This is often attributed, incorrectly, to there being no You can use this activity to get pupils to relate moments to the stability of objects. Classroom Activity Other resources on Force Force Forces and Motion. Force Forces and Motion. Misconceptions Forces and Motion Many students think of a force as a property of objects, not as something that arises when two objects interact Number of Resources 3 Number of References 16 Number of Diagnostic Resources 3.
Forces and Motion Many students are unable to identify correctly the forces acting on each object in a situation where two or more objects interact Number of Resources 2 Number of References 3 Number of Diagnostic Resources 1. Forces and Motion Many students can use the word 'force' to mean a push or a pull and understand that a force is needed to get a stationary object moving Number of Resources 1 Number of References 3 Number of Diagnostic Resources 1.
We've won an award! Learn more. In a previous unit, it was stated that all objects regardless of their mass free fall with the same acceleration - 9. This particular acceleration value is so important in physics that it has its own peculiar name - the acceleration of gravity - and its own peculiar symbol - g. But why do all objects free fall at the same rate of acceleration regardless of their mass?
Is it because they all weigh the same? These questions will be explored in this section of Lesson 3. In addition to an exploration of free fall, the motion of objects that encounter air resistance will also be analyzed.
In particular, two questions will be explored:. As learned in an earlier unit, free fall is a special type of motion in which the only force acting upon an object is gravity.
Objects that are said to be undergoing free fall , are not encountering a significant force of air resistance; they are falling under the sole influence of gravity. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass.
But why? Consider the free-falling motion of a kg baby elephant and a 1-kg overgrown mouse. If Newton's second law were applied to their falling motion, and if a free-body diagram were constructed, then it would be seen that the kg baby elephant would experiences a greater force of gravity.
This greater force of gravity would have a direct effect upon the elephant's acceleration; thus, based on force alone, it might be thought that the kg baby elephant would accelerate faster. But acceleration depends upon two factors: force and mass.
The kg baby elephant obviously has more mass or inertia. This increased mass has an inverse effect upon the elephant's acceleration. The gravitational field strength is a property of the location within Earth's gravitational field and not a property of the baby elephant nor the mouse. All objects placed upon Earth's surface will experience this amount of force 9. Being a property of the location within Earth's gravitational field and not a property of the free falling object itself, all objects on Earth's surface will experience this amount of force per mass.
As such, all objects free fall at the same rate regardless of their mass. Well, here you have a greater mass times the same quantity.
Here you have a smaller mass times the same quantity. So if the mass of the brick is greater than the mass of the feather, it's completely reasonable to say that the force of gravity on the brick is going to be greater than the force of gravity on the feather. So if you do all this, and everything we've done to this point is correct, you might say, hey, there's going to be more force due to gravity on the brick, and that's why the brick will be accelerated down more quickly.
But what you need to remember is that there is more gravitational force on this brick. But it also has greater mass.
And we remember the larger something's mass is, the less acceleration it'll experience for a given force. So what really determines how quickly either of these things will fall is their accelerations. And let's figure out their accelerations. I'll do this in a neutral color. We know that force is equal to mass times acceleration.
So if we want to figure out the acceleration of the brick-- or we could write it the other way. If we divide both sides by mass, we get acceleration is equal to force divided by mass.
And acceleration is a vector quantity, and force is also a vector quantity. And in this situation, we're not using any actual value. But if I were using actual values, I would use negative numbers for downwards and positive values for upwards.
But we're not using any signs here. But you could assume that the direction is implicitly being given. So what's the acceleration of the brick? That's a lowercase b I was writing. The acceleration of the brick is going to be equal to the force applied to the brick divided by the mass of the brick. But the force applied to the brick, we already know, is this business right over here. It is little g on the moon, the gravitational field on the moon, times the mass of the brick, and we're dividing that by the mass of the brick.
So the acceleration on the brick on the moon-- the acceleration that the brick will experience-- is the same thing as that gravitational field expression. This is how quickly it would accelerate on the moon. Now let's do the same thing for the feather. I think you see where this is going. The acceleration of the feather is going to be the force on the feather divided by the mass of the feather.
The force on the feather is g sub m-- g with the subscript m-- times the mass of the feather, and then we're going to divide that by the mass of the feather.
And so, once again, its acceleration is going to be the same quantity. So they are both going to accelerate at the same rate downwards, which tells us that they'll both hit the ground at the same rate. They'll both accelerate from the same point at the same time, and they'll both have the same velocity when they hit the ground. And they'll both hit it at the exact same time, despite one having a larger mass.
So the reality is, because it has a larger mass, it has a larger gravitational attraction to the moon. But because of its mass, that attraction gives it the same acceleration as something with a smaller mass.
So any mass at the same level on the surface of the moon would experience the same acceleration. So now the quite natural question is, wait, Sal, if that's true on the moon it should also be true on Earth. And it would be true on Earth.
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