Physics 221 Midterm

(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 ball is thrown vertically upward, and as it comes down, it is caught at its initial position. Which of the following graphs best represents the velocity of the ball as a function of time?

(C) Line with a decreasing slope

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 ball is launched up a ramp by a spring as shown in the figure. At the time when the clock starts, the ball is near the bottom of the ramp and rolling up the ramp as shown. It goes to the top and then rolls back down.For the graphs shown below, the horizontal axis represents the time. The vertical axis is unspecified. Select the number of the graph that could correctly represent(i) the x-component of the ball's position,(ii) the x-component of the ball's velocity,(iii) the x-component of the ball's acceleration.

(i) 8, (ii) 3, (iii) 2

A toy car is moving on a straight track along the positive x-axis. It stays to the right of the origin. For the situations described below, choose the letter of the correct acceleration vs. time graphs that corresponds to the motion described. (i) The car speeds up at a constant rate, moving away from the origin. (ii) The car slows down at a constant rate, moving away from the origin. (iii) The car moves at a constant speed toward the origin.

(i) B, (ii) D, (iii) E (Graphs with horizontal lines)

Three vectors A, B, and C are shown. The vector sum of these three vectors is S = A + B + C. What is the value of Sx, the x-component or horizontal component of S?Let each box be one unit wide and one unit tall.

-3 units

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

Two packages are dropped from an airplane. A parachute can increase the cross sectional area of each packages by a factor of 79. The parachute on package 1 fails to open, and the terminal speed of package 1 is 16.9 m/s. The parachute on package 2 opens. What is the terminal speed of package 2 in m/s? Enter a number with one digit after the decimal point.

1.9 m/s

Refer to the diagram below. A block with a mass of 1kg is hung from the ceiling of an elevator by a rope. What is the force exerted by rope on the block when the elevator is travelling upward at a constant speed of 2.0 m/s?

10 N

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

You have landed on a planet with no atmosphere. To determine the magnitude of the gravitational acceleration g' near the surface of the planet, you drop a small ball and record the distance fallen y as a function of time t. Here are your data. t(s),y(m) 0,0 0.1,0.05 0.2,0.2 0.3,0.46 0.4,0.86 0.5,1.3 0.6,1.95 0.7,2.45 0.8,3.35 0.9,4.37 1,5.25 Paste this data table into an Excel spreadsheet. Produce a graph of y(m) versus t(s). Add a trendline. Choose Polynomial, Order 2 and under options click "Display equation on chart". Format the trendline label so that it shows 3 decimal places. What is g' based on these data? Enter a number with 3 digits behind the decimal point.

10.716

A 1000 kg block sits on a frictionless table, connected by a massless rope over a frictionless pulley to a 0.01 kg washer hanging off the edge of a table. What is the magnitude of the acceleration of the washer?

10^-4 m/s^2

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

12

A car engine delivers 2.4 * 104 W to its wheels when moving at a constant speed of 19.5 m/s. What is the magnitude of the resistive force (in N) acting on the automobile at that speed? Enter an integer.

1231 N

A Boeing 737 jet plane lands with a speed of 60 m/s (about 135 mi/h) and can decelerate on a wet runway at a maximum rate of 4 m/s2 as it comes to rest. What is the minimum time needed before the plane will come to rest?

15 s

An 4.5 kg object gains height 7.4 while being accelerated from rest to a final speed of 8.3 m/s by an external force. What was the net work (in J) done on the object? Let g = 9.8 m/s2. Enter an integer.

155 J

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 8.3 m/s from a height of 13 m.What is the speed of the stone in m/s when it hits the ground? Use g = 9.8 m/s2.Enter a number with 2 digits behind the decimal point.

17.99 m/s

Use g = 10 m/s2. Water descends from the top of a tall hydroelectric dam. Its gravitational potential energy is eventually converted into electric energy. How much gravitational potential energy is released when 1000 kg of water descend 200 m to the generators?

2 MJ

A stone is thrown directly upward with an initial speed of 9.2 m/s from a height of 10.7 m.After what time interval (in s) does the stone strike the ground? Use g = 9.8 m/s2. Enter a number with 2 digits behind the decimal point.

2.69 s

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

The mass of a a satellite in geostationary orbit is 1,077 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? Enter an integer, Round up or down correctly.

240 N

A car with a mass of 982 kg is moving to the right with a constant acceleration of 2.5 m/s2.What is the total force (in N) on the car?Enter a integer. Round up or down correctly.

2455 N

An ideal spring with a spring constant of 4.5 N/cm and an equilibrium length of 22 cm hangs vertically from the ceiling. A 1.4 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 pointLet g = 9.8 m/s2..

25 cm

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

When struck by a club, a golf ball initially at rest acquires a speed of 31 m/s. If the ball is in contact with the club for 1.17 ms, what is the magnitude of the average acceleration (in m/s2)?(Type a number with 3 significant figures into the box.)

26500 m/s^2

An 2.7 kg object gains height 6 while being accelerated from rest to a final speed of 9.5 m/s by an external force. What was the work (in J) done on the object by this force? Let g = 9.8 m/s2. Enter an integer.

