ch. 6 (physics) //6.1

In Figure 6-1, the block of mass m is at rest on an inclined plane that makes an angle θ with the horizontal. The force of static friction f must be such that

E) f = mg sinθ.

An airplane is flying with constant speed along a horizontal circle. Is the direction of its acceleration constant?

No. The acceleration is directed towards the center of the circle, so the acceleration vector rotates as the body rotates.

A net horizontal force is required for a body to move in a horizontal circle.

TRUE

An ideal pulley changes the direction of the tension in a string without changing its magnitude.

TRUE

For uniform circular motion, the velocity and acceleration vectors are perpendicular to each other at every point in the path.

TRUE

The banking angle for a properly banked curve does not depend on the mass of the car going over it.

TRUE

The coefficient of static friction is always larger than the coefficient of kinetic friction.

TRUE

The force fo static friction between two surfaces is parallel to the surface of contact, and in in the direction that opposes relative motion.

TRUE

The force of kinetic friction between two surfaces is independent of the area of contact between the surfaces.

TRUE

The force of static friction between two surfaces is independent of the area of contact between the surfaces.

TRUE

When a curve is properly banked a passenger in a car traveling on it at the designed speed does not feel a lateral force.

TRUE

A packing crate slides down an inclined ramp at constant velocity. Thus we can deduce that

A) a frictional force is acting on it.

In Figure 6-2 the scale at left is attached to the ceiling and a mass of 1.00 kg hangs from it. It reads 9.81 N. The identical scale at the right is connected by perfect strings passing over perfect pulleys to two 1.00 kg masses hanging vertically at the end of the strings. The scale at right reads

A) exactly 9.81 N.

A flatbed truck is carrying a crate along a level road. The coefficient of static friction between the load and the bed is 0.40. The truck accelerates forward and the crate stays in its place on the truck bed. In what direction is the force that the bed exerts on the crate?

A) forward

A roller coaster car is on a track that forms a circular loop in the vertical plane. If the car is to just maintain contact with track at the top of the loop, what is the minimum value for its centripetal acceleration at this point?

A) g downward

For an object that travels at a fixed speed along a circular path, the acceleration of the object is

A) larger in magnitude the smaller the radius of the circle.

Compare the two situations shown in Figure 6-3. On the left (A), James is holding the rope and keeping the bucket at rest. On the right (B), James ties the rope to the bucket so that it keeps the bucket at rest. In both cases the bucket contains the same quantity of water. In what case is the tension in the rope higher?

A) left

As a car drives with its tires rolling freely without any slippage, the type of friction acting between the tires and the road is

A) static friction.

Two identical masses are attached by a light string that passes over a small pulley, as shown in Figure 6-6. The table and the pulley are frictionless. The masses are moving

A) with an acceleration less than g.

You are driving in your car with a sack of groceries in the seat next to you. You see a light change and you slow down and the sack remains on the seat. Suddenly the car in front of you slams on the brakes, and you are forced to brake harder. The groceries slide off the seat. Explain what happened.

At the lower deceleration the frictional force required to decelerate the sack of groceries was below the maximum static friction between the sack and the seat, but at the higher deceleration the frictional force was not sufficient, so the sack slid.

Consider a particle moving with constant speed such that its acceleration of constant magnitude is always perpendicular to its velocity.

B) It is moving in a circle.

A 16-kg fish is weighed with two spring scales, each of negligible weight, as shown in Figure 6-4. What will be the readings on the scales?

B) The sum of the two readings will be 32 kg.

If a car slows down with the wheels rolling, is the frictional force between the tires and the ground kinetic or static?

B) static

When a car goes around a curve, it has a tendency to skid outwards. Is the frictional force between the tires and the ground that keeps the car from skidding kinetic or static?

B) static

A roller coaster car (mass = M) is on a track that forms a circular loop (radius = r) in the vertical plane. If the car is to just maintain contact with the track at the top of the loop, what is the minimum value for its speed at that point?

C) (rg)1/2

Two masses, m1 and m2, are connected to each other as shown in Figure 6-5. Mass m1 slides without friction on the table surface. Both masses have acceleration of magnitude a as shown. How does the tension in the string compare to the weight, m2 g, of mass m2?

C) The tension is smaller than m2 g.

An object moves in a circular path at a constant speed. Compare the direction of the object's velocity and acceleration vectors.

C) The vectors are perpendicular.

When an object experiences uniform circular motion, the direction of the acceleration is

C) is directed toward the center of the circular path.

When an object experiences uniform circular motion, the direction of the net force is

C) is directed toward the center of the circular path.

The banking angle in a turn on the Olympic bobsled track is not constant, but increases upward from the horizontal. Coming around a turn, the bobsled team will intentionally "climb the wall," then go lower coming out of the turn. Why do they do this?

C) to take the turn at a faster speed

In Figure 6-1, the block of mass m is at rest on an inclined plane that makes an angle θ with the horizontal. The normal force F acting on the block must be such that

D) F = mg cosθ.

Its more difficult to start moving a heavy carton from rest than it is to keep pushing it with constant velocity, because

D) μk < μs.

What type of acceleration does an object moving with constant speed in a circular path experience?

E) centripetal acceleration.

A body moving with constant speed cannot be accelerating.

FALSE

An object moving along a curved path with constant speed does not have a net force acting on it.

FALSE

The force of kinetic friction between two surfaces is dependent on the relative speed of the two surfaces.

FALSE

The stretch of a spring and the force it exerts are inversely proportional.

FALSE

When an object is in translational equilibrium, the net force acting on the object is non-zero.

FALSE

You are driving your car and set your sunglasses on the dashboard. When you make a left turn, the sunglasses go sliding off to the right. Explain.

If the glasses stay in place relative to the car, they are going around a curve and require a force perpendicular to the direction of motion to do this. If the frictional force is too small they do not follow the same path as the car but one that is less curved.

Why does a cyclist tilt her bicycle on a curve?

In order to make the force of the seat normal to the seat. The seat has to provide a vertical force equal to her weight and a horizontal centripetal force that keeps her going around the curve. The resultant is inclined at an angle, and if the bicycle is tilted at that angle she will not feel a tendency to slide off the seat.

State the condition for an object to be in translational equilibrium.

The net force acting on the object must be zero.

Is it possible for an object moving with a constant speed to accelerate? Explain.

Yes, although the speed is constant, the direction of the velocity can be changing.