# This is a question and answer site

Hundreds of solved questions for you to study. The maximum number of questions and answers we have are from Physics.

There are some questions and answers from Chemistry, Mathematics and Statistics as well.

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Study and find out what is physics. Clearly see how to use physics equations and physics formulas.

Some of the topics are: work power and energy, forces and laws of motion, properties of matter

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### Work, Power and Energy

#### Work

Let us look at some questions and their answers:

Question:If you push vigorously against a brick wall, how much work do you do on the wall? Explain.

The formula we use here is W = Fx where W is the work done by a force F which moves through a distance x is the formula which defines work in physics

The answer to this question is decided by the fact that the wall is still. It does not move. Click here to see the solution to this question

Question: A man is using a human powered mower to cut grass. The mower is pushed with a force of 45N directed at an angle of 41 degree below the horizontal direction. What is the work done in pushing the mower 9.1 m across the yard?

The formula we use her is again W = Fx. However there is diffence. The force is acting at an angle, so only a component of it will do actual work on the mower.

Question: Find the work that must be done on a 6000 kg car to decrease its speed from 45 m/s to 25 m/s

The formula to be used here is KE = ½mv2. Where KE is the kinectic energy and m the mass and v the velocity

The question gives us the mass of the car and the initial and the final velocities.

Using the KE formula we can determine the change in KE. This in turn will give us the work done on the car to reduce its speed. Click here to see the answer

#### Power

Question: A 60 kg women runs up a staircase 15 m high ( vertically ) in 20 sec. a ) How much power does she expend. b) What is her horsepower rating.

Power is the work done over time. So the formula is P = Work Done/time. In this question we will get the work done by finding the change in the potential energy PE. The formula for PE is PE = mgh where m is the mass, g the acceleration due to gravity and h is the height. click here to see the answer

Question: A 1000-kg car accelerates uniformly from rest to a speed of 30 m/s in the first 10 s (frictionless road) . Find a) the work done on the car in this time . b) the average power delivered by the engine in the first 10 s . c) the instantaneous power delivered by the engine at t = 2 s .

Work done is found by using the product of force and distance. Average power is determined by finding the work done over time. For instantaneous power we will first have to find out the instantaneous velocity at that time. Click here to see the answer

#### Energy

Question: A 3.0 kg stone is dropped from a height of 100 m. Find its kinetic and potential energies when it is 50 m from the ground.

The formula used here is PE = mgh where m is the mass, g the acceleration due to gravity and h is the height. We use it to determine the potential energy at the two heights. Click here to see the answer

Question: A car coasts down a long hill and then up a smaller one onto a level surface, where it has a speed of 32 ft/sec. If the car started 200 ft above the lowest point in the track. How far above this lowest point is the level surface?

Assume that the car starts from rest and use the formula for conservation of energy. Click here to see the answer

### Force and laws of motion

Question: A 300 N crate is pushed across the room at 2 m/s. When the pushing stops, the crate comes to rest in 3 seconds. How large is the force of friction on the crate?

Find the accelertion by useing the formula v = u + at and then find the force of friction by using the formula F = ma. Click here to see the answer

Question: A runner with a mass of 107 kg going 7 meters per second collides with a runner of 125 kg going in the opposite direction how fast must the 125 kg runner be going to stop the 107 kg runner in his tracks?