physics 221 test 1

A block slides on a track from A to B to C to D. The section from A to B to C is frictionless, but for the section from C to D the coefficient of friction is not zero. Is the block's mechanical (ordered) energy increasing, decreasing, or constant in (a) region AB, (b) region BC, and (c) region CD?

(a) constant (b) constant (c) decreasing

(A) Can two force vectors of unequal magnitude sum to zero net force? (B) Can three force vectors of equal magnitude sum to zero net force?

(a) no, (b) yes

A cart can move to the right or left along a horizontal track (the positive part of the x axis) as shown in the figure below. Neglect friction. A force is applied to the cart. Choose an appropriate force versus time graph for each statement below. (i) The cart moves toward the right (away from the origin) with constant velocity. (ii) The cart moves toward the right and is speeding up at a steady rate (constant acceleration). (iii) The cart moves toward the left and is speeding up at a steady rate (constant acceleration). (iv) The cart moves toward the right, speeds up and then slows down.

(i) E, (ii) A, (iii) B, (iv) G

A boy holds a 40 N weight at arm's length for 10 s. His arm is 1.5 m above the ground. The work done by the force of the boy on the weight while he is holding it is

0

Two cylindrical artificial bones are made of the same material and length, one with twice the radius as the other. When the two have the same tension force applied, the larger bone stretches by what factor compared to the smaller bone

0.25

Another scheme to catch the roadrunner has failed and a safe falls from rest from the top of a 30.0 m high cliff toward Wile E. Coyote, who is standing at the base. Wile first notices the safe after it has fallen 15 m. How long does he have to get out of the way?

0.72 s s = ut + (1/2)at^2 30 = (0)t + (1/2)9.8t^2 t = 2.474 s 15 =(0)t + (1/2)9.8t^2 t = 1.749 s = 2.474 - 1.749 = 0.724 sec

A 753 kg car starts from rest. A constant force of 118 N acts on it. What is the distance (in m) it travels in 3.2 s?

0.8

In how many of the following situations does the car have a westward acceleration? -- The car travels westward at constant speed. -- The car travels eastward and speeds up. -- The car travels westward and slows down. -- The car travels eastward and slows down. -- The car starts from rest and moves toward the east.

1 when a car slows down, its initial speed is greater than the final speed and acceleration becomes negative

An apple at rest weighs 1 N. Now assume the same apple is in free fall near the surface of Earth. Neglect friction. The net force on the apple while it is in free fall is

1 N

Consider motion in one dimension. (The sign of the vector quantities is their direction indicator.) An object moves in the positive x-direction with speed 6.7 m/s for 2 s. It stops for 1.8 s and then moves in the negative x direction with speed 3.5 m/s for 1.9 s. What is the average velocity of this object in units of m/s?

1.18 m/s average velocity = total displacement / total time taken = (6.7*2) - (3.5*1.9) / (2+1.8+1.9) = 1.18 m/s

As a bacterium swims through water it propels itself with its flagella so as to overcome the frictional drag forces and move at, more or less, constant velocity of 100 micrometer/s for periods of time. If the frictional drag force on a bacterium is 0.1 N, how much work does the bacterium do in 1 s?

10 microJoule

A vector of magnitude 20 is added to a vector of magnitude 25. The magnitude of this sum might be

12 add vectors together if going same way, subtract if going opposite ways since 20+25 = 45, and 25-20 = 5, the answer must be between 5 and 45, which must be 12 based on other choices

A 2.1 kg object is brought to a stop from a speed of 3.9 m/s in 0.16 m by a spring system. What is the spring constant k of the spring?

1248

Consider motion in one dimension. (The sign of the vector quantities is their direction indicator.) An object moves in the positive x-direction with speed 4 m/s for 3.9 s. It stops for 1.1 s and then moves in the negative x direction with speed 0.8 m/s for 1.2 s. What is the displacement of this object in units of meter?

14.64 toward + ve x axis distance travelled = speed * time = 4*3.9 = 15.6 toward - ve x axis distance travelled = speed * time = 0.8*1.2 = 0.96 displacement = initial - final = 15.6-0.96 = 14.64

By what factor does the drag force on a car increase as its speed increases from 25 km/h to 100 km/h?

16

A stone is thrown directly upward with an initial speed of 5.7 m/s from a height of 12 m. What is the speed of the stone in m/s when it hits the ground? Use g = 9.8 m/s2.

16.36

An ideal spring with a spring constant of 4.8 N/cm and an equilibrium length of 15 cm hangs vertically from the ceiling. A 0.9 kg mass is attached to it. After all motion has damped out, what is the new length of the spring in cm? Enter a number with 1 digit behind the decimal point Let g = 9.8 m/s2..

