Force and energy problems

In summary: So it should be 1.5*Sin60=1.3m. So the velocity should be about 4.6m/s. Does that make sense to you?In summary, the first problem involves a 6kg block sliding down a frictionless incline and finding its speed after sliding 1.5m from rest and with an initial speed of 2m/s. The second problem involves a single force of 5N acting on an 8kg object and finding its velocity as a function of time, the expression for power input as a function of time, and the power input at a specific time. The solutions involve using conservation of energy and F=ma to find the velocity and power input.
  • #1
nns91
301
1

Homework Statement



1.A 6kg block slides down a frictionless incline making an angle of 60 degree with the horizontal.(a) What is the speed of the block after it has slid 1.5 if it starts from rest. (b) What is its speed after 1.5m if it starts with an initial speed of 2 m/s ?

2. A single force of 5 N acts in the x direction on an 8kg object. (a) If the object starts from rest at x=0 at time t=0, find its velocity v as a function of time.(b) write an expression for the power input as a function of time.(c) What is the power input of the force at time t= 3s



Homework Equations



W=KE
F=ma

The Attempt at a Solution



1. I use W=Kinetic energy to find v. I found down that Wtotal= Wgravity + Wnormalforce=102N.

Then I set 102=1/2 mv^2. But I got marked wrong. Any opinion ??

2. I use F=ma to find the acceleration then use acceleration as the slope of V(t) but I am wrong again. How should I do this ?
 
Physics news on Phys.org
  • #2
nns91 said:

Homework Statement



1.A 6kg block slides down a frictionless incline making an angle of 60 degree with the horizontal.(a) What is the speed of the block after it has slid 1.5 if it starts from rest. (b) What is its speed after 1.5m if it starts with an initial speed of 2 m/s ?

2. A single force of 5 N acts in the x direction on an 8kg object. (a) If the object starts from rest at x=0 at time t=0, find its velocity v as a function of time.(b) write an expression for the power input as a function of time.(c) What is the power input of the force at time t= 3s

Homework Equations



W=KE
F=ma

The Attempt at a Solution



1. I use W=Kinetic energy to find v. I found down that Wtotal= Wgravity + Wnormalforce=102N.

Then I set 102=1/2 mv^2. But I got marked wrong. Any opinion ??

2. I use F=ma to find the acceleration then use acceleration as the slope of V(t) but I am wrong again. How should I do this ?

For 1.
If by "W" you mean kinetic energy, then Wgravity and Wnormalforce are meaningless.
And the energy is not measured in N but in joules.
I suppose you mean to write conservation of energy or work-energy theorem.
Either one will work. The normal force does not do any work (it's normal, right? this means perpendicular)
So, for example the work-energy theorem:
KE_final = KE_initial +Work
Work=mgh (work done by gravity)

For 2.
It seems OK so far. Show your work so we can see where is the mistake.
I assume that by using the acceleration as the slope of V(t) you mean V(t)=a*t.
 
  • #3
2. So here is my work:

F=ma so a=F/m=5/8=0.625 m/s^2

v(t)=0.625t

Is that right ?
 
  • #4
1. That's what I meant to say. So did I do it right or wrong ?

I got W=KE so 102=(mv^2)/2 and solve for v
 
  • #5
nns91 said:
2. So here is my work:

F=ma so a=F/m=5/8=0.625 m/s^2

v(t)=0.625t

Is that right ?

Yes, so far it seems OK to me.
 
  • #6
nns91 said:
1. That's what I meant to say. So did I do it right or wrong ?

I got W=KE so 102=(mv^2)/2 and solve for v

What is 102? From where do you get it ?
 
  • #7
It is the gravitational force's work
 
  • #8
You mean m*g*h?
m=5kg, g=10m/s (aprrox); That will give h=about 2 m??
It slides 1.5 m along the plane. What is the vertical distance corresponding to this?
Use the angle of the incline. Cannot be MORE than 1.5 m.
 
  • #9
no.

I got W=m*g*sin60*2.

2 is the distance it slides, not the vertical
 
  • #10
nns91 said:
no.

I got W=m*g*sin60*2.

2 is the distance it slides, not the vertical

In the problem it says 1.5 m. So why 2?

When you multiply by Sin(60) you get the vertical distance. But it should be 1.5*Sin60.
I think you mixed the distance with the speed (2m/s).
 

1. What is the difference between force and energy?

Force is a measure of how much an object is being pushed or pulled, while energy is the ability to do work. Force is a vector quantity, meaning it has both magnitude and direction, while energy is a scalar quantity, meaning it only has magnitude.

2. How do I calculate the force of an object?

To calculate the force of an object, you need to know its mass and acceleration. The formula for force is F = ma, where F is force, m is mass, and a is acceleration. Make sure to use the correct units (such as kilograms for mass and meters per second squared for acceleration) in your calculation.

3. What is the relationship between force and motion?

According to Newton's first law of motion, an object will remain at rest or in motion at a constant velocity unless acted upon by an unbalanced force. This means that force is necessary to change an object's motion, whether it be to start, stop, or change its direction.

4. Can energy be created or destroyed?

No, according to the law of conservation of energy, energy cannot be created or destroyed, only transformed from one form to another. This means that the total amount of energy in a closed system remains constant.

5. How does potential energy differ from kinetic energy?

Potential energy is the energy an object possesses due to its position or configuration, while kinetic energy is the energy an object possesses due to its motion. Potential energy can be converted into kinetic energy and vice versa, but they are two distinct forms of energy.

Similar threads

  • Introductory Physics Homework Help
Replies
29
Views
785
  • Introductory Physics Homework Help
Replies
3
Views
394
  • Introductory Physics Homework Help
Replies
23
Views
1K
  • Introductory Physics Homework Help
Replies
9
Views
1K
  • Introductory Physics Homework Help
Replies
15
Views
276
  • Introductory Physics Homework Help
Replies
25
Views
955
  • Introductory Physics Homework Help
Replies
17
Views
713
  • Introductory Physics Homework Help
2
Replies
56
Views
1K
  • Introductory Physics Homework Help
Replies
24
Views
839
  • Introductory Physics Homework Help
Replies
2
Views
602
Back
Top