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Have you ever wondered what it means to do work, and how it's related to energy transfer? From lifting a heavy object to running a marathon, we use our bodies to do work every day. In this article, we'll define what work is, how to calculate it, and explore the relationship between work and energy transfer. With examples from everyday life, you'll gain a deeper understanding of the forces that shape our world. Whether you're a student of physics or simply curious about the science behind our daily activities, this article is for you!
Work Done Defined
We can define work done as:
"Work is done when a force is exerted on an object in the direction of its displacement, and the object is moved over a distance"
The object is said to have received work from the force. However, there is no work done, if a force is applied to an object but there is no resulting movement.
How to Calculate Work Done?
The work done on an object is directly proportional to the magnitude of force and the distance an object covers in the direction of the force.
To compute the work done on an object by a force, the following equation is used:
W = F × s
Where:
- W = work done, measured in joules (J) or newton-meters (N m)
- F = force exerted, measured in newtons (N)
- s = displacement of the object, measured in meters (m)
Example 1
A crate is lifted 2 meters vertically by a force of 400 newtons. Calculate the work done by the force on the crate.
Solution
The work done by the force on the crate can be calculated using the formula:
W = F × s
Where:
W = work done by the force F = force applied on the crate = 400 N
s = distance moved by the crate in the direction of the force = 2 m
Plugging in the values:
W = 400 N × 2 m
W = 800 J
Therefore, the work done by the force on the crate is 800 joules (J).
Example 2
A crane lifts a 500 kg block of concrete from the ground to a height of 10 meters. The force applied by the crane is 3000 N and it takes 30 seconds to lift the block. Calculate the work done by the crane.
Solution
To solve this problem, we need to calculate the work done by the crane in lifting the block. The formula for work is:
W = F × s
Where:
W = work done
F = force applied
s = distance moved in the direction of the force
In this case, the force applied is 3000 N and the distance moved in the direction of the force is 10 meters. Therefore:
W = 3000 N × 10 m
W = 30,000 J
The work done by the crane in lifting the block is 30,000 joules (J).
Relationship Between Work Done and Energy Transfer
Whenever work is done, there is a transfer of energy from one store to another. Working, whether mechanical or electrical, is a pathway for energy transfer. The amount of energy transferred, measured in joules, is equal to the amount of work done in joules. Therefore:
Energy transferred (J) = Work done (J).
If a force is applied in the direction of an object's motion, it gains energy and the energy is transferred to its kinetic store. However, if the force is applied opposite to the object's motion, it loses energy and the energy is transferred to the object and its surroundings' thermal store.
Example 1
An object lifted vertically with a 250 N force for a distance of 20 meters. Calculate the amount of work done and energy transfer.
Solution
The work done on the object can be calculated using the formula:
W = F × s
Where W is the work done, F is the force, and s is the distance.
W = 250 N × 25 m = 5000 N m
The energy is mechanically transferred to the gravitational potential store of the object. During this energy transfer, 5000 J of energy was transferred.
Example 2
A man lifts a crate out of a truck. The crate has a mass of 20 kg and the truck bed is 2 meters high. The gravitational field strength is 9.5 N/kg.
a) Describe the energy transfer involved in lifting the crate out of the truck bed.
b) Calculate the amount of energy transferred to the crate during the lift.
Solution
The energy transfer involved in lifting the crate out of the truck bed is from the man's muscular system to the crate's gravitational potential energy store. As the man applies an upward force to the crate, the energy is transferred from the man's muscles to the crate, and the crate gains gravitational potential energy.
a) To calculate the energy transferred to the crate during the lift, we need to first calculate the work done. The formula for work is:
W = F × s
where W is the work done, F is the force applied, and s is the distance moved in the direction of the force.
The force required to lift the crate is equal to the weight of the crate:
F = m × g
where m is the mass of the crate and g is the gravitational field strength.
F = 20 kg × 9.5 N/kg
F = 190 N
The distance moved in the direction of the force is 2 meters.
Therefore, the work done to lift the crate out of the truck bed is:
W = F × s
W = 190 N × 2 m
W = 380 J
b) Since, the energy transferred to the crate during the lift is equal to the work done, which is 380 joules (J).
Examples of Work Done and Energy Transfer From Everyday Life
Here are some of the examples of work done and energy transfer from everyday life:
1. Riding a bike up a hill
When a cyclist pedals up a hill, they are doing work and transferring energy from their body to the bike. The energy is stored in the bike's kinetic and gravitational potential energy stores.
2. Turning on a light switch
When you turn on a light switch, you are doing work and transferring energy from the electrical energy store to the light bulb. The light bulb then emits light, which is energy transferred to the surroundings.
3. Throwing a ball
When you throw a ball, you are doing work and transferring energy from your body to the ball. The energy is stored in the ball's kinetic energy store.
4. Cooking food on a stove
When you cook food on a stove, you are doing work and transferring energy from the stove to the food. The energy is stored in the food's thermal energy store.
5. Charging a phone
When you charge a phone, you are doing work and transferring energy from the electrical energy store to the battery in the phone. The energy is stored in the battery's chemical energy store.
6. Pushing a shopping cart
When you push a shopping cart, you are doing work and transferring energy from your body to the cart. The energy is stored in the cart's kinetic energy store.
7. Running on a treadmill
When you run on a treadmill, you are doing work and transferring energy from your body to the treadmill. The energy is stored in the treadmill's kinetic energy store.
8. Swinging on a swing
When you swing on a swing, you are doing work and transferring energy from your body to the swing. The energy is stored in the swing's kinetic energy store.
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