Mechanics Unit 2

A 90-kg man stands in an elevator that has a downward acceleration of 1.4 m/s2. The force exerted by him on the floor is about:

760 N

Why do raindrops fall with constant speed during the later stages of their descent?

Air resistance just balances the force of gravity

Which of the following is true, according to Newton's second law?

The acceleration of an object is proportional to the net force on it and inversely proportional to its mass.

When the brakes of an automobile are applied, the road exerts the greatest retarding force:

just before the wheels start to slide

A circus performer of weight W is walking along a "high wire" as shown. The tension in the wire is:

much more than W

The driver of a 1000-kg car tries to turn through a circle of radius 100 m on an unbanked curve at a speed of 10 m/s. The actual frictional force between the tires and a slippery road has a magnitude of 900 N. The car:

slides off to the outside of the curve

A block is placed on a rough wooden plane. It is found that when the plane is tilted 30 to the horizontal, the block will slide down at constant speed. The coefficient of kinetic friction of the block with the plane is:

0.577

Two blocks are connected by a string and pulley as shown. Assuming that the string and pulley are massless, the magnitude of the acceleration of each block is:

0.98 m/s^2

A baseball has a terminal speed of 42 m/s in air (ρ = 1.2 kg/m3). What would be its terminal speed in water (ρ = 1.0 x 103 kg/m3)?

1.5 m/s

A 13-N weight and a 12-N weight are connected by a massless string over a massless, frictionless pulley. The 13-N weight has a downward acceleration with magnitude equal to that of a freely falling body times:

1/25

A block slides down a frictionless plane that makes an angle of 30° with the horizontal. The acceleration of the block is:

4.90 m/s^2

A box with a weight of 50 N rests on a horizontal surface. A person pulls horizontally on it with a force of 15 N and it does not move. To start it moving, a second person pulls vertically upward on the box. If the coefficient of static friction is 0.4, what is the smallest vertical force for which the box moves?

12 N

An object placed on an equal-arm balance requires 12 kg to balance it. When placed on a spring scale, the scale reads 12 kg. Everything (balance, scale, set of weights and object) is now transported to the Moon where free-fall acceleration is one-sixth that on Earth. The new readings of the balance and spring scale (respectively) are:

12 kg, 2 kg

An automobile moves on a level horizontal road in a circle of radius 30 m. The coefficient of friction between tires and road is 0.50. The maximum speed with which this car can round this curve is:

12 m/s

A 24-N horizontal force is applied to a 40-N block initially at rest on a rough horizontal surface. If the coefficients of friction are s = 0.5 and k = 0.4, the magnitude of the frictional force on the block is:

16 N

At what angle should the roadway on a curve with a 50m radius be banked to allow cars to negotiate the curve at 12 m/s even if the roadway is icy (and the frictional force is zero)?

16°

A cube has a drag coefficient of 0.8. What would be the terminal velocity of a sugar cube 1 cm on a side in air (ρ = 1.2 kg/m3)? Take the density of sugar to be 1.6 x 103 kg/m3.

18 m/s

A constant force of 8.0 N is exerted for 4.0 s on a 16-kg object initially at rest. The change in speed of this object will be:

2 m/s

Two forces are applied to a 5.0-kg crate; one is 6.0 N to the north and the other is 8.0 N to the west. The magnitude of the acceleration of the crate is:

2.0 m/s^2

A giant wheel, 40 m in diameter, is fitted with a cage and platform on which a man can stand. The wheel rotates at such a speed that when the cage is at X (as shown) the force exerted by the man on the platform is equal to his weight. The speed of the man is:

20 m/s

A crate rests on a horizontal surface and a woman pulls on it with a 10-N force. No matter what the orientation of the force, the crate does not move. Rank the situations shown below according to the magnitude of the frictional force of the surface on the crate, least to greatest.

