Download SCI 111

Practice Problem #1
• A weightlifter bench presses 80kg (approx.
175lbs.) 0.75m straight up.
• a. How much work does she do, assuming
constant velocity, in one lift (just the 0.75m
straight up)?
• b. How many repetitions would she have to
do to burn off 1 hamburger (400 Calories).
Note 1 calorie = 4.186 J.
Solution to Practice Problem #1
Solution to Practice Problem #1
Practice Problem #2
• A person pushes a stalled 2000 kg car from
rest to a final speed of 2.0 m/s. During this
time the car moves 20 m. Neglecting friction
v
between the car and the road, find:
• a. the final acceleration of the car
• b. the horizontal force exerted on the car
f
(Hint: Use Newton’s 2nd Law to find the force using the acceleration.)
• C. the work done on the car
Solution to Practice Problem #2
Practice Problem #3
A person pushes a 10 kg box at a constant velocity over a distance of 4
m. The frictional force between the box and the floor is 30% the weight
of the box. How much work does the person do in pushing the box?
Solution to Practice Problem #3
• The weight of the box is mg = 980 N
• The frictional force Ff = 0.3  980 N = 294 N
• The work done, W = Ff  d = 294 N  4 m = 1,176 J
Practice Problem #4
• An elevator m=800 kg has a maximum load
of 8 people or 600 kg. The elevator goes up
10 stories = 30 m at a constant speed of 4
m/s. What is the average power output of the
elevator motor if the elevator is fully loaded
with its maximum weight? (neglect friction)
(Hint: First determine the time the elevator takes to go up 10 stories, then determine the work the elevator motor
exerted. Finally find the power of the motor.)
Solution to Practice Problem #4
Practice Problem #5
• A 500 kilogram car is driving at 15
meters/second. What's its kinetic
energy?
• Solution
KE = ½ mv2 = ½  500 kg  (15 m/s)2 = 56,250 J
Practice Problem #6
• A cart is loaded with a brick and pulled at constant speed along
an inclined plane to the height of a seat-top. If the mass of the
loaded cart is 3.0 kg and the height of the seat top is 0.45 meters,
then what is the potential energy of the loaded cart at the height
of the seat-top?
• Solution: PE = mgh = 3 kg  9.8 m/s2  0.45m = 13.23 J
Practice Problem #7
A forklift lifting a crate of mass 100 kg at a
constant velocity to a height of 8 m over a time
of 4 s. The forklift then holds the crate in place
for 20 s.
a. How much power does the forklift exert in
lifting the crate?
b. How much power does the forklift exert in
holding the crate in place?
Solution to Practice Problem #7