unit 3
a child pulls on a wagon with a force of 75 N. if the wagon moves a total of 42 m in 3.1 min, what is the average power delivered by the child?
17 W
in an inelastic collision involving an isolated system, the final total momentum is
exactly the same as the initial momentum
in a perfectly inelastic collision
the colliding objects stick together
You throw a ball straight up. Compare the sign of the work done by gravity while the ball goes up with the sign of the work done by gravity while it goes down.
the work is negative on the way up and positive on the way down
An egg falls from a bird's nest in a tree and feels no effects due to the air. As it falls,
only its mechanical energy is conserved
A child does 350 J of work while pulling a box from the ground up to his tree house at a steady speed with a light rope. The tree house is 4.0 m above the ground. What is the mass of the box?
8.9 kg
When you throw a pebble straight up with initial speed V, it reaches a maximum height H with no air resistance. At what speed should you throw it up vertically so it will go twice as high?
square root of 2 • V
Two friends are standing on opposite ends of a canoe that is initially at rest with respect to a frictionless lake. The person in the front throws a very massive ball toward the back, and the person in the back catches it. After the ball is caught, the canoe is
stationary
In the figure, four point masses are placed as shown. Assume that all the numbers in the figure are accurate to two significant figures. What are the x and y coordinates of the center of mass ( or center of gravity) of this arrangement?
(2.3 m, 2.8 m)
a 35-N bucket of water is lifted vertically 3.0 m and then returned to its original position. how much work did gravity do on the bucket during this process?
0 J
a person carrries a 25.0 N rock through the path shown in the figure starting at point A and ending at point B. the total time from A to B is 1.5 min. how much work did GRAVITY do on the rock between A and B? (image has no vertical movement)
0 J
jennifer hits a stationary 0.20kg ball and it leaves her racket at 40 m/s. time lapse photography shows that the ball was in contact with the racket for 40 ms. what avg force did ball exert on racket & what is the ratio of this force to the weight of the ball?
0.2 kN and 100:1
how do you find impulse on a force vs time graph?
area under curve
in a game of pool, the white cue ball hits the #5 ball and stops, while the #5 ball moves away with same velocity as cue ball had originally. both balls have same mass. this type of collision is
elastic
if the work on an object is in the positive direction, the force and displacement must be
going in the same direction
A railroad car collides with and sticks to an identical railroad car that is initially at rest. After the collision, the kinetic energy of the system is
half as much as before
The momentum of an isolated system is conserved
in both elastic and inelastic collisions
in an elastic collision
kinetic energy and momentum are conserved
A rubber ball bounces off the walls with initial speed v and reverses it's directions so it's speed is v right after the bounce. As a result of this bounce, which of the following quantities of the ball are conserved?
kinetic energy of the ball
Two objects, one of mass m and the other of mass 2m, are dropped from the top of a building. If there is no air resistance, when they hit the ground
the heavier one will have twice the kinetic energy of the lighter one.
in an inelastic collision
momentum is conserved but kinetic energy is not
on a horizontal frictionless air table, a puck runs into an ideal horizontal spring that is fastened to the table. the puck compressed the spring by 15 cm before coming to rest. during the compression process, which quantities are conserved?
only the mechanical energy of the puck
If the units of your answer are kg•m2/s3, which of the following types of quantities could your answer be? (There could be more than one correct choice.)
power
if the answer to your calculation has units of kg • m2/s2; what type of quantity could it be? (more than 1)
rotational kinetic energy, torque, and work
A 30-N box is pulled upward 6.0 m along the surface of a ramp that rises at 37° above the horizontal. How much work does gravity do on the box during this process?
-110 J
three objects are moving along a straight line as shown in the figure. Taking the positive direction to be the right what is the total momentum of the system? One has a momentum of positive 40, the other has a momentum of -60, and the third one has a momentum of positive six.
-14.0 kg • m/s
Two ice skaters suddenly push off against one another starting from a stationary position. The 45-kg skater acquires a speed of 0.375 m/s relative to the ice. What speed does the 60-kg skater acquire relative to the ice?
0.28 m/s
in a police ballistics test, 2.00ag bullet traveling at 700 m/s suddenly hits and becomes embedded in a stationary 5.00 kg wood block. what is the speed of the block immediately after the bullet has stopped moving relative to the block?
0.28 m/s
It requires 0.30 kJ of work to fully drive a stake into the ground. If the average resistive force on the stake by the ground is 828 N, how long is the stake?
0.36 m
how much work is required to stretch an ideal spring of spring constant (force constant) 40 N/m from x=0.20 to x=0.25 m if the unstretched position is at x=0.00m
0.45 J
What is the net power needed to change the speed of a 1600-kg sport utility vehicle from 15.0 m/s to 40.0 m/s in 4.00 seconds?
