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Answers to Readers' Questions on Physical Science:

Work

The following 18 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Difference between speed and velocity
• What work is done if friction is zero?
• Confused about forces
• Why is the force multiplied by the distance in work?
• Don't understand material for test
• Why did salt water cushion the falling egg?
• Where did horsepower come from?
• Work required
• Why is work a scalar quantity?
• Work and force problem

Next 10

Difference between speed and velocity

Question

March 9, 2009

1 what is inertia
2 diffrences between speed and velocity

segun - Nigeria

17419

Answer

For information on inertia, see:
http://www.school-for-champions.com/science/motion_laws.htm

Speed is motion in any direction, while velocity has the direction defined.

5 km/hr is speed but 5 km/hr northwest is velocity.

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What work is done if friction is zero?

Question

May 1, 2008

Is it not true that if you move an object of a certain mass a certain distance across a frictionless horizontal floor that you will be doing work on the object only if you constantly accelerate it? In other words, if you moved with a constant velocity over the same distance on the same floor you would be doing zero work on the object. Weight, in this case would not be an issue, only inertia or mass plus horizontal acceleration.

Matthew - Australia

15841

Answer

If you apply a force on an object on a frictionless floor or in outer space, it will accelerate as long as the force is being applied. The work you expend is the force times the distance that you apply the force. As soon as you stop applying the force, the object will continue to move at a constant velocity, due to its inertia. When it is moving on its own, you are doing no work.

This is why work is usually measured when there is a resistive force, such as friction, gravity or a spring that is holding back the object. As soon as you stop applying the force in those situations, the object will slow down and soon stop.

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Confused about forces

Question

April 25, 2008

for the equation W=Fd
i'm confused about what force is
let's says there's a question that gives you the weight of an object and the force applied

eg. A box has a weight of 90N. Someone pushed the box with a force of 120N for a distance of 15m.

Would I do: W=Fd=90x15
or W=Fd=120x15
or would I add the forces together: W=Fd=(120+90)x15

Josh - Canada

15805

Answer

In reality, the work should be against a resistance force. If you lift an object, you are working against the force of gravity. The force required to lift the object equals its weight. If you are pushing a box across the floor, you need to know the coefficient of friction, which you will multiply by the weight to determined to resistive force of friction.

In your example, the work against friction would be W = 120x15. The coefficient of friction would be: fr = 120/90 = 4/3.

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Why is the force multiplied by the distance in work?

Question

March 19, 2008

When a force is applied on a body and the body gets displaced in the direction of the force then this activity is defined as work. The Mathematical relation being W = F.S My question is why did we multiply the two terms Force and Displacement, not that i am saying we should have added them, but why multiply? The answer has to be better than "Because we cant add Force and Displacement as they have different units N+m" or "We cant divide or subtract them because the effect would some how numerically seem to reduce N/m, N-m" or the only operation left that is known to us is multiply" Who ever came up with the relation Hopefully must have had some real logic in the multiplication of these two entities.

Faisal - Pakistan

15568

Answer

Work can be looked at as moving an object a certain distance against a resistive force such as gravity. We can then define this concept of work as the product of the force to move it times the distance moved.

But also, you can say it takes energy to move that object, such that Energy is equivalent to Work. This can be shown by looking at the units. The units of F = ma are kg*m/s^2. The unit of distance is m. Thus, the units of F*S are kg*m^2/s^2, which are the units of energy.

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Don't understand material for test

Question

October 3, 2007

Help! My 5th grader needs to know how to do this for a test and i have no clue.
1. They need to fill a sandbox with 20 bags of sand. Each weighs 50 pounds.
How many newtons of force will be needed to lift the sand?
2. The sand is 5 meters away from the sandbox. How much work will they have
to do to move all the sand to the sandbox?
3. Figure out how much work they must do to move 1 bag of sand?
4. It takes 3 minutes to lift 1 bag of sand and carry it to the sandbox. How
much power does it take to move 1 bag of sand?

