Top Ten Most Interesting / Relevant Concepts of Physics

11) Mass

I found the concept of mass to be very interesting when considered in terms of inertia. More specifically, mass is a measurement of an object’s inertia. Inertia is the tendency of an object to resist changes in motion. Thus, the more mass an object has, the greater its inertia. This means that this object will resist outside forces that try to stop its motion or put it into motion from being at rest. I found this concept interesting as the non-physicist world commonly misuses the term “mass” interchangeably with “weight.” These are not the same concept. I like how mass has so much more to it than an object’s matter. It is actually the physical characteristic of a particular object in regards to Newton’s 1^st Law of Motion, which states that any object in motion tends to stay in motion while an object at rest tends to stay at rest, unless acted upon by an outside force.

2) Force ~ acceleration

According to Newton’s 2^nd Law of Motion, acceleration is directly proportional to force and inversely proportional to mass. As a cross-country runner, I find this information very helpful to know. What Newton is saying is that the greater my mass, the lower my acceleration, while the greater my force, the greater my acceleration. Therefore, if I can decrease my mass as much as possible, then I can increase my acceleration. Also, increasing the force with which I accelerate will boost my acceleration as well. Logically, I would want to buy the lightest, or least massive, shoes, wear clothing that would add the least amount of mass to my total mass, and train to decrease my overall mass while increasing my muscle mass in order to run with greater force. All of these things are just small factors that could contribute to my overall success in a race. Although they may seem logically connected, I had not previously seen the connection between the concepts of force and acceleration or mass and acceleration in such a concise, formulaic way. Newton’s 2^nd Law is very relevant to my life as a runner and I also found such a simple physics concept interesting, as it is so basic yet so important.

3) Hitting a home run/ throwing a ball up at a curve

I thought that learning the physics behind hitting a home run was really interesting. I do not play baseball but it was cool to learn the scientific explanation of something that I do not generally associate with physics, or science at all for that matter. We learned how to solve a problem for the picture below and other similar situations. When a ball is thrown straight up with an initial vertical velocity of 20 m/s, gravity causes the velocity of the ball to decrease as it gets higher. This is because it has a constant acceleration (10 m/s^2) so the after the first second that the ball has traveled vertically through the air, its velocity would then be 10 m/s. It would be traveling at 0 m/s after 2 seconds when it has reached the top of its path. The ball then falls back down and gains velocity at the same constant acceleration. This process is important to keep in mind when considering objects being thrown at a curve. But this is not a complete explanation of that object’s path. You also need to consider the horizontal velocity of the ball. The horizontal velocity is constant throughout the entire path through the air. Therefore, while a baseball is following its curved path from home plate to beyond the fence (a homerun), it has a different vertical velocity but the same horizontal velocity at every second. For this specific problem, I know that the horizontal velocity is 30m/s because I used a handy dandy equation. Velocity = distance / time. Since I know the ball has a hang time of 4 seconds and must travel a horizontal distance of 120 meters, 120 ÷ 4 is 30, so the horizontal velocity of the ball is 30 meters per second. Also, it is important to remember that the vertical height of an object controls the time it spends in the air, so using this information we could also find the vertical height of the ball when it reaches the top of its path but this is another problem for another day. Another thing that we found about the baseball was how fast it had to be hit at the angle given. We see in the picture that the ball was hit at a 45º angle so we can form a right triangle from the vectors of the horizontal and vertical velocities at t= 0 seconds. From here we used the Pythagorean Theorem (a² + b² = c²) to find that the ball’s “actual” velocity was 36 m/s (it would need to be hit at this speed to make a homerun. In baseball friendly terms, the ball would be travelling at about 86mph).

4) Newton’s 3^rd Law

Newton’s 3^rd Law of Motion states that for every force, there is an equal and opposite force. Essentially, if I push against a wall, the wall pushes back on me with the same amount of force in the opposite direction of my force. I found this concept interesting, as I had never heard it before. Also, it is relevant as it can be applied to not only every action-reaction pair you encounter in your daily routine but also to some big things that we take for granted. For example, if we are visiting another city and we decide to take a historic carriage ride tour, the horses are able to pull the carriage thanks to and despite Newton’s 3^rd law. The horse and the carriage pull on each other with equal and opposite forces. The carriage and ground do the same with each other, as do the horse and ground, however, the horse and ground put more force on each other (thanks to friction) and thus the entire system of the horse and carriage move in the direction of the horse.

5) Tides

Tides are one of my favorite concepts that we have studied this year. It is easy to take the ocean’s steady, constant rocking for granted- it was so cool to find out that the rhythmic tides of the sea are actually caused by gravitational force. Again it was cool to see physics applied to something that seems a simple part of our lives. To go into a more specific explanation of the topic, I will first say that tides are not caused by the force between the moon and the Earth or the force between the sun and the Earth. When I say they are caused by gravitational force, I mean the difference of force felt on opposite side of the Earth. The side closest to the moon feels more force because force = 1/d^2 and this part of the ocean is much closer to the moon. The side opposite feels a smaller force because it is over more distance. The center of the Earth feels a force that is in between the two. If a hypothetical 25 Newtons of force are felt on the point of the Earth closest to the moon, 15 N felt in the center, and 5 Newtons felt on the far side, we can subtract these forces from each other to find a force of 10 Newtons. (Side A, 25 N, minus the center, 15 N, equals 10 N on side A towards the moon, and side B, 5 N, minus the center, 15 N, equals -10N). Since B is a negative force, it is pulled in the direction opposite from side A (away from the moon). This causes the Earth’s water mass to form a bulge around the planet. It is high tide in the parts of the world that the bulge is most oblong (A and B) at the same time, while low tide is on the other poles of the Earth, as seen in the picture. I thought this was very interesting as I would never have expected tides to be cause by a bulge in the ocean around the Earth.

