WORK

Which of the following do you think is performing work? Why?

In physics, work is done only when an object moves under the influence of a force.

How will you know if no work is done?

In conclusion, no work is done when:*Force is applied on an object but the object does not move*The direction of the applied force and the direction in which the object moves are perpendicular to each other

Activity 1.8: By Pair ActivityIdentify the following whether work is done on an object or person.

Then, briefly state your reason.1. A boy running across a playground2. A mother dancing with the baby in her arms3. A basket being lifted4. A person in an ascending elevator5. A stone whirled around a horizontal circle6. A big box dragged across the floor7. A man climbing up a tree8. A girl walking up the stairs9. A boy pushing a wall10. A mango fruit falling from a branch

Work is only done when an object moves under the influence of a force. Therefore, it is defined as the product of the magnitude of displacement multiplied by the component of the force parallel to the displacement.In equation:

W = Fd or W = FdcosθW = Fxd = (N)(m)

= Nm or J (joule)

Sample Problems 1. How much work do you do by pushing a sack of rice with a force of 100 N across a distance of 5 meters?

2. Suppose a woman is pushing a cart with a 500 N force along the 7 meters aisle, how much work is done in pushing the cart from one end of the aisle to the other?3. How much work is done by a student who lifts a 5-kg box to a vertical height of 1.5 m?

4. A sailor pulls a boat a distance of 30 m along a dock using rope that makes a 25⁰ angle with the horizontal. How much work does the sailor do on the boat if he exerts a force of 255 N on the rope?5. If the sailor in problem #4 pulled with the same force, and along the same distance, but an angle of 50⁰, how much work would he do?

Practice (Board work)1. A car of mass 900 kg accelerates at 3m/s2 from rest. How much work is done after it has travelled 100 m?2. A book with a mass of 1 kg is on the floor. If the book is lifted from the floor to the top shelf, which is 2 m from the floor, how much work is done on the book?

Activity 3: Solve the following problems1. How much force must Ritchilda exert on the window shade if she exerts 2.5 J of work to lower the window shade to a distance of 0.5 m?

2. After finishing his lab report, Carlo pulls his 75-kg body out of the study room chair and climbs up the 5.0 m-high flight of stairs to his bedroom. How much work does he do in going to his bedroom?

3. How many joules of work are done against a cart when a force of 50 N pushes it 1 km away?

4. A block is pulled by a constant force of 40 N. If the work done by the force on the block is 680 J, what is the displacement of the block?

5. A block displacement cause of a force in a horizontal surface is 13 m and the work done by the force on the block is 15.6 J. Find the force.

POWERTo explain the relationship between power and workTo solve problems relating to power

Observe the pictures below. Which of the boys do you think has greater power and does more work?

Consider the two scenarios. Two boys have to climb up the stairs, as the lift is out of order.

SCENARIO 1 SCENARIO 2*Boy A and boy B have equal mass

*Boy A reached the 4th storey before boy B

* Boy A has larger mass than boy B

* Boy A and boy B reached the 4th storey at the same time

The two boys are of equal mass and travel at the same distance. Therefore, they do the same amount of work

Since boy A has a larger mass, he has to do more work to carry himself up the four storeys

Cont… (SCENARIO 1) (SCENARIO 2)

Power is defined as the rate of work done or rate of energy conversion.

In equation:P or P

Power is measured in joules per second (J/s) or watts (W).

However, since boy A took a shorter time to reach the 4th floor as compared to boy B, we say that boy A has more power

In other words, boy A was able to do more work than boy B in the same amount of time as boy B

Therefore, boy A has more power

During the Teen Power Competition, Jack and Jill ran up the hill, Jack is twice as massive as Jill. Yet Jill ascended the same distance in half the time Jack did. (A)Who expended more work? (B)Who is more powerful? Explain your answers.

