What is the relation between Force, Physical Strength,Enery and Work?

Question

I am sorry ,but, I am not much of science or a physics guy, if this question sounds stupid. Anyways, what is the relation between all of them. Like for example, does applying more force means more Physical Strength,Energy and Work is required. Lets say, if a person hass to move a heavy object. So how will Force,Physical Strength, Energy and Work will relate to each other? Like will the person be required to apply more force or more Physical Strength or more Energy or more Work or all of them to move the object faster?

Answer 1

We have to talk about Physical Strength last, for two reasons: (1) we have to clearly define the other physics terms first, and (2) Physical Strength gets us into biophysics so we'll have to talk about how muscles generate force, etc.

"Force" is a push or a pull or a twist (if a twist, then it is also called "torque"). Force is what is required to accelerate (change the speed or direction) of any object.

In physics, Energy and Work are more-or-less the same thing, convertible into each other. Work is defined as Force times the Distance over which that force was applied. So for example, if a heavy ball (like a bowling ball) is not moving. And you push it with a certain force for a certain distance, you cause it to roll. You have done a "Force times Distance" amount of work on that ball to get it rolling. It continues to roll, and is now rolling with an amount of Energy equal to the work that you did on it. Of course, even with a bowling ball there is some friction between the ball and the ground. That friction also does work on the ball, gradually slowing it down. The amount of work that friction must do on the ball to bring it to a complete stop is exactly equal to the amount of work that was originally done on the ball.

I hope that helps give you a concept. To summarize:

Work = Energy = Force x Distance

Or Force = Energy / Distance.

Regarding that last equation, think of it this way: For a given amount energy that the bowling ball is rolling with, the more force you apply to the ball, the shorter the distance over which you will be able to bring it to a stop.

Now, to understand physical strength, we have to first understand something about how muscles work. This is going to be a very rudimentary, simplified description: Muscles contain fibers made up strands of muscle cells. Each muscle cell has the ability to contract. Muscle contraction is essentially a force applied for a distance (that is, work, or energy). However the force and distance involved for a single muscle cell is very small. The energy needed to generate this cellular movement results from a flow or movement of ions, or electrical charge, not too unlike water falling over a waterfall. The ions flow through certain "openings" or "gates" in the cell membrane when they are opened. However once the ions flow through, we need to somehow get the ions back to the other side to be able to flow through again. This is done by certain types of protein molecules that do work on the ions and pump them back to the other side of the muscle cell's membrane. So you see, there is a lot of work (Force times Distance) going on at a cellular, and molecular (chemical) level. The forces generated by a single ion, and even by a single muscle cell through which millions of ions flow, is very small. However when millions of muscle cells all contract at the same time, they can together generate a significant amount of force (enough to move a bowling ball, and more!)

Now we can talk, in a general and very simplified way, about strength. Lets define, for our discussion, "strength" as the "amount of force" that a given muscle is able to generate. Given the above description of muscle, it should be clear that more cells working together should be able to generate more force, in much the same way that ten people pushing together can generate more force to push a car that just two or three people pushing together. A larger muscle has more cells than a smaller muscle, and so it has greater "physical strength".Hope that helps.

Regarding your comment

"So does that mean if I have more Physical Strength or more Energy or do more work then the force will be higher or I can generate higher force and vice versa?"

To clarify:
As I have defined it, "Physical Strength" means you can generate more "Force".

However, there are many factors here. First, remember Energy is Force times Distance. If you have larger muscles you may generate a larger force, but that says nothing about the distance over which you may be able to generate that force; that is, you may only have a limited amount of energy available for generating that force. If you haven't eaten in a while, you may "feel" weak, lacking the energy to generate that Force for a long time, or a long distance.

Let's look where that force is coming from. The force generated by your muscles comes from biochemical reactions in your muscle cells that use up chemical energy ultimately from the food you eat. Larger muscles, i.e. more muscle cells, can generate more force, but they use up more energy generating that force, simply because there are more of them. Think of it this way: ten people pushing together can generate more force to push a car, but you are going to have to feed lunch to all ten of them. Pushing the car a specific distance, say 50 feet, with a larger force (ten people) uses more energy than pushing it with a smaller force (Energy = Force times Distance; a smaller force for the same distance means less energy). Similarly, a larger muscle can generate a larger force, but it uses up more energy than a smaller muscle generating a smaller force (because there are many more cells that have to be "fed" lunch, so to speak).

