Physics Unit 4 - work, power and energy
0 joules
For an Brecht sitting on a horizontal surface, what is the work done by the normal force?
- a force causes a displacement of the object - components of a force are parallel toa displacement of the object
What are 2 conditions for work to be done?
The total mechanical energy remains the same as long as no external forces such as friction, acts upon it. (MEi = MEf)
What does it mean for the total mechanical energy to be conserved?
Net work equals the change in kinetic energy
What does the work-kinetic energy theorem state?
A car pulling a boat
What is one example of how a moving object could do work?
Increase the object's mass and increase the object's speed
What will increase the kinetic energy of an object?
Force and displacement are opposite (90º > ø > 270º)
When is work negative?
Force and displacement are in same direction (90º < ø < 270º)
When is work positive?
At a constant velocity, net force equals 0. If net force equals 0, then net work equals 0.
A 6 kg object is moving at a constant velocity of 5 m/s, pulled to the right by a 15 N force in the presence of friction. What is the net work done on the object?
Goes down
A net work of negative 50 J is done on an object. What happens to the objects speed?
Goes up
A net work of positive 80 J is done on an object. What happens to the objects speed?
Potential energy
A object at rest may have ____.
Work
A scalar quantity that can be either positive or negative
8 joules
An object has kinetic energy of 32 J. if the object's speed is halved, then it's new kinetic energy will be ___.
-moving above the ground (a non-zero height) -at rest above the ground (a non-zero height)
An object will definitely have potential energy if it is ______.
No
Do forces that are perpendicular to the distance do work on an object ?
The object sped up.
If positive net work is done on an object, according to the work-kinetic energy theorem, what is TRUE of the object's motion?
Increase by a factor of 16
If the speed of an object increases by a factor of 4 and the mass remains constant, what happens to its kinetic energy?
Work is done to slow down a moving car.
In which case is the net work negative?
Increase, decrease, remain the same
Suppose an object speeds up while rolling down an incline. In this situation, the object's kinetic energy would ______, its potential energy would ______, and its total mechanical energy would _____. Assume that only conservative forces are doing work on the object.
Energy
The ability to do work
Gravitational potential energy
The energy an object has due to its location or position.
Kinetic energy
The energy that an object has due to its motion
Power
The rate at which is done; work done on an object per time
Mechanical energy
The total kinetic and potential energy of a system
Person A delivers the largest power because she does work at a faster rate. (Less time)
Two people, A and B, do the same amount of work. Person A does it in half time as person B. Who delivers the largest power?
Force
Variable F, unit N, vector
Power
Variable P, unit E, scalar
Work
Variable W, unit J, scalar ; magnitude of force times magnitude of displacement
Gravity pulls it down
Why does a ball being held above the ground have gravitational potential energy?
When work is done against the force of gravity to lift an object to some height above ground, the work done is stored as potential energy.
Why does a ball held at a height have energy?
The spring will move forward
Why does a compress spring have elastic potential energy?
Because work is done to move an object and the object can do work on other objects that it hits
Why does a moving object have energy?
Kinetic energy
variable KE, unit J, scalar
Gravitational potential energy
variable PEg, unit J, scalar
