (0.03), he quickly calculates the normal force. What is it?

N

2.

(a) If half of the weight of a flatbed truck is supported by its two drive wheels, what is the maximum

acceleration it can achieve on dry concrete where the coefficient of kinetic friction is 0.7 and the

coefficient of static friction is 1.

m/s2

(b) Will a metal cabinet lying on the wooden bed of the truck slip if it accelerates at this rate where

the coefficient of kinetic friction is 0.3 and the coefficient of static friction is 0.55?

Yes

No

(c) Answer both of these questions for the case that the truck has four-wheel drive, and the cabinet is

wooden.

maximum acceleration

m/s2Will it slip?

Yes

No

3.

A contestant in a winter games event pushes a 34.0 kg block of ice across a frozen lake as shown in

Figure 4.29(a). The coefficient of static friction is 0.1 and the coefficient of kinetic friction is 0.03.

Figure 4.29

(a) Calculate the minimum force F he must exert to get the block moving.

N

(b) What is its acceleration once it starts to move, if that force is maintained?

m/s2

4.

A contestant in a winter games event pushes a 31.0 kg block of ice across a frozen lake with a rope

over his shoulder as shown in Figure 4.29(b). The coefficient of static friction is 0.1 and the coefficient

of kinetic friction is 0.03.

Figure 4.29

(a) Calculate the minimum force F he must exert to get the block moving.

N

(b) What is its acceleration once it starts to move, if that force is maintained?

m/s2

5.

A 555-g squirrel with a surface area of 940 cm2 falls from a 4.4-m tree to the ground. Estimate its

terminal velocity. (Use the drag coefficient for a horizontal skydiver. Assume that the cross-sectional

area of the squirrel can be approximated as a rectangle of width 11.6 cm and length 23.2 cm. Note,

the squirrel may not reach terminal velocity by the time it hits the ground. Give the squirrel’s terminal

velocity, not it’s velocity as it hits the ground.)

m/s

What will be the velocity of a 55.5-kg person hitting the ground, assuming no drag contribution in

such a short distance?

m/s

6.

A moonshiner makes the error of filling a glass jar to the brim and capping it tightly. The moonshine

expands more than the glass when it warms up, in such a way that the volume increases by 0.3%

(that is, ΔV/V0 = 3 ✕ 10-3) relative to the space available. Calculate the force exerted by the

moonshine per square centimeter if the bulk modulus is 1.5 ✕ 109 N/m2, assuming the jar does not

break.

N/m2.In view of your answer, do you think the jar survives? (Hint: How many

atmospheres is this?)

Yes

No

7.

Semi-trailer trucks have an odometer on one hub of a trailer wheel. The hub is weighted so that it

does not rotate, but it contains gears to count the number of wheel revolutions—it then calculates the

distance traveled. If the wheel has a 1.03 m diameter and goes through 170,000 rotations, how many

km

8.

An automobile with 0.240 m radius tires travels 80,000 km before wearing them out. How many

revolutions do the tires make, neglecting any backing up and any change in radius due to wear?

revolutions

9.

A fairgrounds ride spins its occupants inside a flying-saucer-shaped container. If the horizontal

circular path the riders follow has a 6.00 m radius, at how many revolutions per minute will the riders

be subjected to a centripetal acceleration 2.10 times that of gravity?

rev/min

10.

Taking the age of Earth to be about 4 ✕ 109 years and assuming its orbital radius of 1.5 ✕ 1011 m has

not changed and is circular, calculate the approximate total distance Earth has traveled since its birth

(in a frame of reference stationary with respect to the Sun).

m

11.

At takeoff a commercial jet has a 45.0 m/s speed. Its tires have a diameter of 0.850 m.

(a) At how many rpm are the tires rotating?

rpm

(b) What is the centripetal acceleration at the edge of the tire?

m/s2

(c) With what force must a determined 10-15 kg bacterium cling to the rim?

N

(d) Take the ratio of this force to the bacterium’s weight.

(force from part (c) / bacterium’s weight)

12.

What is the ideal speed to take a 100 m radius curve banked at a 30.0° angle?

m/s

13.

If a car takes a banked curve at less than the ideal speed, friction is needed to keep it from sliding

toward the inside of the curve (a real problem on icy mountain roads).

(a) Calculate the ideal speed to take a 85 m radius curve banked at 15°.

m/s

(b) What is the minimum coefficient of friction needed for a frightened driver to take the

same curve at 20.0 km/h?

14.

Astrology, that unlikely and vague pseudoscience, makes much of the position of the planets at the

moment of birth. The only known force a planet exerts on earth is gravitational.

(a) Calculate the gravitational force exerted on a 5.00 kg baby by a 90 kg father 0.300 m

away at birth (assisting so he is close).

N

(b) Calculate the force on the baby due to Jupiter if it is at its closest to the earth, some

6.29 ✕ 1011 m away, showing it to be comparable to that of the father. The mass of

Jupiter is about 1.90 ✕ 1027 kg. Other objects in the room and the hospital building also

exert similar gravitational forces. (Of course, there could be an unknown force acting, but

scientists first need to be convinced that there is even an effect, much less that an

unknown force causes it.)

N