3, 2, 1

A 0.2-kg stone is attached to a string and swung in a circle of radius 0.6 m on a horizontal and frictionless surface. If the stone makes 150 revolutions per minute, the tension force of the string on the stone is:

30 N

A 5-kg block is suspended by a rope from the ceiling of an elevator that accelerates downward at 3.0 m/s2. The tension force of the rope on the block is:

34 N, up

A 5-kg concrete block is lowered with a downward acceleration of 2.8 m/s2 by means of a rope. The force of the block on the Earth is:

49 N, up

The standard 1-kg mass is attached to a compressed spring and the spring is released. If the mass initially has an acceleration of 5.6 m/s2, the force of the spring has a magnitude of:

5.6 N

The system shown remains at rest. The force of friction on the block on the slope is:

8 N

A man weighing 700 N is in an elevator that is accelerating upward at 4 m/s2. The force exerted on him by the elevator floor is:

990 N

Three blocks (A, B, C), each having the same mass M, are connected by strings as shown. Block C is pulled to the right by a force that causes the entire system to accelerate. Neglecting friction, the net force acting on block B is:

F/3

A person riding a Ferris wheel is strapped into her seat by a seat belt. The wheel is spun so that the centripetal acceleration is g. Select the correct combination of forces that act on her when she is at the top. Here, Fg = force of gravity, down; Fb = seat belt force, down; and Fs = seat force, up.

Fg = mg, Fb = 0, Fs = 0

Two forces, one with magnitude of 3 N and the other with a magnitude of 5 N, are applied to an object. For which orientation of the forces shown in the diagrams is the magnitude of the acceleration of the object the least?

I: -3Ni + 5Ni

A block of mass m is pulled at constant velocity along a rough horizontal floor by an applied force as shown. The magnitude of frictional force is:

T cosθ

Two blocks are connected by a massless string that passes over a massless pulley, as shown. In the absence of friction, how does the tension force exerted by the string on the 250 N block compare with the tension force exerted by the string on the 350 N block?

The two forces are equal.

In SI units a force is numerically equal to the _____, when the force is applied to it.

acceleration of the standard kilogram

In uniform circular motion,

both the acceleration and the velocity are continually changing direction.

An object of mass m and another object of mass 2m are each forced to move along a circle of radius 1.0 m at a constant speed of 1.0 m/s. The magnitudes of their accelerations are:

equal

A coin is placed on a horizontal phonograph turntable. Let N be the normal force exerted by the turntable on the coin, f be the frictional force exerted by the turntable on the coin, and fs, max be the maximum force of the static friction. The speed of the turntable is increased in small steps. If the coin does not slide, then

f increases and both N and fs, max stay the same

A massless rope passes over a massless pulley suspended from the ceiling. A 4-kg block is attached to one end and a 5-kg block is attached to the other end. The acceleration of the 5-kg block is:

g/9

An object moves around a circle. If the radius is doubled keeping the speed the same then the magnitude of the centripetal force must be:

half as great

The mass and weight of a body:

have the same ratio as that of any other body placed at that location

The term "mass" refers to the same physical concept as:

inertia

A ball is thrown upward into the air with a speed that is greater than terminal speed. On the way up it slows down and, after its speed equals the terminal speed but before it gets to the top of its trajectory:

it continues to slow down

If a satellite moves above the Earth's atmosphere in a circular orbit with constant speed, then:

its acceleration is toward the Earth

You stand on a spring scale on the floor of an elevator. Of the following, the scale shows the highest reading when the elevator:

moves upward with increasing speed

The coefficient of kinetic friction:

none of the above

Acceleration is always in the direction:

of the net force

In uniform circular motion,

the acceleration and the velocity are always perpendicular.

The "reaction" force does not cancel the "action" force because:

the act on different bodies

A heavy ball is suspended as shown. A quick jerk on the lower string will break that string but a slow pull on the lower string will break the upper string. The first result occurs because:

the ball has inertia

A ball of mass m is thrown downward from the edge of a cliff with an initial speed that is three times the terminal speed. Initially the drag force on it is

upward and greater than mg

A 1-N pendulum bob is held at an angle q from the vertical by a 2-N horizontal force F as shown. The tension in the string supporting the pendulum bob is:

√5 N