ch. 7 & 8

Which one of the following statements is WRONG? (a) Doubling the speed of an object makes its kinetic energy four times larger. (b) A force that slows down a moving object, does negative work on the slowing object. (c) The kinetic energy of a body depends upon the speed of motion of the reference frame in which the kinetic energy is measured. (d) A force exerted perpendicular to the motion of a particle does work on that particle as it moves. (e) A car stops from a speed of 50 mph by braking so that the car slows with constant deceleration. Neglecting air friction, the work done on the car by friction with the ground is independent of the magnitude of this deceleration.

(d)A force exerted perpendicular to the motion of a particle does work on that particle as it moves.

Which one of the following statements is WRONG? (a) Doubling the speed of an object makes its kinetic energy four times larger. (b) A force that slows down a moving object, does negative work on the slowing object. (c) The kinetic energy of a body depends upon the speed of motion of the reference frame in which the kinetic energy is measured. (d) A force exerted perpendicular to the motion of a particle does no work on that particle as it moves. (e) A car stops from a speed of 50 mph by braking so that the car slows with constant deceleration. Neglecting air friction, the work done on the car by friction with the ground depends upon the magnitude of this deceleration

(e) A car stops from a speed of 50 mph by braking so that the car slows with constant deceleration. Neglecting air friction, the work done on the car by friction with the ground depends upon the magnitude of this deceleration.

A 10 kg block moving in a straight line is slowed down by a force that varies with position as shown in the figure at the right. If the force is directed opposite to the direction of motion, how much work is done by this force as the block moves from the origin to x=10m

-24 J

The total mechanical energy of an object is the _______.

-KE plus the PE of the object at any instant during its motion -the initial KE plus the initial PE of the object

which of the following is a differential equation that correctly describes Newton's second law for a simple harmonic oscillator of mass m and restoring force constant k?

-kx = m (d^2x/dt^2)

An unstretched ideal spring hangs vertically from a fixed support. A 0.4 kg object is then attached to the lower end of the spring. The object is pulled down to a distance of .35 m below the unstretched position and released from rest at time t=0. A graph of the subsequent vertical position y of the lower end of the spring was initially unstretched. At which of the following times is the upward velocity of the object the greatest?

.25s

A worker pushes a wheelbarrow with a force of 50 N over a level distance of 5.0 m. If a frictional force of 43 N acts on the wheelbarrow in a direction opposite to that of the worker, what net work is done on the wheelbarrow?

35 J

horsepower

373

Teacher squirrels

39.05 J

A mass m = 10 kg is projected up a 53o ramp with initial speed vo = 10 m/s. If it is subject to a constant friction force of 20 N, to what maximum height h does it rise?

4.0 m

An electrical motor provides .50 W of mechanical power. How much time will it take the motor to lift a 0.1 kg mass at constant speed from the floor to a shelf 2.0 m above the floor?

4.0s

A skier with a mass of 100 kg drops off a ramp of snow at a height of 4.0 m above the ground. What is the skier's gravitational potential energy at this point? What is the skier's vertical speed the moment he hits the ground? Assume no external work.

4000 J; 8.9 m/s

In the figure at the right, a stone of mass 5 kg is at rest on a spring, having caused the spring to compress by 20 cm from its unstretched length. If the stone is removed, and the spring is now compressed by 60 cm from its unstretched length, what potential energy is then stored in the spring?

45 J

A glider is gliding through the air at a height of 416 m with a speed of 45.2 m/s. The glider dives to a height of 278 m. Determine the glider's new speed. KEi + PEi + Wext = KEf + PEf

69.3 m/s

A particle of mass 10 kg, starting from rest, has an amount of work done on it that gives it a final speed of 5 m/s. If, instead, twice as much work had been done on it, its final speed would have been about:

7 m/s

A women weighing 50 N walks up 120 stairs, each of height .25m and then back down 60 stairs. How much net work does she do against gravity?

750J

An elevator motor lifts 715 kg of mass to the height of the fourth floor of an office building (11 meters above ground level) at a constant speed in 9.35 seconds. Determine the power rating of the motor.

84.118 W

Bart runs up a 2.91 meter high flight of stairs at a constant speed in 2.15 seconds. If Bart's mass is 65.9 kg, determine the work which he did and his power rating

891.9 W

In the figure at the right, a stone of mass 10 kg is at rest on a spring, having caused the spring to compress by 20 cm from its unstretched length. If the stone is removed, and the spring is now compressed by 60 cm from its unstretched length, what potential energy is then stored in the spring?

90 J

(A) A force is conservative if the work it does on a particle that moves through a round trip is zero; otherwise it is nonconservative. (B) A force is conservative if the work done by it on a particle that moves between two points is the same for all paths connecting those points; otherwise it is nonconservative. (C) A potential energy can be defined for any force. Which one of the following answers is completely correct?

A and B are correct, but C is wrong

Here are three statements: (A) A force is conservative if the work it does on a particle that moves through a round trip is zero; otherwise it is nonconservative. (B) A force is conservative if the work done by it on a particle that moves between two points is the same for all paths connecting those points; otherwise it is nonconservative. (C) A potential energy can only be defined for a conservative force. Which one of the following answers is completely correct?

All correct

During the Powerhouse lab, Jack and Jill ran up the hill. Jack is twice as massive as Jill; yet Jill ascended the same distance in half the time. Who did the most work? Who delivered the most power?

Jack does more work than Jill. Jack must apply twice the force to lift his twice-as-massive body up the same flight of stairs. Yet, Jill is just as "power-full" as Jack. Jill does one-half the work yet does it one-half the time. The reduction in work done is compensated for by the reduction in time.

