F=ma

Discussion in 'Physics' started by Guinness, Oct 25, 2012.

Dec 31, 2009
81
1
Hi,
I am just a beginner when it comes to physics. But I am curious as to is there a difference between the Force and Kinetic Energy of a object impacting another?

Say a object hits a solid stationary surface, like a solid steel ball hitting a steel wall. Is there a difference between the force it impacts the wall and the kinetic energy it impacts the wall with or are they the same thing?

If they are different, what is the difference?

Sorry for what is such a basic question, but I have seen discussions of force and kinetic energy and they seen to be 2 different things! But I am confused as to what the difference is, as I thought the energy of a object hitting the other would be just one value and would not need different terms to describe it.

2. Shayan22h Member

Oct 24, 2012
30
3
force and energy re completely two different concepts but they re directly related as F.d= W and W= E so u see that the amount of Force applied on an object multiplied by the distance that the object is traveled due to the force (assumption : force and distance perpendicular) gives u the work done on the object so on the other words u could say this work gained by the object equals to the energy of the object that is obtained by force.
hope this could help

Guinness likes this.
3. vortmax Member

Oct 10, 2012
103
18
There is a lot you can learn by just studying the equations and units involved. For instance, consider force. Force is defined as:

F = ma

From this equation, we can see that if you apply a force (F) to an object with mass (m), it will accelerate with acceleration (a).

The base unit of energy is the Joule, which in basic units is Kg m^2/s^2. We can divide this unit up into three parts: Kg * m/s^2 * m which is mass * acceleration * distance.

If we look back at the force equation, we can see that

or, if you apply a force to an object (accelerate it) over a distance, then it builds energy (this is actually the equation for work). You can think about it like water in a cup. When the object is stationary, the cup is empty. When you apply a force, you start filling the cup with water. Once you stop filling the cup, the water is still in there...the object still has energy, and will keep that energy until it is removed (by an opposing force).

Now for a collision. Assume the steel ball is moving at constant speed (no force, no acceleration, but it does have energy) and strikes the steel wall. For it to bounce back, its velocity must change....velocity is a vector quantity, so changing direction of travel is an acceleration even if the speed does not change.

So in this collision, the ball starts at Vi, must reach 0 and then hits Vf going the other direction. This is an acceleration and thus there is a force involved....this force is equal and opposite between the ball and the wall. In a real system, this force compresses the ball and the wall slightly until all the kinetic energy of the ball is stored as potential energy (like a spring). At this point, the ball is not moving but this only lasts a small fraction of a second. Now the ball and wall start to relax back to their normal states, translating potential energy back into kinetic energy and accelerating the ball in the opposite direction. If this energy transfer process is 100% efficient, the ball will leave with the exact same speed (but opposite direction). However in reality, material are not perfect and some energy is lost...heating up the ball and wall and producing that 'clink' sound that you hear.

4. WBahn Moderator

Mar 31, 2012
18,088
4,917
As already pointed out, they are completely different critters. Take that same steel ball and let it impact a pillow instead. It has the same energy, but the forces involved will be dramatically different.

Consider two identical wineglasses, one dropped on a ceramic floor and the other dropped on a thick carpet from a height of, say, about four feet. You would expect the one dropped on the ceramic floor to shatter while the one dropped on the carpet to have a good chance of surviving. Although both have to change velocity by the same amount, the first has to do so in a tiny distance of perhaps a fraction of a millimeter, while the second has much more time to do so because, as the carpet depresses, the glass has perhaps something on the order of a centimeter to come to a stop. Since the acceleration is much less, the force producing that acceleration is also much less. If the ratio of the two distances is 100, then the forces differ by the same factor (of course, I'm assuming that the acceleration is constant from initial contact to when it comes to a halt -- this is a lousy assumption, but it is good enough for this discussion).

That's a little bit like asking what the difference is between temperature and pressure - in some circumstances there can be a relationship between them, but it is hard to talk about the "differences" between them. It is best to talk about them separately as what they are. Just as it would be best to talk about what an engine is and what a transmission is instead of talking about the differences between them.

Good. That's because they are.

The energy doesn't need a different term. But energy is not all there is when talking about collisions. There's also force and momemtum and the degree of elasticity.