16.8

How much work (in J) must a person do to lift 4 bundles of shingles each weighing 20 pounds 18 feet onto a roof? 1 pound = 4.448 N, 1 foot = 0.305 m

1954

Two packages are dropped from an airplane. A parachute can increase the cross sectional area of each packages by a factor of 75. The parachute on package 1 fails to open, and the terminal speed of package 1 is 21.5 m/s. The parachute on package 2 opens. What is the terminal speed of package 2 in m/s?

2.5

The mass of atoms is often given in atomic mass units (u). (1 u = 1.66*10-27 kg) The mass of a hydrogen atom is 1 u, the mass of a carbon atom is 12 u, and the mass of a oxygen atom is 16 u. Calculate the mass, in kg, of a water molecule, H2O.

2.99*10^-26 H = 1 u, O = 16 u 2(1u)+1(16u) = 18u = 18u (1.66*10^-27 / 1u) = 2.99*10^-26 kg

Consider motion in one dimension. (The sign of the vector quantities is their direction indicator.) An object moves in the positive x-direction with speed 1.9 m/s for 2 s. It stops for 1.7 s and then moves in the negative x direction with speed 3.8 m/s for 4.5 s. What is the total distance traveled by the object in units of meter?

20.9 distance traveled toward + ve axis = speed * time = 1.9 * 2 = 3.8 m distance traveled toward - ve axis = speed * time = 3.8 * 4.5 = 17.1 = 3.8 + 17.1 = 20.9 m

A car with a mass of 750 kg is moving to the right with a constant acceleration of 2.8 m/s. What is the total force (in N) on the car?

2100

The mass of a a satellite in geostationary orbit is 1108 kg. The mass of Earth is 5.97*1024 kg. Because of the force Earth exerts on the satellite, it accelerates. The magnitude of the acceleration is 0.223 m/s2, and it is directed towards Earth. What is the magnitude of the force in N the satellite exerts on Earth?

247

Suppose you do 1000 J of work on a 4 kg object, and all the work goes into lifting it above the ground. How high above the ground will the object be when all the work has been done?

25 m

A 3.5 kg object has kinetic enery 9254 J. What is the magnitude of its momentum (in kg m/s)?

255

In SI units, what is the magnitude the net force acting on a 1921 kg car that accelerates uniformly along a straight line from a speed of 6 m/s to a speed of 17 m/s in 8 s?

2641

A car engine delivers 2.9 * 104 W to its wheels when moving at a constant speed of 9.3 m/s. What is the magnitude of the resistive force (in N) acting on the automobile at that speed?

3118

A 100 kg runner with a speed of 5.8 m/s makes a sharp turn with a radius of 1 m to avoid a collision. What is the magnitude of the frictional force (in N) the ground exerts on the runner during the turn?

3364

A 5 kg object is sliding on a surface with a coefficient of kinetic friction of 0.52. If at t = 0 it has a speed of 21 m/s, at what time (in s) will it come to rest? Let g = 9.8 m/s2.

4.1

A person walks first at a constant speed of 6 m/s along a straight line from point A to point B and then back along the line from B to A at a constant speed of 4 m/s. What is her average speed over the entire trip? Hint: Assume the distance between A and B is d. How much distance does the person cover on her trip? How long does it take her to cover this distance?

4.8 m/s average speed = total distance / total time =2 / (1/6 + 1/4) = 4.8 m/s

An 4.9 kg object gains height 6.8 while being accelerated from rest to a final speed of 7.5 m/s by an external force. What was the work (in J) done on the object by this force?

464

A ball is projected horizontally with speed 23.2 m/s from the top of a 21.2 m high building. Neglecting drag, what is its horizontal distance (in m) from the bottom of the building when it hits the ground? Let g = 9.8 m/s2.

48.3

A spring is stretched by 4.1 cm when a force of 2 N is applied. If a force of 4.7 N is applied, how many additional cm does the spring stretch?

5.53

A driver in a car accelerating towards the right has an apparent weight with components 937 N pointing down and 535 N pointing towards the left. What is the magnitude of the acceleration of the car in m/s2? Enter a number with 1 digit after the decimal point. Let g = 9.8 m/s2.

5.6

A person weighing 718 N stands in an elevator that decelerates at a rate of 1.5 m/s2 while moving upward. What is the magnitude of the force in N exerted on the person by the floor of the elevator while it is decelerating?

608

In SI units, what is the change in the magnitude of the momentum of a 1196 kg car that accelerates uniformly along a straight line from a speed of 9 m/s to a speed of 16 m/s in 6 s?

