Physics exam 1

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? Only when pulling it towards you. You are not doing work on the saw, the saw is doing work for you. Only when pushing it away from you. When pushing it away from you and when pulling it towards you.

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

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

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 4 2 1 0.5 0.25

0.25

A 1280 kg car starts from rest. A constant force of 516 N acts on it. What is the distance (in m) it travels in 2 s?

0.8 1. f=ma (find a) 2. d=Vi+(axt^2/2)

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

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 the force when something is free falling is it's weight

Two packages are dropped from an airplane. A parachute can increase the cross sectional area of each packages by a factor of 34. The parachute on package 1 fails to open, and the terminal speed of package 1 is 7.5 m/s. The parachute on package 2 opens. What is the terminal speed of package 2 in m/s?

1.3 m/s V1/V2=√(A2/A1) 1. 7.5/V2=√(34) 7.5/V2=5.83 V2=7.5/5.83 V2= 1.3

How much work (in J) must a person do to lift 4 bundles of shingles each weighing 14 pounds 19 feet onto a roof? 1 pound = 4.448 N, 1 foot = 0.305 m

1443.465 1. 4(14)=56 lb *4.448 N=249.088 N 2. 19 ft *.305 m= 5.795 m 3. 5.795*249.088=1443.465

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 2.9 kg object is brought to a stop from a speed of 2.6 m/s in 0.11 m by a spring system. What is the spring constant k of the spring?

1620 2.9(2.6²)/(0.11²)=1620

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

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? 250 m 1000 m 100 m 25 m 500 m

25 m

An ideal spring with a spring constant of 4.6 N/cm and an equilibrium length of 23 cm hangs vertically from the ceiling. A 1.9 kg mass is attached to it. After all motion has damped out, what is the new length of the spring in cm?

27 1. F=k*∆x ∆x=f/k= (1.9*9.8)/4.6=4.05 2. x+∆x=23+4.05=27

A 5 kg object is sliding on a surface with a coefficient of kinetic friction of 0.72. If at t = 0 it has a speed of 28 m/s, at what time (in s) will it come to rest? Let g = 9.8 m/s2.

3.97 1. friction force=kinetic friction* (mass*gravity) 2. -F=ma (solve for a) 3. Vf=Vi+a(t) (solve for t)

A ball is projected horizontally with speed 13.3 m/s from the top of a 25.5 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.

30.3 sqrt (2*25.5/9.8)=2.28 13.3*2.28=30.3

A car engine delivers 2.2 * 104 W to its wheels when moving at a constant speed of 7.3 m/s. What is the magnitude of the resistive force (in N) acting on the automobile at that speed?

3013.7 power=force*speed 2.2*10⁴=x*7.3 x=3013.7

An 3.7 kg object gains height 6.4 while being accelerated from rest to a final speed of 8 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.

350.464 work=∆KE+∆PE ∆KE=1/2mv²= 1/2(3.7)(8²)=118.4 ∆PE=mgh= 3.7(9.8)(6.4)=232.064 118.4+232.064=350.464

The mass of a a satellite in geostationary orbit is 1766 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?

394 mass*acceleration

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

A 99 kg runner with a speed of 6.5 m/s makes a sharp turn with a radius of 0.9 m to avoid a collision. What is the magnitude of the frictional force (in N) the ground exerts on the runner during the turn?

4648 force=mass*(speed ^2)/radius

A driver in a car accelerating towards the right has an apparent weight with components 785 N pointing down and 411 N pointing towards the left. What is the magnitude of the acceleration of the car in m/s2? Let g = 9.8 m/s2.

5.1 (411*9.8)/785

An object's velocity in the positive x-direction changes from 4.6 m/s to 12.3 m/s in 4.3 s. A force of constant magnitude 11 N is acting on it. What is the object's mass in kg?

6.15 1. a=(final-initial)/t 2. F=ma

A spring is stretched by 6.8 cm when a force of 2 N is applied. If a force of 4 N is applied, how many additional cm does the spring stretch?

6.8 1. F=k*x1 (find k) 2. x2= F/k (find new x) 3. x2-x1 (subtract old x from new x)

Two children pull a sled as shown. The sled moves with constant velocity v. Find the frictional force on the sled. 90.6 N 50 N 45 N 42 N 0

90.6 N

A car with a mass of 259 kg is moving to the right with a constant acceleration of 3.7 m/s2. What is the total force (in N) on the car?

