Physics Chapter 9 Flashcards
A force sets an object in motion. When the
force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force distance, and what quantity can this change?
Work; object’s energy
Work is required to lift a barbell. How many
times more work is required to lift the barbell three times as high?
Three
Which requires more work, lifting a 10-kg
load a vertical distance of 2 m or lifting a
5-kg load a vertical distance of 4 m?
Both the same, 200 J
How many joules of work are done on an
object when a force of 10 N pushes it a distance of 10 m?
10 N times 10 m = 100 J
How much power is required to do 100 J of
work on an object in a time of 0.5 s? How
much power is required if the same work is
done in 1 s?
(100 J)/(0.5 s) 5 200 W
(100 J)/(1 s) 5 100 W
What are the two main forms of mechanical
energy?
PE and KE
a. If you do 100 J of work to elevate a bucket
of water, what is its gravitational potential
energy relative to its starting position?
b. What would the gravitational potential
energy be if the bucket were raised twice
as high?
a. 100 J
b. 200 J
A boulder is raised above the ground so that its potential energy relative to the ground is 200 J. Then it is dropped. What is its kinetic energy just before it hits the ground?
200J
Suppose you know the amount of work the brakes of a car must do to stop a car at a given speed. How much work must they do to stop a car that is moving four times as fast? How will the stopping distances compare?
16 times as much work;
16 times the distance
How does speed affect the friction between a road and a skidding tire?
Speed does not affect friction.
What will be the kinetic energy of an arrow having a potential energy of 50 J after it is shot from a bow?
50J
What does it mean to say that in any system the total energy score stay the same?
Energy is conserved.
In what two ways can a machine alter an input force?
It can change its magnitude
and/or direction
In what way is a machine subject to the law of energy conservation? Is it possible for a machine to multiply energy or work input?
Work out cannot exceed work
in; no.
What does it mean to say that a machine has a certain mechanical advantage?
It can multiply force by a
certain amount.
What is the efficiency of a machine that requires 100 J of energy to do 35 J of work?
35%
Distinguish between theoretical mechanical advantage and actual mechanical advantage.
How would these compare if a machine were 100% efficient?
TMA—no friction, AMA—with
friction; same
What is the efficiency of her body when a cyclist expends 1000 W of power to deliver mechanical energy to the bicycle at the rate of 100 W?
10%
Calculate the work done when a force of 1 N moves a book 2 m.
2J
Calculate the work done when a 20-N force pushes a cart 3.5 m.
70J
Calculate the work done in lifting a 500-N barbell 2.2 m above the floor. (What is the
potential energy of the barbell when it is lifted to this height?)
PE= 1100J
Calculate the watts of power expended when a force of 1 N moves a book 2 m in a time interval of 1 s.
2W
Calculate the power expended when a 20-N force pushes a cart 3.5 m in a time of 0.5 s.
140W
How many joules of potential energy does
a 1-kg book gain when it is elevated 4 m?
When it is elevated 8 m? (Let g = 10 N/kg.)
40J, 80J