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A review of what forces are and how they cause objects to move or stop.
Have you ever wondered why a heavy door stays shut until you lean into it, or why a soccer ball eventually stops rolling even on a perfectly flat field?
In physics, a force is simply a push or a pull acting upon an object. Forces are everywhere! When you brush your teeth, you apply a force to the toothbrush. When you sit on a chair, your body applies a force to the seat. Forces are measured in a unit called Newtons, abbreviated as . A force does not exist on its own; it is the result of an interaction between two objects. If you stop pushing a swing, the interaction ends, and you are no longer applying that specific force.
Quick Check
If you are opening a heavy desk drawer toward your body, are you applying a push or a pull?
Answer
You are applying a pull.
1. You stand behind a stationary grocery cart. 2. You apply a push force of forward. 3. Because there is no force pushing back, the cart moves forward in the direction of your push.
Quick Check
If you push a door with of force and your friend pushes the other side with of force, will the door move?
Answer
No, because the forces are balanced and cancel each other out.
Objects are naturally 'lazy'—they want to keep doing exactly what they are already doing. This property is called inertia. To change an object's motion, you must apply an unbalanced force. An unbalanced force can do three things: 1. Start an object moving (like kicking a ball). 2. Stop an object from moving (like catching a ball). 3. Change the direction of a moving object (like hitting a tennis ball with a racket). Without a force, a moving object would theoretically keep moving forever in a straight line!
In a game of tug-of-war, Team A pulls left with . Team B pulls right with . 1. Calculate the net force: . 2. Because the net force is zero, the rope stays perfectly still. This is a state of equilibrium.
You slide a book across a wooden table. You give it a quick push (), but it eventually slows down and stops. Why? 1. As the book slides, the surface of the table rubs against the book. 2. This creates a hidden force called friction () acting in the opposite direction of the motion. 3. The friction force is an unbalanced force that changes the book's motion from moving to stopped.
Which of the following is the best definition of a force?
If an object is moving at a constant speed in a straight line, what can we say about the forces acting on it?
A force is required to make a moving car turn a corner, even if the car stays at the same speed.
Review Tomorrow
In 24 hours, try to explain to a friend why a ball rolling on grass stops sooner than a ball rolling on smooth ice using the word 'force'.
Practice Activity
Go around your house and find three examples of 'pulls' and three examples of 'pushes' you perform during your chores.