958 f=ma

Which of the following statements about vectors and scalars are TRUE? A scalar quantity has a magnitude and a vector quantity does not. A vector is a large quantity and a scalar is a small quantity. The quantity 9.8 m/s/s is an acceleration value and as such is a vector quantity. A vector quantity has a direction and a scalar does not. The quantity 20 m/s, north is a speed and as such is a scalar quantity.

A vector quantity has a direction and a scalar does not.

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. The thrown object. It depends on the masses of the objects. The dropped object.

Both objects have the same acceleration

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. F and a pointing downward, v pointing to the right

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.

E= straight line on x axis A= straight line above the x axis B=straight line below x axis G= downward straight line

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 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: 2100 N, Fall: 700 N Lift: 490 N, Fall: 1400 N Lift: 910N, Fall: 0 N Lift: 490 N, Fall: 0 N Lift: 910 N, Fall: 700 N

Lift: 910N, Fall: 0 N

A person weighing 563 N stands in an elevator that decelerates at a rate of 1.2 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?

N=494; N=mg-ma 1. 563(1-(1.2/9.8))

A ball is thrown straight upward. It rises in the air and then falls back to earth. Neglect friction. It loses energy throughout its motion. Its total energy is largest at maximum height. On the way up, kinetic energy is converted to potential energy. It loses mechanical energy as it rises and gains it back as it falls. None of the above.

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

Which of the following observations about the frictional force is incorrect? The magnitude of the force of kinetic friction is always proportional to the normal force. The magnitude of the force of static friction is always proportional to the normal force. The magnitude of the force of static friction is always proportional to the external applied force. The direction of the force of kinetic friction is always opposite to the direction of the relative motion of the object with respect to the contact surface. The direction of the force of static friction is always opposite to the direction of the impending motion of the object with respect to the contact surface. All of the above statements are correct!

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

In an inertial frame, an object cannot remain at rest unless which of the following holds? The net force acting on it is zero. There are no forces at all acting on it. The net force acting on it is constant. There is only one force acting on it.

The net force acting on it is zero.

A box is at rest on a flat table. Which of the following is true? The normal force and the gravitational force are an action-reaction pair. The normal force and the gravitational force are equal in magnitude and opposite in direction. Gravity exerts a force on the box that depends on how big the normal or support force is. The magnitude of normal force depends on the coefficient of static friction between the book and the table.

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

A force of 20 N in the positive x‐direction accelerates an object giving it a = 2 m/s2. When the object is moving with velocity v = 10 m/s, an additional force of 10 N in the negative x‐direction is applied. Which statement is true? The object now moves with constant velocity. The object slows down and comes to a stop. The object now accelerates with acceleration a = 1 m/s2 in the positive x‐direction. The object now accelerates with acceleration a = ‐1 m/s2 in the negative x‐direction.

The object now accelerates with acceleration a = 1 m/s2 in the positive x‐direction.

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. Yes, this is possible, but then the net force must change direction. Yes, this is possible, and a car that starts from rest, speeds up, slows to a stop, and then backs up is an example. No, this is not possible because it is always speeding up or always slowing down, but it can never turn around. No, this is not possible because it is always speeding up.

Yes, this is possible, and a rock thrown straight up is an example.

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 A hits the ground before ball B. Ball B his the ground before ball A. Since ball B travels farther, it must have higher initial speed v0. Since ball A reaches a greater height, it must have higher initial kinetic energy.

b hits the ground before a

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 The net force on both cars is zero. Both cars are acted on by the same non-zero net force. Car 2

car 1

In order to do a positive amount of work you must change an object's position. exert a force and not move. exert a force and just move. exert a force and move in the direction of the force. exert a force or move.

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

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.

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. his weight is still essentially unchanged but his mass is less than on earth. his weight and mass have both decreased significantly. his weight and mass have both increased significantly. neither his weight nor his mass have changed much.

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

A bungee jumper jumps from a bridge. Describe what happens to the jumper's kinetic and (gravitational and elastic) potential energy right after the jumper has reached the bottom of his descent and is pulled back up? kinetic energy increases, potential energy stays the same kinetic energy increases, potential energy decreases kinetic energy decreases, potential energy increases kinetic energy increases, potential energy increases kinetic energy stays the same, potential energy increases

kinetic energy increases, potential energy decreases

(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?

no yes

Is it possible to have motion in the absence of a force? No. Only in a non-inertial frame. Only in outer space. Only in a moving frame. None of the above.

none of the above

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.

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

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 the action force is greater than the reaction force. they are in the same direction. the reaction force exists only after the action force is removed. the reaction force is greater than the action force. they act on different bodies.

they act on different bodies

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 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. 60 J. 4 J. 90 J. 6 J.

zero b/c he's not moving so he's not doing work

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