Now on to velocity, speed, acceleration and force. Velocity and speed are two different things, but the difference is very small. Velocity gives more information than speed does, because it tells us how fast something is moving in a specific direction. Speed is how fast something is going, but says nothing about the direction of that motion.. Acceleration says how much the velocity is changing in a specific direction.
If something has a constant velocity, say moving south at 65 mph, there is no acceleration. Now how can you make something accelerate? To accelerate, an object needs to feel a force, that means a pull or a push. Force is mass times acceleration. This means that force is the amount of stuff times how hard it is being pushed or pulled. When something falls, it falls because of gravity. Because that object feels a force, it accelerates, which means its velocity gets bigger and bigger as it falls.
The strength with which the Earth pulls on something in the form of gravity is a type of acceleration. Earth pulls on everything the exact same amount. Everything gets accelerated towards the Earth exactly the same way. The force that objects feel may be different because they have different masses, but the acceleration on Earth they experience is exactly the same.
Weight is the force that acts on the mass due to gravity, because it is how much stuff there is times the acceleration at which is pulled towards the Earth, or any planet or moons. Because Earth gives everything the exact same acceleration, objects with different masses will still hit the ground at the same time if they are dropped from the same height.
The first time you say that, no one will believe you because everyone has dropped a marble and a feather at the same time and they hit the floor at different times. That is not because of differences in the acceleration - which is constant on Earth, it is because air is pushing against the object in the opposite direction the Earth is pulling. This force is caused by air resistance. The less massive the object is, the more the force of air resistance slows the object down as it falls.
If two objects were dropped on the moon, where there is no air, they would fall at the same rate no matter how much they differ in mass. The shape of the object can impact how much it is affected by air resistance.
For example, if you drop a piece of paper horizontally, it has a lot of surface that is exposed to the air resistance.
But if you drop the paper vertically, on the thin side, then there is less surface exposed to the air resistance. This means that, in that position, the paper will feel less push from the air and the same pull from the Earth. Two pieces of paper with the same mass dropped from the same height but with one in the horizontal position and the other in the vertical position will not hit the floor at the same time.
Astronaut Neil Armstrong did an experiment on the moon to convince everyone that Galileo was right, that two objects of different mass and shape -in this case a feather and a hammer - in the absence of air resistance will hit the ground at the same time.
Experiment 2 that you will be performing, two objects of different masses that roughly experience the same air resistance will be dropped and hopefully convince your kids that mass has nothing to do with how things fall. The idea is to encourage them to be creative, to understand how to design experiments, and to think like scientists and engineers.
They are given a set of materials that they can use to do their experiments. This is to prompt them, but they should be allowed to use other materials in their design. As the teacher, you can ask prompting questions to get them to think about the different aspects of the experiments. Below are the full instructions for one possible design.
The goal of the experiment is for students to understand that mass is not a factor that affects how objects fall, that they notice the shape matters and why it matters. Crumpling the paper or changing the direction in which the paper is dropped can support those ideas. Maybe they hit the ground at the same time because they have the same gravitational force on them? First, they can't have the same gravitational force because they have different masses see above.
Second, let's assume that these two balls have the same force. With the same force, the less massive one will have a greater acceleration based on the force-motion model above. Here, you can see this with two fan carts. The closer one has a greater mass, but the forces from the fans are the same. In the end, the less massive one wins. No, the two objects with different mass hit the ground at the same time because they have different forces.
If we put together the definition of the gravitational force on the surface of the Earth and the force-motion model, we get this:. Since both the acceleration AND the gravitational force depend on the mass, the mass cancels. Objects fall with the same acceleration—if and only if the gravitational force is the only force. The gravitational field is not constant.
I lied. Your textbook lied. We lied to protect you. We aren't bad. But now I think you can handle the truth. The gravitational force is an interaction between two objects with mass.
For a falling ball, the two objects with mass are the Earth and the ball. The strength of this gravitational force is proportional to the product of the two masses, but inversely proportional to the square of the distance between the objects.
As a scalar equation, it looks like this. A couple of important things to point out since you can handle the truth now. The G is the universal gravitational constant. It's value is super tiny, so we don't really notice the gravitational interaction between everyday objects.
The other thing to look at is the r in the denominator. This is the distance between the centers of the two objects. Since the Earth is mostly spherically uniform in density, the r for an object near the surface of the Earth will be equal to the radius of the Earth, with a value of 6, kilometers huge.
So, what happens if you move 1 km above the surface of the Earth? The r " goes from 6, km to 6, km—not a big change. Even if you go ALL the way up to the altitude of the International Space Station orbit km , there isn't a crazy huge change. Here, I will show you with this plot of gravitational field vs. Oh, and here is the python code I used to make this —just in case you want it.
For just about all "dropping object" situations, we can just assume the gravitational force is constant. OK, now we are getting into the fun stuff. What if you drop an object and you can't ignore the air resistance? Then we have a more complicated problem, because there are now TWO forces on the falling object. There is the gravitational force see all the stuff above , and there is also an air resistance force. As an object moves through the air, there is a force pushing in the opposite direction of motion.
This force depends on:. The part that makes this complicated is the dependency of the air resistance on the speed of the object. Let's consider a falling object with significant air resistance. How about a ping-pong ball? When I let go of this ball, it is not moving. This means there is zero air resistance force and only the downward gravitational force. This force causes the ball to increase in speed in the downward direction —but once the ball is moving, there is now air resistance force pushing up.
This makes the net force a little bit smaller, and thus you get a slighter increase in speed. Eventually the air drag and gravitational force have equal magnitudes.
The ball then falls at a constant speed—this is called terminal velocity. Since the net force on a falling object with air resistance isn't constant, this is a pretty tough problem. Really, the only practical OK, not really the only way to model this is with a numerical calculation that breaks the motion into tiny steps during which the force is approximately constant. How about a model of a falling ping-pong ball? Here you go.
Click the pencil icon to see and edit the code, and click Play to run it. View Iframe URL. You can see that the ping-pong ball almost reaches a constant speed after dropping a distance of 10 meters.
I put a "no air" object in there for reference. If you want to see what happens if you change the massgo ahead and change the code and re-run it.
It's fun. Now we get to the interesting question. If I drop two objects from the same height, does the heavier one hit the ground first? The answer is "sometimes. Drop 1: A basketball and bowling ball. Here is a slow-motion view of this actual thing. 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. Because the 9. Gravitational forces will be discussed in greater detail in a later unit of The Physics Classroom tutorial. As an object falls through air, it usually encounters some degree of air resistance.
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