If an object is at rest, it certainly does NOT possess this form of energy.

Kinetic Energy

What is the SI unit for work? What are other units?

Nm; kg x m^2/s^2, J, foot x lb

Is work done when a teacher applies a force to a wall and becomes exhausted?

No because the wall is not displaced. A force must cause a displacement in order for work to be done.

Two physics students, Will and Ben, are in the weightlifting room. Will lifts the 100 lb barbell over his head 10 times in one minute; Ben lifts the 100 lb barbel over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power?

They do the same work. Ben is most powerful.

a certain one-dimensional conservative force is given as a function of x by the expression F=-kx^3, where F is in newtons and x is in meters. A possible potential energy function U for this force is

U = 1/4 kx^4

What is the SI unit for power? What are other units?

Watt; j/s, horse power(hp)

Is work scalar? What does the + or - sign mean?

Yes because it is ignorant of direction; how much energy is added or removed from the object.

Is work done when a weightlifter lifts a barbell above her head?

Yes because there is a force applied to the barbell and the force causes a displacement of the barbell.

Force required to lift an object at a constant speed

is equal to the weight of the object

Which would ALWAYS be true of an object possessing kinetic energy of 0 joules?

it is at rest

Which would ALWAYS be true of an object possessing a potential energy of 0 joules?

it is on the ground

Kinetic Energy is the energy of

motion

Potential Energy is the energy of

position

Power is defined as the

rate at which work is done

Which of the points represented in the diagram above correspond to stable equilibrium position(s)?

straight across

energy

the ability to do work

Which of the points represented in the diagram above correspond to equilibrium position(s)?

triangle

A person pushed a 25 kg mass a distance of 100 m along a 37degrees frictionless incline as shown at the right. How much work did the person do?

1.5x10^4 J

Suppose you want to ride your mountain bike up a steep hill. Two paths lead from the base to the top, one twice as long as the other. Compared to the average force you would exert if you took the short path, the average force you exert along the longer path is

1/2 as much

a 50 kg platform diver hits the water below with a kinetic energy of 5000 joules. the height (relative to the water) from which the diver dove was approx.

10 meters

A box with mass m is sliding along on a friction-free surface at 9.87 m/s at a height of 1.81 m. It travels down the hill and then up another hill. Find the speed at the bottom of the hill.

11.6 m/s

A 7 kg block moves in a straight line under the influence of a force that varies with position as shown in the figure at the right. If the force is directed along the direction of motion, how much work is done by this force as the block moves from the origin to x = 10 m?

12 J

A woman weighing 50 N walks up 120 stairs, each of height 0.25m. How much work does she do against gravity?

1500 J

An unstretched ideal spring hangs vertically from a fixed support. A 0.4 kg object is then attached to the lower end of the spring. The object is pulled down to a distance of .35 m below the unstretched position and released from rest at time t=0. A graph of the subsequent vertical position y of the lower end of the spring was initially unstretched. What is the spring constant of the spring?

16 N/m

On a recent adventure trip, Anita Break went rock-climbing. Anita was able to steadily lift her 80 kg body 20 meters in 100 seconds. Determine Anita's power rating during this portion of the climb

160 W

A person pushed a 50 kg mass a distance of 100 m along a 30o frictionless incline as shown at the right. How much work did the person do?

2.5x10^4 J

In 60 seconds, you lift twenty 6 kg bricks from the ground to the top of a 1 m high table. What is your average power output?

20 W

Tim, with mass 61.4 kg, climbs a gymnasium rope a distance of 3.4 m. The acceleration of gravity is 9.8 m/s2. How much potential energy does Tim gain?

2045.85 J

The figure at the right shows the force as a function of distance x acting on a 30 kg mass. The work that this force does on the body between x = 4 m and x = 10 m is about:

21 J

A 50 kg cart on a roller coaster is released with a speed of 9 m/s at a height of 60 m above the ground. If the roller coaster is frictionless, about what will be the cart's speed at a height of 40 m above the ground?

22m/s

A 1423-kg car is moving along a level highway with a speed of 26.4 m/s. The driver takes the foot off the accelerator and the car experiences a slowing force of 901-N over a distance of 106 m. Determine the speed of the car after traveling this distance. KEi + PEi + Wext = KEf + PEf

23.7 m/s

a worker pushes a wheelbarrow with a force of 20 N over a level distance of 25m. If a frictional force of 10 N acts on the wheelbarrow in a direction opposite to the of the worker, what net work is done on the wheelbarrow?

250J

A 50 kg cart on a roller coaster is released from rest at a height of 55 m above the ground. If the roller coaster is frictionless, about what will be the cart's speed at a height of 10 m above the ground?

30m/s

Depends upon object mass and object height.

Potential Energy

Two machines (e.g., elevators) might do identical jobs (e.g., lift 10 passengers three floors) and yet the machines might have different power outputs.

Power is the rate at which work is done. The two machines are doing the same amount of work, yet doing the work at different rates. One machine might do the job very quickly and the other very slowly. The machine that does the work in the least amount of time (quickly) is most powerful.

Conservation of Mechanical Energy formula for nonconservative forces

TMEi + Wext = TMEf

Conservation of Mechanical Energy formula for conservative forces

TMEi = TMEf

At what point in its motion is the KE of a pendulum bob a maximum?

at the lowest point

If an object moves in such a manner as to conserve its total mechanical energy, then _________.

the sum of the kinetic energy and the potential energy remains the same throughout its motion