Leona - USA

14469

Answer

We have 20x50 = 1000 pounds. Now weight is a force, but pounds are in the English system, and the teacher wants it in Newtons, which is in the metric or SI system of measurement.

There are 0.22 pounds per Newton, so divide 1000 by 0.22 to get the Newtons.

Work equals force times distance. Strictly speaking, it should be the distance moved against a resistive force, such as gravity. If they lift an object, that is work. But if they carry it, that is not work. The teacher may be defining it somewhat incorrectly.

I hope that helps.

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Why did salt water cushion the falling egg?

Question

April 25, 2006

ok, i have this science fair at my school, and i have to make a package for an egg that when dropped from two storis, it will not break.(the egg is raw)
i have found a package that works, but i dont know why. it is a tin can filled with salt water, in which the egg floats, covered with tin foil. i need to know the physics behind this; i cant see how salt absorbs the shock of a two story fall. Please get back to me as soon as possible, i will be very grateful. thank you.
-jenny

Jenny - USA

10942

Answer

The solution to this experiment is to find a package that will absorb and distribute the force when the egg hits the ground. You want something to cushion the blow, so that the egg won't crack.

You have a clever solution to the problem. The salt in the water causes the egg to float. This is because salty water is more dense than the egg. When the can hits the ground, the force is spread through the water. That has to do with the elasticity of the water. So it cushions the crash. The egg will bob around in the water, but it will not hit anything hard that can crack it.

Note that if the egg was at the bottom of the can, it could easy crack from the crash.

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Where did horsepower come from?

Question

April 22, 2006

sir,
i know that 1horse power is eqal to 746 watts. but what is the reason that this unit is termed as 1horse power? is the power in 1horse is 746 watt? if the ans is yes. then tell me how it was determind by the scientist who was discover this relation. if the ans is no why we termed it as 1horse power?

gurjinder - India

10907

Answer

Just like the English measure of distance is 1 foot, which was the length of the king's foot, the old English measure of power is the work that a horse could do. The problem is that just as each king had different sized feet, so too does each horse have different strength. That is why it is better to convert to such a unit as a watt for power.

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Work required

Question

October 12, 2005

I needed help with this question please.

A force of 2.2 X 10(to the 4th power)N is needed to lift a 2270 kg stone block.using the equation work= force X distance, determine how much work is required to lift such a block a distance of 2 m. express your answer in units of j (1 J = 1 N X m).

christina - USA

8524

Answer

Work = 2.2*10^4N * 2m = 4.4*10^4 N-m = 44000 J.

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Why is work a scalar quantity?

Question

March 13, 2005

why work is taken as a scalar quantity as it is always in some specific direation.

zia - Pakistan

6317

Answer

That is a confusing definition.

Work = Force times distance. The force is a vector, since it is in a given direction. If you would define distance as a scalar number, then work would have to be a vector in the direction of the force.

But for some reason, they define distance as a vector. When two vectors are multiplied together, they call it a "dot product" and the result is a scalar.

That means that work is just a number or a scalar.

I don't agree with that viewpoint, but it is what is accepted. At least you understand the reasoning.

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Work and force problem

Question

February 7, 2005

Having a problem figuring this one out. (Please answer and explain how you got your answer.

A student lifts 110kg mass in the work-out rm. Tom lifts the mass a distance of 1.5 meters.
A. How much force is required to light the mass?
B. How much work does he perform?
I answered
A. f=ma f= 110kg(1.5) f=165n
B. w=fd w= 165(1.5) w=247.5j
Please explain where I made my mistake.
Thank you
Lyndsey Barnett

Lyndsey - USA

5883

Answer

Force to lift a mass = mass X gravity acceleration = 110kg x 9.8 m/s^2 = 1078n
Work against gravity = force x distance = 1078 x 1.5m = 1617 J

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