8372

Two pitchers are standing side by side, and simultaneously throw baseballs A and B. The balls follow the parabolic trajectories shown. Which of the following statements is true?

Ball B his the ground before ball A.

Consider the following five graphs (note the axes carefully). Which of these represent(s) one-dimensional motion at constant acceleration, including a = 0?

I, II, IV, and V only

Two moving objects have masses m1 and m2, respectively. If m2 = 4m1, and both have the same kinetic energy, which has more momentum?

Object 2 with mass m2.

A ball is thrown straight upward. It rises in the air and then falls back to earth. Neglect friction.

On the way up, kinetic energy is converted to potential energy.

Which of the following statements about distance and/or displacement are TRUE? (1) Distance is a vector quantity and displacement is a scalar quantity. (2) If a person walks in a straight line and never changes direction, then the distance and the magnitude of the displacement are exactly the same. (3) The diagram below depicts the path of a person walking from position A to B back to C to and then to D. The distance traveled is 90 yds. (4) For the same diagram below, the displacement of the person walking from A to B to C to D is 50 yds.

Only statements 2 and 3 are true

Which of the following observations about the frictional force is incorrect?

The magnitude of the force of static friction is always proportional to the normal force.

A box is at rest on a flat table. Which of the following is true?

The normal force and the gravitational force are equal in magnitude and opposite in direction.

A woman pulls a crate up a rough, inclined plane at constant speed. Which one of the following statements concerning this situation is false?

The work done on the object by gravity is zero.

Why do elastic balls bounce so well?

They store energy through compression, like a spring.

You are cutting wood with a hand saw. You have to push the saw away from you as it moves away from you and pull it towards you as it moves towards you. When are you doing positive work on the saw?

When pushing it away from you and when pulling it towards you.

Can an object's velocity change direction when its acceleration is constant?

Yes, this is possible, and a rock thrown straight up is an example. an object's velocity can change direction when its acceleration is constant

You drop an object from a bridge and simultaneously, you throw a-other object downwards at 10 m/s. As the objects are falling, which object has the larger acceleration? (Ignore air resistance.)

both objects have the same acceleration

Car 1 starts from rest along a straight highway with constant acceleration a = 1 m/s2. Car 2 passes car 1 with constant velocity of 120 km/hr. Which car is acted on by a larger net force?

car 1

A small metal cylinder rests on a circular turntable, rotating at a constant speed, as illustrated in the diagram above. Which of the sets of vectors best describes the velocity, acceleration, and net force acting on the cylinder at the position indicated in the diagram?

d

In order to do a positive amount of work you must

exert a force and move in the direction of the force.

Your class is rather unhappy with the instructor, and they pitch in and decide to fund a sabbatical for him to go to Mars. Unbeknownst to the students the following semester, the instructor has made long distance course arrangements so the "show can go on". The instructor begins talking about mass, weight and related things. He correctly makes which following statement:

his mass is still essentially unchanged but his weight is less than on earth.

A clever physics student is able to sneak a scale on a popular ride at an amusement park. Before the ride begins to move, the scale reads 700 N for the student's weight. As the ride lifts, it initially accelerates upward at 3 m/s2. At the top, the riders are held in suspense before the gondola is allowed to free fall downward with acceleration equal to g. What should the scale read for the student's apparent weight during the lift and the fall?

lift: 910 N, fall: 0 N

Rockets receive their forward propulsion through

pushing against the exhaust

After clearing the bar on a high jump, you land softly on a giant mattress. Landing on a mattress is more comfortable than landing on a sand heap of equal size because

the force the mattress exerts on you to stop your descend is much less than the force the sand heap would exert on you.

In an inertial frame, an object cannot remain at rest unless which of the following holds?

the net force acting on it is zero

An object is thrown straight up. At the top of its path

the velocity is zero and the acceleration is equal to g pointing down.

Newton's 3rd law is also called the law of action and reaction. The "reaction" force does not cancel the "action" force because

they act on different bodies

Two carts A and B are moving towards each other as shown in the diagram. They collide and come to rest at the point of impact. From this observation we can conclude that the carts

were moving with momenta equal in magnitude and opposite in direction.

Refer to the diagram below. Blocks I and II, each with a mass of 1 kg, are hung from the ceiling of an elevator by ropes 1 and 2. What is the force exerted by rope 1 on block I when the elevator is travelling upward at a constant speed of 2.0 m/s?

~10 N

A mass m is lowered gently onto a vertical spring of length L with spring constant k until it just touches the spring. Let y be the distance the spring is compressed and v be the velocity of the mass. The mass is released from rest. Which of the following is the equation for conservation of energy?

½mv2 + ½ky2 + mg(L - y) = mgL