281 J

You walk in a given direction for 10 m during the first 3 seconds of a trip and then 20 m during the next 7 seconds. Your average speed is equal to

3 m/s.

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 9.3 m/s for 3.4 s. It stops for 1.4 s and then moves in the negative x direction with speed 2.8 m/s for 2.6 s. What is the average velocity of this object in units of m/s? Enter a number with two digits behind the decimal point.

3.29 m/s

A girl pulling a sled exerts a 20 N force horizontally for 40 s. How much power does she generate while moving the sled 60 m?

30 W

A ball is projected horizontally with speed 16.2 m/s from the top of a 18 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. Enter a number with 1 digit behind the decimal point.

31 m

Vector A has magnitude 10 and vector B has magnitude 15. What is NOT a possible magnitude of vector C = A + B?

4

An object's velocity in the positive x-direction changes from 4.5 m/s to 10.8 m/s in 2.5 s.A force of constant magnitude 12 N is acting on it. What is the object's mass in kg? Enter a number with two digits behind the decimal point.

4.76 kg

You operate a 120 W light bulb for 1 hour. How much energy did you use?(1 W = 1 J/s)

432000 J

Which position versus time graph does NOT represent motion with constant acceleration?

5 graph with bump in the line

Use g = 10 m/s2. A crane lifts a 200 kg object to a height of 20 m in 8 s. What is its power output?

5000 W

An object's velocity in the positive x-direction changes from 3.6 m/s to 9.1 m/s in 3.4 s.A force of constant magnitude 10 N is acting on it. What is the object's mass in kg? Enter a number with two digits behind the decimal point.

6.18 kg

A spring is stretched by 7.7 cm when a force of 2 N is applied. If a force of 3.7 N is applied, how many additional cm does the spring stretch? Enter a number with two digits behind the decimal point.

6.55 cm

A 1200 kg car's speed changes from 0 to 20 m/s in 4 s. What is the average power of the car motor during this time period? Neglect friction.

60 kW

A 1400 kg car accelerates uniformly from rest to 60 mph in 6 s. Find the net force needed to produce this motion.

6259 N

The magnitude of the force vector with (x, y) components (5 N, 5 N) is

7.1 N.

A 5 kg object is sliding on a surface with a coefficient of kinetic friction of 0.36. If at t = 0 it has a speed of 25 m/s, at what time (in s) will it come to rest? Let g = 9.8 m/s2. Enter a number with one digit after the decimal point.

7.1 s

A driver in a car accelerating towards the right has an apparent weight with components 749 N pointing down and 612 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.

8 m/s^2

A 1,062 kg car starts from rest. A constant force of 727 N acts on it. What is the distance (in m) it travels in 4.9 s? Enter a number with two digits behind the decimal point.

8.22 m

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 2.5 m/s for 2.1 s. It stops for 4.6 s and then moves in the negative x direction with speed 1.0 m/s for 3.4 s.What is the total distance traveled by the object in units of meter?Enter a number with two digits behind the decimal point.

8.65 m

A 4.9 kg object is brought to a stop from a speed of 2.0 m/s in 0.15 m by a spring system. What is the spring constant k of the spring? Enter an integer. Round up or down correctly.

871

How much work (in J) must a person do to lift 4 bundles of shingles each weighing 16 pounds 11 feet onto a roof? 1 pound = 4.448 N, 1 foot = 0.305 m Enter an integer. Round up or down correctly.

955 J

A particle is moving with constant speed along the circular path shown. Its velocity vector is shown at two different positions. What is the direction of the acceleration when the particle is at point X?

A

Which one of the following situations is not possible?

A body travels with a constant velocity and a time-varying acceleration.

Which system is in a state of stable equilibrium?

A marble in the bottom of a spherical bowl

Three trajectories labeled A, B, and C are shown below. For which trajectory is the vertical velocity component at launch the greatest?

All three trajectories have the same initial vertical velocity component at launch.

Run the simulation Energy Skate Park Basic (Links to an external site.) and choose Playground. Construct a track as shown, small mass, no friction. You have two choices for the skateboard. It can either run in a groove and be fixed to the track (left choice) or it can be allowed to come off the track (right choice). Start the skater in the position shown with the skateboard allowed to come off the track. (W track) Which of the statements below are correct for this track and starting condition? You can choose more that one.

At some point on the left side of the track the skater leaves the track and executes projectile motion for some time interval. Even though there is no friction, mechanical energy is not conserved.

A ball is fired by a cannon from the top of a cliff as shown in the figure below. Which of the path would the cannonball most closely follow?

B

The graph shows position as a function of time for two trains running on parallel tracks. Which is true?

Both trains have the same velocity at some time before tB.

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 Force and Acceleration pointed down, Velocity pointed to the right.

You have watched one too many old cartoons and decide to drive around town with a giant spring that is 4 m long and attached to the front of your car. You decide to help a semi truck that is stuck and try to push them with your car. You take a running start, and when you contact the truck the spring in your car compresses 3 m. During which portion of the compression did you do the most work on the spring?

