# Two questions, work-kinetic energy related

• BifSlamkovich
In summary, the conversation discusses two physics problems involving a block sliding down an incline. The first problem involves finding the rate of friction force when the block's speed is 3.0 m/s, given the coefficient of kinetic friction and the angle of the incline. The second problem involves calculating the kinetic energy of the block at point B, given the kinetic energy at point A and a force acting on the block between the two points. The student is having trouble solving both problems and requests assistance.
BifSlamkovich

## Homework Statement

1)A 5.0-kg block slides down a surface inclined 30 degrees with the horizontal. If the coefficient of kinetic friction for the block and the surface is equal to 0.25, at what rate is the friction force on the block doing work at an instant when the speed of the block is 3.0 m/s?

2) Consideran incline with two locations on the slope, A and B. point A on the incline does not necessarily occur at the top of the incline but does occur before B.
A 2.0-kg block slides down a frictionless incline from p.A to p.B. A force (magnitude P = 3.0 N) acts on the block between A and B, as shown. Points A and B are 2.0 m apart. If the kinetic energy of the block at A is 10 J, what is the kinetic energy of the block at B?

## The Attempt at a Solution

i keep getting incorrect numbers (the questions are multiple choice).

For the first one I used P=Fk and W=F*Δx but i dont know what i'm doing wrong. For the second one, i used KEf=KEi+WAB but again, wrong answer. Please help!

1) The rate of work done by friction can be calculated using the formula W = Fd, where W is work, F is force, and d is distance. In this case, the force of friction is equal to the coefficient of kinetic friction (µ) multiplied by the normal force (N) exerted on the block. The normal force can be found using the formula N = mgcosθ, where m is the mass of the block, g is the acceleration due to gravity, and θ is the angle of inclination.

Therefore, the rate of work done by friction can be calculated as follows:

W = µNcosθ * v

Where v is the velocity of the block. Substituting the given values, we get:

W = (0.25)(5.0 kg)(9.8 m/s^2)cos30 * 3.0 m/s

W = 10.3 J/s

Therefore, the rate of work done by friction on the block at that instant is 10.3 J/s.

2) The kinetic energy of the block at point A is given as 10 J. At point B, the block is still moving and has kinetic energy. This energy is equal to the work done on the block by the force P between points A and B.

Using the work-energy theorem, we can calculate the work done by the force P as:

W = Fd = Pd

Where d is the distance between points A and B, which is given as 2.0 m.

Therefore, the work done by the force P is 3.0 N * 2.0 m = 6.0 J.

Since the kinetic energy at B is equal to the work done on the block, the kinetic energy at B is also 6.0 J.

## 1. What is work in the context of physics?

In physics, work is defined as the amount of energy transferred to or from an object by a force acting on it. It is calculated by multiplying the magnitude of the force by the distance the object moves in the direction of the force.

## 2. How is work related to kinetic energy?

Work and kinetic energy are closely related in the sense that work is the means by which energy is transferred to an object, thereby changing its kinetic energy. The work-energy theorem states that the net work done on an object is equal to the change in its kinetic energy.

## 3. Can work be negative in the context of kinetic energy?

Yes, work can be negative in the context of kinetic energy if the force acting on the object is in the opposite direction of its motion. This means that the force is doing work against the object, reducing its kinetic energy.

## 4. How do you calculate work and kinetic energy?

To calculate work, you multiply the magnitude of the force by the distance the object moves in the direction of the force. To calculate kinetic energy, you use the formula KE = 1/2 * m * v^2, where m is the mass of the object and v is its velocity.

## 5. What are some real-life examples of work and kinetic energy?

Some common examples of work and kinetic energy include throwing a ball, riding a bike, and lifting a weight. In each of these cases, work is done on the object, transferring energy and changing its kinetic energy. Other examples include a car accelerating, a roller coaster moving downhill, and a person running.

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