Written Activity 1.9: By Pair

Sample Problems

A. Dan climbs a flight of stairs in 1.5 min. If he weighs 450 N and the stairs is 10 m from the ground, how much power will he develop?B. Eugene, who weighs 450 N, runs up a flight of steps. If there is a total of 10 steps and each step is 0.20 m high, calculate Eugene’s power if he takes five seconds to run up the flight of steps at constant speed.

Try This:1. Andy and Bryan each lift a 150 kg barbell to a height of 1.5 m off the ground. Andy lifts his barbell in 1.0 s and Bryan lifts his 2.0 s. (a) Who does more work and (b) who exerts more power?2. A boy of 450 N of weight climbs a wood steps with a length of 3 m. the boy takes 6 s to get to the end of the steps. How much power is done by the boy in this activity?

A.S # 1.9: Problem Solving

1. A machine is able to lift 200-kg of bricks vertically to a certain height above the ground in 50 s. If the power of the machine is 1200 W, to what height are the bricks lifted?2. An electric motor lifts an elevator 9 m in 15 s by exerting an upward force of 12,000 N. What power does the motor produce?3. A box that weighs 575 N is lifted a distance of 20 m straight up by a cable attached to a motor. The job is done in 10 s. What power is developed by the motor?4. An electric motor develops 65,000 W of power as it lifts a loaded elevator 17.5 m in 35 s. How much force does the motor exert? 5. An object of mass 1 kg is lifted up vertically through 5 m in 10 s. Find the power of the machine.

ENERGY

What is ENERGY?-the capacity to do work. In order to do work, energy is transferred between objects or being converted from one form to another-the unit of energy is in joule (J) In order to do work, energy is transferred between objects or being converted from one form to another. Law of Conservation of Energy, “Energy cannot be created or destroyed, but can be converted from one form to another”.

Kinetic and Potential EnergyKinetic Energy – the energy possessed by bodies or objects in

motion

Potential Energy - associated with forces that depend on the position or configuration of a body and its surroundings

- stored energy

What energy conversions are exhibited in the following situations?

1. Hammering a nail in the wood2. Sunlight in solar panels3. Playing basketball4. Photosynthesis in plants5. Television

Kinetic Energy

Anything that is moving has kinetic energy (KE). It depends on the mass and the speed of the moving body. Observe the equation for the KE: KE2

Potential Energy Gravitational potential energy (PEg) is an energy the object possesses at a height. The change in gravitational potential energy is equal to the work done in lifting the object to the ground. This can be calculated by multiplying the force needed to lift the object by the distance it had to be moved. In equation,

(gravitational PE) PEg = m.ag.h

Sample Problems:A. If a 0.2-kg ball is thrown with a velocity of 6 m/s, what is its KE?Given: mass = 0.2 kg; v = 6 m/s FBD:Find: KE = ?Solution: KE2 = ½ (0.2kg)(6m/s)2

= 3.6 kgm2/s2 or 3.6 J

B. What is the potential energy of a 0.2 kg ball lifted to a height of 5.0 m above the ground?Given: m= 0.2 kg ; h = 5.0 m FBD:Find: PE = ?Solution: PE = mgh = (0.2 kg)(9.8 m/s2)(5.0m) = 9.8 Nm or 9.8 J

Activity 2: Board work

1. Calculate the kinetic energy of a 1,000 kg car travelling at 60 km/hr.

2. A 1.5 kg book on top of a shelf has a PE of 29.4 J. How high is the shelf?

3. An object has a KE of 14 J and a mass of 17 kg, how fast is the moving object?

Activity # 1.10: Seat Work1. A car has a mass of 2,000 kg and is traveling at 28 m/s. What is the car’s KE?2. An object has a kinetic energy of 25 J and a mass of 34 kg, how fast is the moving object?3. A 60-kg person walks from the ground to the roof of a 74.8 m tall building. How much gravitational potential energy does he have at the top of the building?4. A basket of fruits which is on top of a 1.5 m high table has PE of 44.1 J. What is the mass of the basket of fruits?