275 kW
A block of mass m=34 kg and speed V is behind block of mass M=81 kg and speed of 0.50m/s, as shown in the figure. The surface is frictionless and the blocks collide and couple. After the collision, the blocks have a common speed of 0.90m/s. What is the magnitude of the impulse on the 34kg block due to the collision?
32 N•s
as shown in the figure, a bullet of mass 0.010 kg moving horizontally suddenly strikes a block of wood of mass 1.5 kg that is suspended to a pendulum. the bullet lodges in the wood, and together they swing upward a vertical distance of .4 m. the length of the string is 2.0 m. what was the speed of the bullet before it struck the wooden block?
420 m/s
Person X pushes twice as hard against a stationary brick wall as person Y. Which one of the following statements is correct?
Both do zero work
A 4.0 kg object is moving with speed 2.0 m/s. A 1.0 kg object is moving with speed 4.0 m/s. Both objects encounter the same constant braking force, and are brought to rest. Which object travels the greater distance before stopping?
Both objects travel the same distance
Three cars (car L, car M, and car N) are moving with the same speed and slam on their brakes. The most massive car is car L, and the least massive is car N. If the tires of all three cars have identical coefficients of kinetic friction with the road surface, for which car is the amount of work done by friction in stopping it the greatest?
Car L
You slam on the brakes of your car in a panic, and skid a certain distance on a straight, level road. If you had been traveling twice as fast, what distance would the car have skidded, under the same conditions?
It would have skidded 4 times farther.
A truck has four times the mass of a car and is moving with twice the speed of the car. If Kt and Kc refer to the kinetic energies of truck and car respectively, it is correct to say that
Kt = 16Kc.
A very light ping-pong ball moving east at a speed of 4 m/s collides with a very heavy stationary bowling ball. The Ping-Pong ball bounces back to the west, and the bowling ball moves very slowly to the east. Which object experiences the greater magnitude impulse during the collision?
Neither; both experienced the same magnitude impulse.
A rocket explodes into two fragments, one 25 times heavier than the other. The magnitude of the momentum change of the lighter fragment is
The same as the momentum change of the heavier fragment
if a force on an object is in the negative direction, the work it does on the object must be
The work could be either positive or negative, depending on the direction the object moves.
Three cars, car X, car Y, and car Z, begin accelerating from rest at the same time. Car X is more massive than car Y, which is more massive than car Z. The net accelerating force exerted on each car is identical. After 10 seconds, which car has the most amount of momentum?
They all have the same amount of momentum
during world war I, Germany used a "big bertha" Canon to hurl shells into Paris 30 miles away. This gun also had a very long barrel. What was the reason for using a long barrel in these guns?
to allow the force of the expanding gases from the gunpowder to act for a longer time
Swimmers at a water park have a choice of two frictionless water slides, as shown in the figure. Although both slides drop over the same height h, slide 1 is straight while slide 2 is curved, dropping quickly at first and then leveling out. How does the speed v1 of a swimmer reaching the bottom of slide 1 compare with v2, the speed of a swimmer reaching the end of slide 2?
v1 = v2
when you drop a pebble straight up with initial speed V, it reaches the ground with kinetic energy K if there is no air resistance. from what height should you drop it so it will reach the ground with twice as much kinetic energy?
2H
A bead is moving with a speed of 20 m/s at position A on the track shown in the figure. This track is friction-free, and there is no appreciable air resistance. What is the speed of the bead at point C?
34 m/s
A 1321-kg car climbs a 5.0° slope at a constant speed of Assuming that air resistance may be neglected, at what rate (in kW) must the engine deliver energy to the drive wheels of the car? A) 25 kW B) 287 kW C) 38 kW D) 48 kW
38 kW
A tiger is running in a straight line. If we double both the mass and speed of the tiger, the magnitude of its momentum will increase by what factor?
4
If you want to double the momentum of a gas molecule, By what factor must you increase its kinetic energy?
4
If you want to double the momentum of a gas molecule, by what factor must you increase its kinetic energy?
4
A 30-N stone is dropped from a height of 10 m and strikes the ground with a speed of 13 m/s. What average force of air friction acted on the stone as it fell?
4.1 N
A 0.10-kg ball, traveling horizontally at 25 m/s, strikes a wall and rebounds at 19 m/s. What is the magnitude of the change in the momentum of the ball during the rebound?
4.4 kg m/s
In the figure, a ball hangs by a very light string. What is the minimum speed of the ball at the bottom of its swing (point B) in order for it to reach point A, which is 1.0 m above the bottom of the swing?
4.4 m/s
A golf club exerts an average horizontal force of 1000 N on a 0.045-kg golf ball that is initially at rest on the tee. The club is in contact with the ball for 1.8 ms. What is the speed of the golf ball just as it leaves the tee?
