Topic 2.3: Work, Energy, Power (practice questions) Flashcards
When a car is moving at a speed v in a straight line the total force opposing its motion is F. The product Fv is a measure of the A. power developed by the car. B. rate of change of momentum of the car. C. work done by the car against the frictional forces. D. rate of change of kinetic energy of the car.
A. power developed by the car.
An object of mass m1 has a kinetic energy K1. Another object of mass m2 has a kinetic energy K2 . If the momentum of both objects is the same, the ratio K1/K2 is equal to
A. m2/m1
B. m1/m2
C. (m2/m1)1/2
D. (m1/m2)1/2
A. m2/m1
Which one of the following units is a unit of energy?
A. eV
B. W s−1
C. W m−1
D. Nms-1
A. eV
A rocket is fired vertically. At its highest point, it explodes. Which one of the following describes what
happens to its total momentum and total kinetic energy as a result of the explosion?
Total Momentum Total Kinetic Energy
A. unchanged increased
B. unchanged unchanged
C. increased increased
D. increased unchanged
A. total momentum is unchanged and total kinetic energy increases
The graph below shows the variation with displacement d of the force F applied by a spring on a cart.
The work done by the force in moving the cart through a distance of 2 cm is
A. 10 x 10-2 J
B. 7 x 10-2J
C. 5 x 10-2J
D. 2.5 x 10-2 J
C. 5 x 10-2J
Which of the following quantities are conserved in an inelastic collision between two bodies?
Total linear momentum Total kinetic energy on the bodies on the bodies
A. yes yes
B. yes no
C. no yes
D. no no
B. total linear mometum is the only one conserved in an inelastic collision between two bodies
The diagram below shows the variation with displacement x of the force F acting on an object in the direction of the displacement.
Which area represents the work done by the force when the displacement changes from to x1 to x2?
A. QRS
B. WPRT
C. WPQV
D. VQRT
D. VQRT
An engine takes in an amount E of thermal energy and, as a result, does an amount W of useful work. An amount H of thermal energy is ejected. The law of conservation of energy and the efficiency of the engine are given by which of the following?
Law of conservation of energy Effieciency
A. E= W + H W
B. E= W + H W/E
C. E + H = W W/H
D. E + H + W W/(E - H)
B. law of conservation of energy is represented by E = W + H, and effieciency is represented by W/E
A machine lifts an object of weight 1.5 x 103 N to a height of 10 m. The machine has an overall efficiency of 20 %. The work done by the machine in raising the object is
A. 3.0 x 103J
B. 1.2 x 104J
C. 1.8 x 104J
D. 7.5 x 104J
D. 7.5 x 104J
Joe is standing on the surface of a frozen pond and he throws a ball horizontally. Considering Joe and the ball together, which one of the following correctly describes the change in the magnitude of the
momentum and the change in the kinetic energy of Joe and the ball immediately after the ball is thrown?
Magnitude of momentum__Kinetic energy of of Joe and ball__Joe and ball
A. no change increases
B. increases increases
C. no change no change
D. increases no change
A. The is no change in magnitude of momentum of Joe and the ball, but the kinetic energy of Joe and the ball increases
The variation with time of the vertical speed of a ball falling in air is shown below.
During the time from 0 to T, the ball gains kinetic energy and loses ∆Ep gravitational potential energy. Which of the following statements is true?
A. ∆Ep is equal to the gain in kinetic energy.
B. ∆Ep is greater than the gain in kinetic energy.
C. ∆Ep is equal to the work done against air resistance.
D. ∆Ep is less than the work done against air resistance.
B.The change in gravitational potental energy is greater than the gain in kinetic energy.
The diagram below represents energy transfers in an engine.
The efficiency of the engine is given by the expression
A. Ew/Ein
B. Ew/Eout
C. Eout/Ein
D. Eout/Ew
C. efficiency is the energy put out over the energy in
An object of mass m is initially at rest. An impulse I acts on the object. The change in kinetic energy of the object is
A. I2/2m
B. I2/m
C. I2m
D. 2I2m
A. The change in kinetic energy is equal tothe impulse squared divided by 2 time the mass
The graph below shows the variation with displacement d of the force F acting on a particle
The area that represents the work done by the force between d = 0 and d = dmax is
A. s − r.
B. r.
C. s.
D. s + r.
D. s + r
A spring is compressed by a force F.
For a compression e, the force F is given by F = ke. When the compression force is removed, the spring returns to its original length in time t. The best estimate for the power developed by the spring
during its expansion is
A. ke/2t
B. ke/t
C. ke2/2t
D. ke2/t
C. ke2/2t
A force stretches a wire that is fixed at one end. The value of this force increases from zero to a
maximum value and then returns to zero. The graph below shows the variation with force F of the extension x of the wire.
Which area, or areas, represents the net work done on the wire by the force?
A. Area P
B. Area Q
C. Area R
D. Area Q and area R
B. Area Q
A body moving along a straight-line has mass 3.0 kg and kinetic energy 24J. The motion is then opposed by a net force of 4.0 N. The body will come to rest after travelling a distance of
A. 2.0 m.
B. 6.0 m.
C. 8.0 m.
D. 12 m.
B. 6.0m
Two objects collide inelastically. For this system of two objects
A. only momentum is conserved.
B. only kinetic energy is conserved.
C. both momentum and kinetic energy are conserved.
D. neither momentum nor kinetic energy are conserved.
A. Only momentum is conserved
An object of weight 50 N is dragged up an inclined plane at constant speed, through a vertical height of 12 m. The total work done is 1500 J. The work done against friction is
A. 2100 J.
B. 1500 J.
C. 900 J.
D. 50 J.
C. 900 J
The graph below shows the variation with load F of the length L of a spring.
Which of the following expressions gives the force per unit extension (the spring constant) of
the spring?
A. F1/L1
B. F2/L2
C. (F2-F1)/L2
D. (F2-F1)/(L2-L1)
D. The spring constant is the second force subtracted by the first force divided by the second lenght divided by the first length.
The graph below shows the variation with displacement x of the force F acting on an object. The force F always acts in the same direction as the displacement.
At point Q, the displacement is xQ and the force is FQ.
Which of the following gives the work done by the force on the body as the displacement increases from zero to xQ and then returns to zero?
A. Zero
B. 1/2FQxQ
C. FQxQ
D. 2FQxQ
A. Zero
Water flows out from a tank down a pipe, as shown below
The pipe is always full of water. Which of the following gives the change in the kinetic energy and in the gravitational potential energy
of the water as the water flows down the pipe?
kinetic energy__gravitational potential energy
A. constant decreases
B. constant increases
C. increases decreases
D. increases increases
A. the kinetic energy will stay constant while the gravitational potential energy will decrease