During the third meter.

Run the simulation Energy Skate Park Basic (Links to an external site.) and choose Playground. Construct a track as shown, small mass, no friction. You have two choices for the skateboard. It can either run in a groove and be fixed to the track (left choice) or it can be allowed to come off the track (right choice). Start the skater in the position shown with the skateboard allowed to come off the track. (looped track 2, skater starts lower on this one) Which of the statements below are correct for this track and starting condition? You can choose more that one.

Even though there is no friction, mechanical energy is not conserved. At some point on the right side of the track the skater leaves the track and executes projectile motion for some time interval.

A friend is casually talking with you about physics concepts and brings up the following claim. "They say that average speed for a trip is total distance divided by total time, which is true. Then they say that if the trip consists of a person traveling at two different speeds for two different times then you can get the average speed by just finding the average speeds for the two parts of the trip and then take their average." Is their claim true or false?

False

A stone is dropped from the roof of a single story building to the surface of the earth.

It speeds up because of an almost constant force of gravity acting upon it.

A car's speed changes from 20 mph to 60 mph. By what factor does its kinetic energy change?

It will increase by a factor of 9.

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: 910N, Fall: 0N

Run the simulation Energy Skate Park Basic (Links to an external site.) and choose Playground. Construct a track as shown, small mass, no friction. You have two choices for the skateboard. It can either run in a groove and be fixed to the track (left choice) or it can be allowed to come off the track (right choice). Start the skater in the position shown with the skateboard allowed to come off the track. (looped track) Which of the statements below are correct for this track and starting condition? You can choose more that one.

Mechanical energy is conserved. The skater returns to his starting position.

A ball is thrown straight upward. It rises in the air and then falls back to earth. Neglect friction. What is true about its energy?

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

The positions of two blocks at successive 0.20-second time intervals are represented by the numbered squares in the figure below. The blocks are moving towards the right. Are the blocks accelerating?

Only the upper block is accelerating.

A rock is thrown straight up from the surface of the Earth.Which one of the following statements describes the energy transformation of the rock as it rises?Neglect air resistance.

The KE decreases and the PE increases.

Assume there is a completely straight highway from city A to city B, which is 200 km long. You drive along the highway from A to B and back to A in 4 hours. What are the average speed and average velocity of that trip?

The average speed is 100 km/hr and the average velocity is zero.

A train is moving with constant velocity v along a straight portion of the track. A boy sitting in the train facing forward carefully places a ball on the floor, just in front of his seat.

The ball stays where the boy has placed it, because no net force is acting on it, and therefore it keeps on moving with constant velocity.

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 mass M, attached to a piece of string OM, is whirled round over a person's head in a horizontal circle. At the position shown in the diagram below, the string suddenly breaks. In which direction does the mass M move horizontally after the string breaks?

The mass M moves in the direction III.

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

The net force acting on it is zero.

A 10 kg and a 4 kg mass are acted on by the same magnitude net force (which remains constant) for the same period of time. Both masses are at rest before the force is applied. After this time, the 10 kg mass moves with a speed v1 and the 4 kg mass moves with a speed v2. Which of the following is true?

The ratio v1/v2 is equal to 2/5.

The graph shows the net force acting on an astronaut during a rocket launch into a stable orbit. During which time interval was the acceleration of the rocket the greatest?

U-V

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.

Is it possible to have motion in the absence of a force?

Yes, but only non-accelerated motion

Is it possible to have motion in the absence of a net force?

Yes.

Two objects are traveling around different circular orbits with constant speed. They both have the same centripetal acceleration, but object A is traveling twice as fast as object B. The orbit radius for object A is

four times the orbit for object B.

You paddle in a Kayak across a calm mountain lake. Each time you pull the paddle backward through the water, from the front of the boat to its rear, the water exerts a force on the paddle that is

in the forward direction, towards the front of the boat.

You paddle in a Kayak across a calm mountain lake. Each time you pull the paddle backward through the water, from the front of the boat to its rear, the water exerts a force on the paddle that is

in the forwards direction, towards the front of the boat

In the absence of air resistance, a ball of mass m is tossed upward to reach a height of 20 m. At the 10 m position, half way up, the net force on the ball is

mg

In the absence of air resistance, a ball of mass m is tossed upward to reach a height of 20 m. At the 15 m position, 3/4 of the way up, the net force on the ball is

mg.

Your hands push on a heavy box to slide it across the floor with constant velocity. The other force of the action/reaction pair is

the box pushing backward against your hands.

A car traveling at 100 km/hr strikes an unfortunate bug and splatters it. The magnitude of the force of impact is

the same for both

An elevator is being lifted up an elevator shaft at a constant speed by a steel cable as shown in the figure above. All frictional effects are negligible. In this situation, forces on the elevator are such that

the upward force by the cable is equal to the downward force of gravity.

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.

A woman exerts a constant horizontal force on a large box. As a result, the box moves across a horizontal floor at a constant speed v0. If the woman doubles the constant horizontal force that she exerts on the box, and the force resisting the motion of the box does not change, the box then moves

with constantly increasing speed

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