Lesson 2 Work and Energy
Operational Definitions of Work and Force
Force and work are related to one another, and it is important to understand each one.
C2.2 Baseball hit
A force is defined as a push or pull on an object. Force is measured in the units of newtons (N).
An object at rest tends to stay at rest, unless acted upon by an outside force. What does this mean?
Think of a soccer player waiting to start a play by kicking a ball that is at rest into play. The ball is at rest because there are no unbalanced forces acting on it. When the player kicks the ball, an unbalanced force is applied to it over a distance, so it moves.
An unbalanced force is when the force acting in one direction is greater than the force acting in the opposite direction.
An object at rest tends to stay at rest, unless acted upon by an outside force. What does this mean?
Think of a soccer player waiting to start a play by kicking a ball that is at rest into play. The ball is at rest because there are no unbalanced forces acting on it. When the player kicks the ball, an unbalanced force is applied to it over a distance, so it moves.
An unbalanced force is when the force acting in one direction is greater than the force acting in the opposite direction.
C2.3 Soccer ball at rest
C2.4 Gravel pathway
Now, think of the player walking home from soccer practice kicking the ball along the gravel pathway. When she kicks the soccer ball, why does it not just keep travelling forever on the pathway? Why does it slow down and stop every few metres
until the player kicks it again?
An object in motion tends to stay in motion, moving along at a constant speed and in a straight line, unlesss acted upon by an outside force. How does this apply to the soccer ball on the gravel pathway?
An object in motion tends to stay in motion, moving along at a constant speed and in a straight line, unlesss acted upon by an outside force. How does this apply to the soccer ball on the gravel pathway?
The friction
that occurs between the moving soccer ball and the uneven surface of the gravel pathway, slows the motion of the ball down. An unbalanced force (friction) is applied to the ball in the opposite direction to the ball’s movement, slowing
the speed of the ball. Without the resistive force, the ball could keep moving.
When the ball is kicked from rest, energy from the soccer player’s movement is transferred to the ball, allowing it to move. When the soccer ball is travelling along the gravel pathway, energy from the ball is transferred to the ground through friction, preventing it from continuing to move.
Basically, if energy is used to apply a force to an object over a distance and the object then changes in some way, force and energy are related.
When the ball is kicked from rest, energy from the soccer player’s movement is transferred to the ball, allowing it to move. When the soccer ball is travelling along the gravel pathway, energy from the ball is transferred to the ground through friction, preventing it from continuing to move.
Basically, if energy is used to apply a force to an object over a distance and the object then changes in some way, force and energy are related.
C2.5 Children playing with a wagon
Work
is done on an object whenever a force moves an object a distance and the distance is in the direction of the force.
Work is done on an object if three conditions are met:
- There must be movement of the object.
- There must be a force exerted on the object.
- The force and the distance the object travels must be in the same direction.
Read This
Please read pages 156 and 157 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on what effect unbalanced forces can have on an object depending upon its initial conditions and what work is
and how we determine if work is done. Remember, if you have any questions or do not understand something, ask your teacher!
Practice Questions
Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses
(where necessary) to study from.
- For each of the following scenarios, indicate whether unbalanced forces are present that will affect the movement of the object.
- A book is shoved across a table and slows down.
- A canoe is floating in a lake.
- A child climbs onto one end of an unoccupied seesaw.
- A book is shoved across a table and slows down. (unbalanced forces)
- A canoe is floating in a lake. (balanced forces)
- A child climbs onto one end of an unoccupied seesaw. (unbalanced forces)
- For each of the following scenarios, indicate whether work is being done.
- A caretaker pushes a mop across the floor.
- A person in a wheelchair coasts down a ramp.
- A horse pulls a carriage during a race.
- A boy carries a book from his desk to the shelf.
- A caretaker pushes a mop across the floor. (Work is done.)
- A person in a wheelchair coasts down a ramp. (Work is not done by the person in the wheelchair—coasting does not require a force to be exerted.)
- A horse pulls a carriage during a race. (Work is done on the carriage by the horse.)
- A boy carries a book from his desk to the shelf. (Work is not done by the boy on the book—the force required for the book to be held and carried is up while the boy travels forward with the book. So the force acting on the book and the direction of movement are not in the same direction.)