Download SPH 3U Dynamics Lesson 1: Introduction to Forces and Free

SPH 3U Dynamics Lesson 1: Introduction to Forces and Free-Body Diagrams
Kinematics (unit one) is the study of the motion of objects.
Dynamics, our second unit, is the study of the causes of motion.
Motion is caused by forces. Applying a force to something might result in it moving. For example, if you kick a
soccer ball, your foot exerts a force on the soccer ball, and it will move. If you kick a car, however, the force
your foot exerts on the car is not enough to move the car.
Force is measured in Newtons, N. 1 Newton is equal to 1 kg of mass accelerating at 1 m/s2. Newtons are a
vector quantity; forces have directions.
Some types of forces:
Applied forces: Forces of something pushing or pulling on something else. For example, you kicking the soccer
ball.
Tension: The force exerted by a rope that is tied to an object and is being pulled on. For example, a boat
towing people on a tube attached by a rope. The rope exerts a force of tension on the tube (and the boat can
make the ride more fun by changing directions).
Friction: Friction is a force that opposes motion. It is caused by the binding of any two surfaces together while
they are in contact. Whichever direction something is moving in, friction is in the opposite direction. For
example, a person who fell off his or her bike sliding across the road.
Gravity: Gravity is the force that pulls all of us towards the centre of the Earth. In space, gravity attracts any
objects together. The larger the object, the greater the force of gravity.
Gravity is always acting on objects on Earth. We can calculate the force of gravity using the equation
Fg = mg;
the force of gravity is equal to the mass of the object in kg times the acceleration due to
gravity (in dynamics its represented just by g) which is still 9.8 m/s2.
Newton’s 3rd Law: We are going to cover each of Newton’s Laws in general, but one aspect comes up here.
Newton’s 3rd Law states that for every force there is an equal and opposite reaction force. It can be a little odd
to think about at first.
If gravity is acting on all of us right now, why are not we not getting pulled into the centre of the Earth?
Normal Force: When gravity pulls us downwards, we exert a force on whatever surface we are standing on.
That surface exerts an equal force in the opposite direction (up) back on us. This force is the normal force.
For an object resting on a horizontal surface, the normal force and gravity will balance, so it will not move or
fall.
Free-Body Diagrams (FBD): Diagrams of Forces using a dot as the object that has forces acting on it, and
arrows with numbers to show the size and direction of the forces.
Ex 1) Mr. Schweitzer weighs 83 kg and is standing on the floor of the classroom. Calculate the force of gravity
and the normal force acting on him, then draw a FBD of the situation.
Net Force: The Net Force on an object is the result of all the forces acting on it. Just like in the kinematics unit,
we have to do math in one direction only. Often, up and right are labeled positive and down and left are
labeled negative.
Ex 2) Find the Net Force on Mr. S from the above question.
Four Fundamental Forces: Physicists know about 4 types of forces that exist in the universe.
Gravity: Attractive force between objects.
Electromagnetic: Force between electric charges and between magnetically charged objects and fields. It can
attract or repel.
Strong Nuclear force: Holds protons and neutrons together in the nuclei of atoms.
Weak Nuclear force: When subatomic particles transform into other particles the weak nuclear force controls
their interactions.
Bonus: What kind of force is it when Mr. S pushes his binder across the countertop?
Do questions #1-2, 5,15, 7, 9, 12 on pg 122 (they’re out of order on purpose).