PHYSICS

[cerafina.info]

HI Every one,

I was doing some word problems from the Physics course of MECHANICS and came up with some questions I couldn't do... It would really helpful if someone can answer them!


1) A lift of mass 1200 kg is being pulled vertically upwards at a steady speed of
a) 1.0 m/s
b) 2.0 m/s
What is the tension in the cable in each case.



2) In an experiment an air rifle pellet is fired into a block of modelling clay that rests on a table.

The air rifle pellet remians inside the clay block after the impact. As a result of the collison, the clay block slides along the table in a straight line and comes to rest. Further data relating to the experiment are given below:

Mass of air - rifle pellet = 2.0g
Mass of clay block = 56 g
Velocity of impact of air - rifle pellet = 140 m/s
Stopping distance of clay block = 2.8 m
Initial speed of the clay block after the air - rifle pellet strikes it = 4.8 m/s.



a) Calculate the average frictional force that the surface of the table exerts on the clay block whilt the clay block is moving.

b) Discuss the energy transformations that occur in the clay block and the air rifle pellet from the moment the air - rifle pellet strikes the block until the clay block comes to rest.

c) The clay block is dropped from rest from the edge of the table and falls vertically to the ground. The table is 0.85m above the ground. Calculate the speed with which the clay block strikes the ground.



You don't have to answer all of them together but if you do it would be great ! Thanks a lot!

 

[dark+matters]

Hello! I only had time for one.

1) Did you draw a freebody diagram? What principles are involved in a problem like this?

Spoiler

Newton's first law of motion: An object moves in a straight line and at a constant speed except to the extent that it interacts with other objects.

We need to define what the system is in this problem and what the surroundings are.

Spoiler

System: elevator. Surroundings: Earth, cable.

The forces of the Earth and the cable have to cancel out since no acceleration is involved if we are following the elevator as the system. What kind of forces does Earth exert on a mass near its surface? Gravity: 9.8 N/kg. What is force? Mass times acceleration.

What information are we given to solve for force? Well, velocity's units don't match up with acceleration's and we're not going to be dividing by time at any point, so that's a good clue that something isn't going to fit if we throw velocity in there.

Spoiler

So just use F=mg for both. Force depends on acceleration, not velocity.

 

[dark+matters]

Hello! I'm back. And I need to procrastinate. The anxiety I feel about wanting my artistic commissions to be perfect dims in light of posting on this forum. I don't know if I'm right about any of this, by the way. Other people need to correct me if I'm wrong.

So, with regard to the motion of large objects like these, what fundamental principles are in play in this situation?

Spoiler

The energy principle: energy cannot be created or destroyed, but it can change form.

Is energy conserved or not in this collision between two objects? This relates to part b.

Spoiler

Energy is conserved if there is no change in the internal energy of the system. But there has been a change in the internal energy of these objects. <--(hint, hint about things you could say for part b.) They've been deformed. There's friction and therefore heat loss.

The formula for the conservation of energy is:

change in the energy of the system= Work + other energy transfers (like heat or deformity)

What kinds of energy would we have learned about at a mechanics level of instruction?

Spoiler

Kinetic and potential. Together, these make up mechanical energy. Is mechanical energy conserved? Sure. There's no potential energy involved in parts a and b, because potential energy comes from the transfer of energy from a higher place to a lower one (but there is energy due to the object's position in part c)!

So what else is conserved in this situation?

Spoiler

Just like in problem 1), momentum is conserved. Momentum is conserved in all collisions, even if energy is not conserved in any inelastic collision. If you want to Google this kind of collision, it is called a "maximally inelastic collision."

Let's take a look at work done upon the system of the clay/bullet object. What is work?

Spoiler

Work is Force times distance, measured in Joules, or kilograms times meters squared, divided by seconds squared. Do we have a force acting upon the clay/bullet system? Yes. We have the force of friction. What is the formula for calculating friction force?

Spoiler

Friction force is the coefficient of friction times the normal force, but we don't know the coefficient of friction. We do have variables for mass times acceleration though, another way to describe any force. Free body diagram the bullet/clay. There is force acting upon it in two directions: moving left in the x direction and moving down in the y direction. However, I don't think we'll need any of this information.

So let's just combine what we know about this situation into an equation using the parts we have and see if the units will cancel out and combine into something that sounds reasonable for part a.

Spoiler

What's the formula for kinetic energy? (1/2)mv^2.

So...

initial KE_bullet + zero KE of the block, initially = final KE of the mass of the bullet+the mass of the block + distance*friction force. Solve for friction force (and don't forget to convert your grams to kilograms).

l

And for c... what are the four kinematic equations for a free-falling body near the surface of the Earth? Hmmm? Do we have the variables necessary to use those equations? (hint: yes. Yes, we do.)

Hope this helps. Are you in high school or college? I wish I had help when I took this class. I go to physicsforums.com for help sometimes too, although they have very strict rules about how to help people with their homework.