Forces are introduced and the variety of forces are explained.
Forces are introduced and the variety of forces are explained. (1:30) Newton’s 1 Law is introduced and so are Free-Body Diagrams. (3:05) Newton’s 2nd Law is stated and the concept of defining a system is discussed. (6:07) Next, Newton’s 3rd Law, the easiest to state, but hardest to apply is presented. (7:33) Finally, a universal problem solving method for all force problems is laid out, and frictional forces are explored. (9:53)
The Question of the Day asks: (11:02)
If you double the mass of a system without changing the net external force, what will the effect on acceleration be?
a) acceleration is doubled
b) acceleration is halved
c) there is no change to the acceleration
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Hi and welcome to the APsolute RecAP: Physics 1 Edition. Today’s episode will focus on Dynamics including Newton’s Laws and Solving Force Problems.
Lets Zoom out:
Unit 2 – Dynamics
Topic 2.1-2.6
Big Idea – Force Interactions
Up until this point, we have been describing the motion of objects, but now we will shift gears and take a look at the reason objects are set in motion to begin with! Forces. A golfer applies a force to a golf ball when he initially hits the ball off of the tee. As a result, the golf ball accelerates from rest to a high velocity in a very small amount of time. But as it turns out, objects don’t need to move to have forces acting on them. In fact, just sitting in your chair, you have at least two forces acting on you. That is - unless your chair is in the international space station, in which case you only have one.
Lets Zoom in:
A force is a push or a pull. We measure forces in Newtons. 1 Newton is defined as 1 kg m/s^2. Forces come in two varieties, contact forces and field forces. Contact forces include applied forces, tension force, normal force, and frictional force. Field forces include gravitational force, electric force, and magnetic forces. An applied force can be a batter hitting a ball, a person pushing on a box to move it, or any other time something or someone is adding force that wouldn’t normally be applied to an object. A tension force is when a string or a cable applies a force to an object. Normal force is due to a surface pushing on an object and always acts perpendicular to the surface itself. Frictional force acts parallel to a surface and comes in two varieties: static and kinetic friction. Static friction occurs when an object and the surface it is on are motionless relative to one another. Kinetic friction or sliding friction occurs when two surfaces slide against each other. Of the field forces, we will only be focusing on the gravitational force which acts in the downward direction and can be found by multiplying the mass of an object by little “g” the gravitational acceleration of -9.8 m/s^2. Another name for gravitational force is weight.
Isaac Newton worked a great deal with forces, and as a result, 3 scientific laws bear his name. Newton’s 1st Law states that objects maintain their velocity unless acted on by a net external force. We say that objects have inertia, or a tendency to want to maintain their velocity. The 1st Law is often referred to as the Law of Inertia. And, what the heck is a net external force? Well, force is a vector so, if you have a force to the right and an equal force to the left, we would say the net force is 0 N. Therefore, there is no net force, and the object will maintain its velocity. If however, one of those forces is greater than the other, there is a net force in that direction and the object’s velocity will change. The object will accelerate in the direction of the net force. Another way to think of net force is that it is the sum of the forces acting on an object. It helps to split forces into X and Y directions, so I like to make a table to separate the forces in each direction.
Numerous forces can be acting on an object at any given moment, so it is important to have a good way of diagramming all of these invisible pushes and pulls. In Physics, we have such a diagram. It is known as the Free-Body or Force Body Diagram. Ask anyone who has taken a physics course and they will confirm how important being able to draw FBDs are. A Free-Body Diagram should have an arrow for each force present that starts on the center of mass of the object and is drawn outward in the direction that the force is applied. The FBD for a projectile flying through the air toward the right on its way to its peak height is simply a single downward arrow representing the weight or gravitational force acting on it while it is in free-fall. Remember, that is what free-fall means, only gravity acts.
Newton’s 2nd Law states that the acceleration of a system is proportional to the net force acting on the system and inversely proportional to the mass of the system. But, what’s a system? A system is anything you define as the system, but the system cannot change midway through a problem. For example, if a woman is standing on a paddleboard in the water, and she walks forward, the paddleboard will go backwards beneath her. You can look at her as the system, and she walks forward because the friction of the board pushed her forward. You can look at the board as the system which is thrust backward because the friction of her feet push it that way. OR… you can say the woman-board combination is the system in which case the center of mass of the system never actually moves since there is no net force acting on that system externally. All of the friction forces described are within the object listed as the system. The 2nd law states that if we sum all of the forces acting on the defined system, whatever that may be, it will be equal to the product of the mass of the system and the system’s acceleration. Often the system is one object, and this is pretty straight forward, but sometimes there are combinations of objects. The beauty of a scientific law is that it holds true no matter what. These laws are universal.
Newton’s 3rd Law states that for every action force, there is an equal and opposite reaction force. This is the easiest law to state, but the most difficult to feel deep in your bones. I believe a better way to state the 3rd law is to say, “If object A pushes on object B, then object B pushes in the opposite direction with an equal amount of force on object A.” A great example of Newton’s 3rd Law is when you walk. In order to accelerate from rest to a forward velocity, you must push backward on the ground. Although you push backward, you accelerate forward. Well, the second law says your acceleration needs to be in the same direction as the net force, so what’s going on here? You, object A, pushed backward on the ground, object B, and the ground then applied an equal force to you in the forward direction. You only accelerate if the ground can push you! If you were on ice, you’d have a harder time accelerating!
Tension in a rope is another great example of the third law. If you pull on one end of a rope, then whatever you tied the rope to pulls in the opposite direction with an equal amount of force. If your friction with the ground wasn’t greater than the friction of the object you were trying to pull then you wouldn’t be able to accelerate that object to move it. HMMMM!
Speaking of friction, I should probably point out the other equations pertaining to this unit, the ratio of frictional force to normal force is equal to a value specific to any pair of surfaces known as the frictional coefficient. That’s right, any two surfaces have just one value that allows you to identify that pair. These exact values need to be determined experimentally since this number is so specific to the particular pair of materials. It should also be noted that the maximum static frictional force is necessary in order to determine the coefficient of static friction. The kinetic coefficient of friction is determined using the kinetic frictional force or sliding frictional force. These coefficients can be the same, but USUALLY the static coefficient is higher than the kinetic coefficient. This is due to the object having inertia and obeying Newton’s 1st Law. If an object is at rest, it wants to remain that way, so it is harder to overcome static friction.
Dynamics problems can be confusing to start out, so it can be very helpful to have a roadmap to solving force problems. This set of directions has been handed down generation to generation for eons for you to be able to use and learn from… I’m just kidding, but it is really helpful. It is 3 easy steps. FIRST, draw the FBD. SECOND, sum the forces acting on the system to determine the net force. THIRD, set the net force equal to mass of the system times acceleration. THAT’S IT! 3 steps!
To recap……
There are 3 laws of motion discovered by Isaac Newton, and knowng all of them is essential to understanding the way objects move in nature. There are a variety of forces, and each one has its own nuances. Solving force problems can be tricky at first, but utilizing an easy to remember technique of problem-solving can help A LOT. Draw an FBD, sum the forces, set the net forces equal to m*a.
Coming up next on the APsolute RecAP Physics 1 Edition, we go round and round the topic of uniform circular motion.
Today’s Question of the Day focuses on dynamics.
Question: If you double the mass of a system without changing the net external force, what will the effect on acceleration be?
a) acceleration will be doubled
b) acceleration will halved
c) acceleration will be unchanged