Answer 2

Force, energy, and work are all just different ways for us to describe motion.

Energy (more specifically kinetic energy) is a measure of how fast an object's motion is. If an object has non-zero velocity, it has energy by definition.

Force is a measure of how fast an object's rate of motion is changing. If an object's velocity changes, it's experiencing a force by definition.

Work is a measure of how much force you use to move an object. If an object moves because of a force, work is being done on it by definition.

When you want to move something faster (or more massive), you can say any of these: "it needs more energy" / "it needs more force applied to it" / "it needs more work done on it." And yes, if you're doing it manually, you can also say "it needs more physical strength to move it."

Answer 3

Force

The tendency to make something accelerate. Newton's 2nd law:

$$\sum \vec F=m\vec a$$

Physical strength

Can be different things. E.g.

• hardness $H$: a materials resistance against "bumbs" and indentations in the surface,
• yield stress $\sigma_y$: the stress a material can withstand before starting to deform
• ultimate tensile stress $\sigma_{max}$: the maximum stress a material can withstand after which fracture is almost certain
• etc.

Most of these are in some way force per area. So typically strength is no more than an upper limit to force before some unwanted change in a material happens.

Energy

Can be understood as a number telling how likely a force is to be exerted. There are many kinds E.g.

• Kinetic energy $K=\frac{1}{2}mv^2$: Motion enegy
• Potential energy $U$: Stored energy that when released might make something move
• Thermal energy: The molecular version of kinetic energy
• Internal energy $U$: Another kind of stored energy - in this case in the form of molecular bonds e.g. (but also kinetic energy, thermal energy etc.).

Work

Work is a kind of energy transfer. There are two main kinds:

• Work $W$: Energy being transfered by something moving something else.
• Heat $Q$: Energy being transfered by something warming up something else.

A body doesn't contain work, like it contains kinetic energy e.g. Work is rather only the name given to energy in transit - energy being transfered.

$$W=Fx$$

Only a force can do work. And we only call it work, it something has moved ($x$ is displacement)

does applying more force means more Physical Strength,Energy and Work is required

More force means higher tendency of something to move.

• Yes, If you are pushing on a material surface, then this requires larger strength to avoid the surface particles to move and the material to deform.
• No, higher force does not imply more energy. Push harder downwards onto the ground and the normal force grows as well to withstand your push. No energy is stored in this system.
• No, work is only present when something is being moved. Push hard on a wall, and no work is done nomatter your effort.

Lets say, if a person hass to move a heavy object. So how will Force,Physical Strength, Energy and Work will relate to each other?

If the object is heavy, it has a large weight $w$ because of gravity.

• To lift it, you must apply a larger force $F$. From Newton's 2nd law above, since you want it to accelerate upwards (you want a non-zero $a$), the total sum of forces (the resulting force) must point upwards. So $F$ point up must be larger than $w$ pointing down, so that $\sum F=F-w=ma$ is positive.

• Physical strength is not an issue here. You are not trying to deform the object. If on the other hand in this case physical strength means how strong the person is (which more is a fysiological/biological than a physical term) then of course the physical strength must be large enough to allow the person to apply to necessary force $F$. Physical strength is simply an upper limit - so whatever force that needs to be applied must just be lower than this limit.

• You must add energy to this system in order to move the object higher up. This is called gravitational potential energy $U_g=mgh$ and depends on the heigth $h$, you wish to lift it.

• ... and this energy must be added in the form of work $W=Fh$, where the displacement in this case is $x=h$.

... to move the object faster?

When energy has to be applied faster, we can look into either

• the acceleration $a$ in Newton's 2nd law $\sum F=ma$, which has to be larger. This requires larger force as well.
• Or we can look at power $P=W/t$. Power is work over time, meaning work done during the time $t$. Do you want the work (the lifting) to be done faster, you do not need to add more energy, you just need to add it faster. We can put the work expression $W=Fx$ into this formula:

$$P=\frac{W}{t}=\frac{Fx}{t}$$

Do you want the lifting done faster, then $t$ is smaller. This raises the power $P$ that is required. And in order for $P$ to increase, $F$ must increase. Just as Newton's 2nd told as well.