40 m/s
When you drop a pebble from height H, it reaches the ground with speed V if there is no air resistance. From what height should you drop it so it will reach the ground with twice speed?
4h
A rock falls from a vertical cliff that is 4.0 m tall and experiences no significant air resistance as it falls. At what speed will its gravitational potential energy (relative to the base of the cliff) be equal to its kinetic energy?
6.3 m/s
an 80-kg man is skating northward and happens to suddenly collide with a 20-kg boy who is ice skating toward the east. immediately after the collision, the man and boy are seen to be moving together at 2.5 m/s in a direction 60 degrees north of east. how fast was the boy moving just before the collision?
6.3 m/s
a super dart of mass 20g, traveling at 350 m/s, strikes a steel plate at an angle of 30 degrees with the plane of the plate. it bounces off the plate at the same angle but at a speed of 320 m/s. what is the magnitude of the impulse that the plate gives to the bullet?
6.7 N•s
A 1200-kg car moving at 15.6 m/s suddenly collides with a stationary car of mass 1500 kg. If the two vehicles lock together, what is their combined velocity immediately after the collision?
6.9 m/s
A 1500-kg car accelerates from rest to 25 m/s in 7.0 s. What is the average power delivered by the engine? (1 hp = 746 W) A) 60 hp B) 70 hp C) 80 hp D) 90 hp
60 hp
a 2200-kg auto moving northward at 12.0 m/s runs into a 3800-kg truck which is also moving northward, but at 5.00 m/s. if the vehicles lock bumpers, how fast are they moving just after the collision?
7.57 m/s
a 475-gram ball is traveling horizontally at 12.0 m/s to the left when it is suddenly struck horizontally by a bat, causing it to reverse direction and initially travel at 8.5 m/s to the right. if the bat produced at average force of 1275 N on the ball, for how long (in ms) was it in contact w the ball?
7.637 ms
a block of mass m=4.4 kg, moving on a frictionless surface with speed vi= 9.2 m/s, makes a sudden perfectly elastic collision with a second block of mass M as shown in the figure. the second block is originally at rest. just after the collision the 4.4 kg block recoils with a speed of vf=2.5 m/s. what is the mass M of the second block?
7.7 kg
a person carries a 2.00 N pebble through the path shown in the figure starting at point A and ending at point B. (time is 6.75 min) how much work did gravity do on rock between a and b?
-30.0 J
A 2.0-kg ball moving eastward at 3.0 m/s suddenly collides with and sticks to a 4.0-kg ball moving northward at 2.0 m/s. What is the magnitude of the momentum of this system just after the collision?
10 kg m/s
a force acts on an object, causing it to move parallel to the force. the graph in the figure shows this force as a function of the position of the object. how much work does the force do as the object moves from 0 to 15 m?
125 J
A force produces power P by doing work W in a time T. What power will be produced by a force that does six times as much work in half as much time?
12p
Two frisky otters slide down frictionless hillsides of the same height but different slopes. The slope of the hill of otter 1 is 30°, while the slope of the hill of otter 2 is 60°. If both start from rest, which otter is moving faster when she reaches the bottom of her hill?
Both otters have the same speed at the bottom.
Two cyclists who weigh the same and have identical bicycles ride up the same mountain, both starting at the same time. Joe rides straight up the mountain, and Bob rides up the longer road that has a lower grade. Joe gets to the top before Bob. Ignoring friction and wind resistance, which one of the following statements is true?
The amount of work done by Joe is equal to the amount of work done by Bob, but the average power exerted by Joe is greater than that of Bob.
Consider two less-than-desirable options. In the first you are driving 30 mph and crash head-on into an identical car also going 30 mph. In the second option you are driving 30 mph and crash head-on into a stationary brick wall. In neither case does your car bounce back from the thing it hits, and the collision time is the same in both cases. Which of these two situations would result in the greater impact force on your car?
The force would be the same in both cases.
A very elastic rubber ball is dropped from a certain height and hits the floor with a downward speed v. Since it is so elastic, the ball bounces back with the same speed v going upward. Which of the following statements about the bounce are correct?
The magnitude of the ball's momentum was the same just before and just after the bounce.
A 5-kg ball collides inelastically head-on with a 10-kg ball, which is initially stationary. Which of the following statements is true? (There could be more than one correct choice.)
The magnitude of the change of velocity the 5-kg ball experiences is greater than that of the 10-kg ball AND The magnitude of the change of the momentum of the 5-kg ball is equal to the magnitude of the change of momentum of the 10-kg ball
A small car meshes with a large truck in a head-on collision. Which of the following statements concerning the momentum during the collision are correct?
The momentum of the car-truck system is conserved, but the momentum of each one separately is not conserved.
A rocket explodes into two fragments, one 25 times heavier than the other. The magnitude of the momentum change of the lighter fragment is
The same as the momentum change of the heavier fragment.