6) Tangential vs. Rotational velocity

The difference between tangential and rotational velocity is that rotational velocity refers to the total speed of rotation for the object as a whole and is the same at every point on the object, whereas tangential velocity increases the further from the center of the rotating object you get. I found this interesting and relevant because this concept is present on Ferris wheels and other similar things.

7) Rotational inertia – Why do runners bend their legs instead of keeping them straight?

Speaking of rotation, I felt that rotational inertia was particularly relevant to my life. I love running and playing sports so it was interesting to learn physics concepts about my athletic interests. Rotational inertia is an object’s tendency to resist rotation- much like normal inertia and also self-explanatory. Rotational inertia is increased the more distributed an object’s mass is. For example when an ice skater is spinning, she has her arms out, which spreads her mass and gives her greater rotational inertia so she spins slower. When she pulls her arms in, her inertia decreases and she spins faster. Runners bend their legs when they run for a similar reason. Consider your hip to be the axis of rotation in regards to your leg because that is where they rotate. The close to your hip your leg can be, the less rotational inertia you will have and thus the more rotational velocity, so the faster you can rotate your hip joint and you can run faster that way. This was interesting because I never considered there was a scientific reason to this other than that is how your leg works.

 8)  Why do airbags keep us safe?

This question is arguably one of the most relevant issues that we dealt with in our physics course this year. Not only can it be answered in two different ways, using two different concepts, it is also a real-world application of physics, which I find more interesting than memorizing formulas. The first way that we answered this question was in terms of momentum and impulse. During a car crash, your body hits something and goes from moving to not moving. The change in momentum is the same no matter how you are stopped because you are going from moving to not moving (p=mv). Change in momentum is equal to impulse (∆p=J), so the impulse is also the same no matter how you are stopped. Since impulse = force x ∆time, when the time over which the impulse occurs is very long, the force of the impulse is very small. When you collide with a hard surface, it takes a short time to stop you, thus the impulse has a greater force on you. Conversely, an airbag is soft and absorbs the impact so it takes a long time to stop you and there is less force exerted on you. The smaller the force, the smaller the injury. This is why airbags keep us safe in terms of work and momentum. The second way to answer this question is in terms of work and energy. You go from moving to not moving regardless of what you hit, therefore the change in kinetic energy I the same no matter how you stop. KE = ½ mv^2. ∆KE = KE final – KE initial, and ∆KE = work. Since ∆KE is the same, the work done is also the same regardless of what you hit. Work = force x distance. Work must remain the same, but there can be more or less force or stopping distance depending on what you hit in the crash. The dashboard of your car is quite hard and firm, thus it stops you over a short distance when you hit it- this means there is a lot of force and a greater risk of injury. When you hit an airbag, it is soft and absorbs the impact so it takes a greater distance to stop you. This decreases the force on you and thus reduces the risk of injury.

9) Lightning/ Lightning Rods

Lightning is of course something that we see quite often and yet never really understand. We learned that when it is thunder storming, the clouds get very heavy and rub together. The friction causes the clouds to polarize with the negative charges on the bottom and the positive charges on the top. Through induction, the Earth’s surface polarizes with the positive charges along the top of the surface (because opposite charges attract) and negative charges deeper into the previously neutral ground. The air between the ground and clouds wants to equalize and eventually the charges jump across this space to create equilibrium. Energy is released as heat and light- this is lightning. I never knew that the ground was involved so heavily in this process. Lightning rods function as a way to protect our houses from lightning strikes while the charges are equalizing. The rods are metal conductors that stick up from our roofs and provide the lightning the path of least resistance to the ground. The lightning hits the rod and flows through it to the ground, rather than using a house or person to reach the ground. I always thought lightning rods were just metal sticks for lightning to hit instead of hitting people; it was both relevant and interesting to learn that more complex physics concepts are involved in their function.

10) Capacitors (camera flash)

Capacitors are plates that store charge. They transfer electrons between each other- it takes time to build up these charges, however. Once the charge is fully built up, the capacitors release strong currents all at once (including a release of energy). Then they have to build the charge back up all over again. I found this concept especially interesting, as capacitors are what are used to create the flash in a camera. Being very interested and involved in photography, I latched on to this concept. I know understand why you cannot snap multiple consecutive pictures very fast when you are using flash- it needs time between shots for the charges to build back up. Also, the flash itself is the energy released as light when the charges are transferred. It is so cool to view my camera as more than a little toy- it is a complex physics-related object. I would love to explore other physics concepts in photography.