Work
A 5 kg box is pushed across a horizontal floor at a constant speed by a horizontal force of 25 N for 4 m.
If the frictional force is 25 N, what is the net work done on the box?
0 J
A ball of mass 2 kg is moving at a speed of 3 m/s.
What is its kinetic energy?
9 J
A 2 kg book is lifted to a shelf 1.5 m above the floor.
What is the book’s gravitational potential energy (GPE) relative to the floor?
30 J
give an example of a conservative force
gravity
A person lifts a 10 kg box to a height of 2 m in 4 seconds.
What is the average power exerted?
49 W
This theorem relates work to energy
The work-energy theorem
A car has a kinetic energy of 20,000 J and a mass of 1,000 kg.
What is the car’s speed?
6.32 m/s
A 0.5 kg ball is held at a height of 2 m, and another identical ball is held at 4 m.
How much more potential energy does the higher ball have?
10 J
give an example of a nonconservative force
gravity, air resistance, friction, heat
A car engine does 50,000 J of work in 5 seconds.
What is the power output of the engine?
10,000 W
A worker pulls a wagon with a force of 40 N at an angle of 30° above the horizontal. The wagon moves 5 m horizontally.
How much work does the worker do?
173.2 J
A 4 kg cart moves at 2 m/s, and a 2 kg cart moves at 4 m/s.
Which cart has more kinetic energy?
The 2 kg cart
A spring with a constant k=200 N/m and is stretched 0.1 m from its equilibrium position.
Find its elastic potential energy.
1 J
A 1 kg ball is dropped from a height of 5 m.
Ignoring air resistance, what is the speed of the ball just before it hits the ground?
10 m/s
A person pushes a 100 N crate across a floor at a constant speed of 2 m/s.
What is the power exerted by the person?
200 W
A student slides a book across a desk, and friction exerts a force of 2 N opposite the motion. The book moves 1.5 m before stopping.
How much work does friction do?
-3 J
A net force does 150 J of work on a 3 kg object starting from rest.
What is the object’s final speed?
10 m/s
A 5 kg rock is lifted 3 m vertically.
How much work is done against gravity, and how does that relate to its potential energy gain?
150 J
A roller coaster car of mass 500 kg starts from rest at the top of a 30 m hill. At the bottom of the hill, what is its speed, assuming no friction?
24.5 m/s
Power is the rate of change of ______ with respect to time?
Energy transfer
A spring is stretched from its natural length by 0.2 m. The spring constant is k = 100 N/m.
How much work is done to stretch the spring this far?
2 J
A 1,200 kg car increases its speed from 10 m/s to 20 m/s.
How much work (energy) is required to cause this change in motion?
180,000 J
A 2 kg object is dropped from a height of 10 m and then caught when it is 4 m above the ground.
How much potential energy is lost?
120 J
A 2 kg block slides down a 4 m high frictionless ramp. At a height of 1 m above the ground, what is the block’s speed?
7.75 m/s
compare and contrast work, energy, and power
open ended