1. TRIAL EXAM 3: CONSERVATION OF ENERGY
Multiple Choices:
1. A force of 80 lbs is used to push a 160 lb crate 12 ft across level warehouse floor. How much work is done?
a. 920 lb-ft b. 830 lb-ft c. 960 lb-ft d. 800 lb-ft
2. In Problem #1, what is the change in the crate’s potential energy?
a. Remains the same b. Increased c. Decreased d. Infinite
3. In raising 300 kg bronze statue, 12000 kJ of work is performed. How high is it raised?
a. 5.1 m b. 4.1 m c. 6.2 m d. 5.3 m
4. A hoist powered by 30 hp motor is used to raise a 600 kg bucket of concrete to a height of 90 m. If the efficiency is 70%, find the time required.
a. 38.3 sec b. 30.1 sec c. 35.2 sec d. 33.8 sec
5. A horse has power output of 2 hp when it pulls a wagon with a force of 500 N. What is the wagons velocity?
a. 2.98 m/s b. 2.50 m/s c. 2.10 m/s d. 2.22 m/s
6. Find the kinetic energy of a 1200 kg car whose velocity is 32 m/s.
a. 6.88 x 105 J b. 7.31 x 105 J c. 6.14 x 105 J d. 7.22 x 105 J
7. At her highest point, a girl swing is 8 ft above the ground, and at her lowest point, she is 3 ft above the ground. What is her maximum velocity?
a. 20 ft/sec b. 18 ft/sec c. 15 ft/sec d. 22 ft/sec
8. If a spring is stretched 3 cm by suspended object having a mass of 0.88 kg, then the spring constant is ____.
a. 325 N/m b. 254.1 N/m c. 202.3 N/m d. 287.8 N/m
9. How much work is done by an automobile engine in accelerating the automobile and driver with total mass of 2.5x103 kg from rest to 30 m/s over a
distance of 350 m?
a. 1.125 x 106 J b. 2.221 x 106 J c. 1.402 x 106 J d. 1.872 x 106 J
10. A boy starts from rest on his sled and slides down the hill. As he reaches the lower level, the speed of the sled is 15 m/s. If the lower level is 20 m
below the starting point and the boy and sled together have a mass of 65 kg, calculate the total work done by friction (the non-conservative force
acting) as the sled goes down the hill.
a. -3.28 x 103 J b. -4.22 x 103 J c. -5.44 x 103 J d. -6.21 x 103 J
Identification:
1. _____ is the quantity that defined to be the product of the magnitude of force to the displacement, which is in the direction of the displacement.
2. _____ is the quantity that measures the rate at which work is being done.
3. _____ is the measure of the ability of the particle to do work because of its motion.
4. _____ a principle which states that during any time interval the work done by the resultant force on an object is equal to the change in the kinetic
energy of that object during the same interval.
5. _____ is the ability to do work because of the particle’s configuration.
6. _____ is the energy that began as a potential energy and became energy of organized motion or also known as the energy of disorganized motion.
7. _____ is the sum of the kinetic and potential energy of a particle.
8. _____ is the metric unit of work
9. _____ is a force rule that considers a weightless spring that is stretched within its elastic limit.
10. _____ is the general term used to define the energy stored in a spring.
Problem Solving:
1. The spring bumper is used to arrest the motion of the 4-lb energy loss between the plate and block during the collision.
block, which is sliding toward it at v=9 ft/s. As shown, the spring Answer: 1.9034 ft
is confined by the plate P and wall using cables so that its
length is 1.5 ft. If the stiffness of the spring is k=50 lb/ft, find
the required un-stretched length of the spring so that the plate
is not displaced more than 0.20 ft after the block collides into it.
Neglect friction, the mass of the plate and spring, and the
2. Roller coasters are designed so that the riders will not
experience more than 3.5 times their weight as a normal force
against the seat of the car. Determine the velocity at its lowest
point if the car has a speed of 5 ft/s at the crest of the drop.
Neglect friction. Answer: 84.3149 ft/sec
3. The 0.5-lb ball is shot from the spring device shown. The spring
has a stiffness k=10 lb/in and four cords C and plate P keep the
spring compressed 2-in when no load is on the plate. The plate
is pushed back 3-in from its initial position, so that s=30-in. If it
is then released from rest, determine the speed of the ball
when it leaves the surface of the smooth inclined plane.
Answer: 32.3355 ft/sec
BONUS PROBLEM:
The 2-kg collar is attached to a spring that has an un-stretched length of 3
m. If the collar is drawn to point B and released from rest, find its speed
when it arrives at point A. Answer: 2.4495 m/s