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True or False? Common Misconceptions In Newtonian Physics

By Karen J. | Tuesday, December 28, 2021

Physics tends to be hard to grasp at first. For better or for worse, it’s something that’s all around us. We know what gravity is, and we know what speed is. We know that apples fall downwards from trees and that heavier objects are harder to pick up. The point is, even if we haven’t taken a physics course before, we already have our own ideas of it in our heads. It’s often advantageous to have prior knowledge, but it becomes a problem, especially on the conceptual side of foundational topics, when those preconceptions are wrong. So without further ado, here are 5 common misconceptions in key foundational physics topics:

Force is Needed to Keep an Object in Motion

When we throw a ball upwards, where does it go? We know the ball will go up first, and then fall back downwards due to the force of gravity. So is the force of gravity keeping the ball in motion? Actually, if we removed the force of gravity and all other forces such as air resistance, the ball would keep going up forever, with the same initial velocity and in the same direction you had thrown it in. This means the force of gravity is actually changing the motion of the ball, causing it to change direction and fall downwards with increasing speed---hence, the Law of Inertia (all objects in motion stay in motion in a straight line and at a constant speed, and all objects at rest stay at rest unless acted on by external unbalanced forces). To correct this misconception, we can say that unbalanced forces are needed to change the motion of an object, rather than to keep an object in motion in the first place.

Zero Velocity Implies Zero Acceleration

A common mistake when learning the foundations of physics is confusing acceleration and velocity. You’ve probably encountered the generic test question: “When a ball is thrown directly up in the air on Earth, what is the acceleration of the ball at the highest point?”. Maybe you were tempted to say that the acceleration is 0 m/s2 because the velocity is 0 m/s. If that were the case, the ball would never come back down. Think about it. If there is no acceleration, or no change in velocity, how can the ball, with zero velocity at the top of its vertical trajectory, change its velocity to fall back down?

You can also think about it like this: knowing that the force of gravity is causing the downwards acceleration of the ball, it wouldn’t make sense for gravity (or acceleration) to suddenly stop existing when the ball is at its maximum height. I’m sure you weren’t strong enough to throw the ball out of Earth’s gravitational field. For the most part, gravity on Earth is gravity everywhere on Earth.

Constant Speed Implies No Acceleration

This is a tricky one. The key is to remember that velocity is a vector that has two components: magnitude (speed) and direction. If speed, or the magnitude of velocity is constant, but the direction is changing, there is still acceleration. This is a key concept of uniform circular motion, in which an object traveling along a circular path at a constant speed or magnitude of velocity is still experiencing acceleration (as a result of the centripetal force) as its direction of velocity is constantly changing.

Positive Acceleration Always Implies Increasing Speed

This is only true if acceleration and velocity are occurring in the same direction. Again, remember that velocity is a vector; it has both magnitude (speed) and direction. We represent velocity in the negative (leftwards or downwards) directions with a negative (-) sign. Let’s say we are traveling at -10 m/s (to the left). If our acceleration is in the positive direction, such as 5 m/s2, our velocity will change from -10 m/s to -5 m/s in the first second. Here, the magnitude of velocity, or speed, has actually decreased because velocity has increased 5 in the positive direction.

Heavier Objects Fall Faster Than Lighter Objects

When air resistance is negligible, and you drop an elephant and a mouse from the same height off a cliff (not advised), which one hits the ground first? You might think the elephant does because it’s bigger, but both hit the ground at the same time. Why? Because the acceleration due to gravity, about 9.81 m/s2 downwards, is 9.81 m/s2 downwards for both animals. This goes back to the earlier statement that for the most part, gravity on Earth gravity everywhere on Earth.

It might be difficult to think about, but this misconception stems from the tendency to confuse acceleration and force, or the acceleration due to gravity and weight. Remember Newton’s Second Law, F=ma. While the elephant is more massive, say 1000 kg, and the mouse is less massive, say 0.1 kg, the acceleration they both experience is still 9.81 m/s^2 downwards. This acceleration, times their masses, will produce different forces, or different weights: the elephant will weigh about 9810 Newtons while the mouse will weigh a mere 0.981 Newtons. Even without the hypothetical math, we know intuitively that one is definitely heavier than the other, but this difference in weights does not change the shared acceleration they both experience because weight is not the same thing as acceleration.

Conclusion

When studying physics, it’s important to identify and resolve your misconceptions so they don’t hold you back from learning the “right version” of concepts. Hopefully, some of your misconceptions have been cleared, and you feel more confident about your understanding of foundational concepts. If you ever do need help though, we physics tutors at Schoolhouse.world are more than happy to help!

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