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An introduction to what forces are, how they are measured, and how to represent them using vector diagrams.
Why does a ball eventually stop rolling even if no one touches it, and why don't you float away into space right now?
In physics, a force is simply a push or a pull exerted on an object. Forces are the 'movers and shakers' of the universe; they cause objects to speed up, slow down, change direction, or even change shape. We measure the strength of a force using a unit called the Newton (N), named after Sir Isaac Newton. To give you a sense of scale, holding a small apple in your hand requires about of upward force to keep it from falling. Forces are vectors, meaning they have both a magnitude (strength) and a direction. You can't just say you pushed a door with ; you have to specify that you pushed it forward or backward.
Let's look at how we quantify simple actions: 1. Lifting a backpack: You apply an upward force of approximately . 2. Pressing a doorbell: You apply a forward push of about . 3. A professional soccer player kicking a ball: They can exert a force of over !
Quick Check
If you are pushing a heavy box but it isn't moving, are you still exerting a force?
Answer
Yes, a force is being exerted even if the object's motion doesn't change.
Forces are categorized based on whether the objects must touch. Contact forces occur when two objects are physically touching. Examples include friction (which resists motion), tension (pulling a rope), and the normal force (the surface pushing back against you). On the other hand, non-contact forces act over a distance through a field. You don't need to touch the Earth to feel gravity pulling you down, and a magnet can pull a paperclip without touching it. These 'action-at-a-distance' forces are invisible but incredibly powerful.
Identify the forces in this scenario: A magnet pulls a metal car across a carpeted floor. 1. The magnet's pull is a non-contact force (magnetic force). 2. The carpet rubbing against the wheels is a contact force (friction). 3. The Earth pulling the car downward is a non-contact force (gravity).
Quick Check
Which type of force is responsible for a skydiver falling toward the Earth?
Answer
Gravity, which is a non-contact force.
Three students are playing tug-of-war. 1. Team A pulls left with a force of . 2. Team B has two people: one pulls right with and the other pulls right with . 3. Calculate the total force for Team B: to the right. 4. Calculate the net force: to the right.
Which of the following is the standard unit used to measure force?
Which of these is an example of a non-contact force?
In a force vector diagram, a longer arrow represents a weaker force.
Review Tomorrow
Tomorrow morning, try to name three contact forces and three non-contact forces you encountered during your breakfast routine.
Practice Activity
Draw a 'free-body diagram' of a book sitting on a table. Draw one arrow for gravity pulling down and one arrow for the table pushing up. Are the arrows the same length?