Work Basics
This is the physics definition of work.
Work is the transfer of energy that occurs when a force causes an object to move in the direction of the force.
Write the equation used to calculate work.
W=Fd (when force and motion are in the same direction).
This type of energy is the energy of motion.
Kinetic energy.
Mechanical energy is the sum of these two types of energy.
Kinetic energy and potential energy.
Power describes how fast this quantity is transferred or used.
Energy (or work).
True or False: A student holding a heavy backpack motionless is doing work on the backpack.
False — no work is done because there is no displacement.
A 10 N force pushes a box 3 m in the same direction. How much work is done?
W=(10)(3)=30J
Write the formula for kinetic energy.
KE=1/2mv2
Write the equation for mechanical energy.
ME=KE+PE
Write the equation for power.=tW
P=W/t
What two conditions must be met for a force to do work on an object?
A force must be applied, and the object must move in the direction of the force.
A 5 N force pulls a sled 4 m, but the force is applied at an angle. Is the work greater than, less than, or equal to Fd? Explain.
Less than Fd, because only the component of the force parallel to the motion does work.
Calculate the kinetic energy of a 2.0 kg object moving at 4.0 m/s.
KE=1/2(2.0)(4.0)2=16J
An object has 20 J of kinetic energy and 30 J of potential energy. What is its mechanical energy?
ME=50J
A machine does 600 J of work in 3.0 s. Calculate its power.
P=600/3.0=200W
A force is applied to an object, but the object does not move. How much work is done, and why?
Zero work is done because there is no displacement.
A friction force of 6 N acts on a box that slides 5 m. Calculate the work done by friction.
W=−(6)(5)=−30J
This type of energy depends on position or height.
Potential energy.
If mechanical energy is conserved, what happens to energy when kinetic energy decreases?
Potential energy increases by the same amount.
Is friction an internal force or an external force? Explain.
External force — it acts on the system from outside and changes mechanical energy.
A box is pushed to the right while friction acts to the left. Is friction doing work on the box? Explain.
Yes — friction does negative work because it acts opposite the direction of motion.
A 20 N force acts perpendicular to the direction of motion. How much work is done?
Zero joules, because the force is perpendicular to the displacement.
Calculate the gravitational potential energy of a 3.0 kg object lifted 5.0 m.
PE=mgh=(3.0)(9.8)(5.0)=147J
A ball is thrown straight up. At the highest point, what is its kinetic energy and mechanical energy compared to the start?
Kinetic energy is zero, and mechanical energy is the same as at the start.
Mechanical energy is conserved only when this type of force is absent.
Nonconservative forces (such as friction).