A small car meshes with a large truck in a head on collision. Which of the following statements concerning the magnitude of the momentum change during the collision is correct?
The small car and the truck experience the same momentum change
A girl throws a stone from a bridge. Consider the following ways she might throw the stone. The speed of the stone as it leaves her hand is the same in each case, and air resistance is negligible. Case A: Thrown straight up. Case B: Thrown straight down. Case C: Thrown out at an angle of 45° above horizontal. Case D: Thrown straight out horizontally. In which case will the speed of the stone be greatest when it hits the water below?
The speed will be the same in all cases.
You throw a baseball straight up. Compare the sign of the work done by gravity while the ball goes up with the sign of the work done by gravity while it goes down.
The work is negative on the way up and positive on the way down.
A lightweight object and a very heavy object are sliding with equal speeds along a level frictionless surface. They both slide up the same frictionless hill. Which rises to a greater height?
They both slide to the same height.
a heavy rock and a light rock are dropped from the same height and have no air resistance. which is correct? (more than 1) a) when they reach ground, heavier has more KE than lighter b) heavier reaches ground first c) both have same speed when reaching ground d) both have same kinetic energy when reach ground e) when released, both had same gpe
a c
a heavy stone and a light stone are released from rest in such a way that they both have same amount of gpe as they are released. air resistance is negligibly small. which is correct about the stones? (more than 1) a) both have same KE as they reach the ground b) stones released from same height c) initial height of light stone is greater than initial height of heavy stone d) as it reaches the ground, light is traveling faster than heavy stone e) stones both have same speed as they reach the ground
a c d
two identical grasshoppers jump into the air with the same initial speed and experience no air resistance. grasshopper a goes straight up, but grasshopper b goes up at a 66 degree angle above the horizontal. which of the following statements about these grasshoppers is correct? (more than 1) a) at their highest point B is moving faster than A b) at highest point, both have same gpe. c) at their highest point, both have the same amount of kinetic energy d) at the highest point, both have same mechanical energy e) at highest point, A has more potential energy than B
a d e
A heavy dart and a light dart are launched vertically by identical ideal springs. Both springs were initially compressed by the same amount. There is no significant air resistance. Which of the following statements about these darts are correct? (There could be more than one correct choice.) a) at max height, both darts have same gpe b) heavy goes higher than light c) both reach same max height d) both began moving up w same inital speed e) light goes higher than heavy
a e
A heavy frog and a light frog jump straight up into the air. They push off in such away that they both have the same kinetic energy just as they leave the ground. Air resistance is negligible. Which of the following statements about these frogs are correct? (There could be more than one correct choice.) a) heavier frog moving faster than light frog just as they leave ground b) lighter frog goes higher than heavier frog c) just as they leave ground, lighter frog going faster than heavier frog d) both reach same max height e) heavier frog goes higher than lighter frog f) both leave ground w same speed
c d
A heavy dart and a light dart are launched horizontally on a frictionless table by identical ideal springs. Both springs were initially compressed by the same amount. Which of the following statements about these darts are correct? (more than 1) a) both darts move free of spring w same speed b) heavy dart had more epe than light dart c) lighter dart leaves spring moving faster than heavy dart d) darts both have same kinetic energy just as they move free of spring e) both had same initial epe
c d e
a heavy sled and a light sled both moving horizontally with the same KE, suddenly slide onto a rough patch of snow and eventually come to a stop. the coefficient of kinetic friction between the sleds and the rough snow is the same for both of them. (more than 1 correct) a) same distance on snow before stopping b) heavy sled will slide farther on the rough snow than the light slide c) the friction from the snow will do the same amount of work on both sleds d) the light sled will slide farther on the rough snow than the heavy sled e) on the rough snow, the change in KE will be the same for both sleds
c d e
you are standing on a skateboard, initially at rest. A friend throws a very heavy ball towards you. You have two choices about what to do with the other ball: either catch the ball or deflect it back to your friend with the same speed as it was originally thrown. Which choice would you make in order to maximize your speed on the skateboard?
deflect the ball back
Jill does twice as much work as Jack does and in half the time. Jill's power output is
four times Jack's power output.
A stone is held at a height h above the ground. A second stone with four times the mass of the first one is held at the same height. The gravitational potential energy of the second stone compared to that of the first stone is
four times as much
Joe and Bill throw identical balls vertically upward. Joe throws his ball with an initial speed twice as high as Bill. If there is no air resistance, the maximum height of Joe's ball will be
four times that of Bill's ball.
a rubber ball and a lump of clay have equal mass. they are thrown with equal speed against a wall. The ball bounces back with nearly the same speed with which it hit. The clay sticks to the wall. Which one of these objects experiences the greater momentum change?
the ball
Two objects of different masses have momentum of equal, non-zero magnitude. Which object has more kinetic energy?
the lighter object