What is the contact force for a 20kg object with an acceleration of 2.7?

[jamiebean]

Homework Statement

A lift is moving upward with an acceleration a=2.7ms-2. There is a block of mass m=20 kg on the floor of the lift. What is the contact force F between the floor and the block in newton? the acceleration due to gravity is 9.8ms-2. The answer's unit is N.

Refer to the figure below.

Relevant Equations

F=ma

I have watched a couple of videos regarding contact force, but I am still confused in solving this problem.

which is its contact force?
Fc=ma
=20kg x 9.8=196N ??
then what's the acceleration a=2.7 for?

im kind of self-taught right now.. my professor does not explain these simple questions. plus I am new to physics..

 

[PeroK]

then what's the acceleration a=2.7 for?

Exactly! Does the acceleration have an effect on the contact force?

 

[Gaussian97]

jamiebean, the contact force between the lift and the block is the force that the lift is doing to the block.
You know the force that is acting on the block due to gravity, so answer the following questions:
1- What is the acceleration of the block?
2- What must be the RESULTANT force acting on the block?
3- What is then the contact force?

 

[Merlin3189]

which is its contact force?
Fc=ma =20 kg x 9.8 N/kg =196N ??

If the lift were stationary, what would the contact force be?
When the lift accelerates, what difference does it make?

 

[jamiebean]

jamiebean, the contact force between the lift and the block is the force that the lift is doing to the block.
You know the force that is acting on the block due to gravity, so answer the following questions:
1- What is the acceleration of the block?
2- What must be the RESULTANT force acting on the block?
3- What is then the contact force?

1.the acceleration is 2.7ms-2 according to the question?

2. is the resultant force 20kg x 2.7ms-2?

3. 20kg x g(9.8) ??

 

[Gaussian97]

1 and 2 perfect, but 3 not really; try to answer
4. What are the forces acting on this problem? (in vector form please)
5. How is the resultant force defined?

 

[jamiebean]

1 and 2 perfect, but 3 not really; try to answer
4. What are the forces acting on this problem? (in vector form please)
5. How is the resultant force defined?

jamiebean, the contact force between the lift and the block is the force that the lift is doing to the block.
You know the force that is acting on the block due to gravity, so answer the following questions:
1- What is the acceleration of the block?
2- What must be the RESULTANT force acting on the block?
3- What is then the contact force?

1 and 2 perfect, but 3 not really; try to answer
4. What are the forces acting on this problem? (in vector form please)
5. How is the resultant force defined?

3: the contact force is the normal force acting upwards the lift from the ground.

4. how to write in vector form?
how to calculate the normal force? f=20kg x -9.8 ??

5.it is the vector sum of all forces.

 

[jamiebean]

Exactly! Does the acceleration have an effect on the contact force?

should be yes? but how?

 

[jamiebean]

If the lift were stationary, what would the contact force be?
When the lift accelerates, what difference does it make?

if the lift is stationary, the contact force is 20kg x -9.8 ? as the contact force is upwards
when the lift accelerates, how does the contact force change?

 

[PeroK]

should be yes? but how?

How many forces are acting on the block?

 

[jamiebean]

How many forces are acting on the block?

3?
the acceleration upwards, the normal force and the weight of the lift ?

 

[PeroK]

3?
the acceleration upwards, the normal force and the weight of the lift ?

Only two, I'm afraid. Acceleration isn't a force. Acceleration is the result of any net force (or "resultant" force.)

 

[jamiebean]

Only two, I'm afraid. Acceleration isn't a force. Acceleration is the result of any net force (or "resultant" force.)

so the two forces are normal force and weight?
is my solution correct?:

net force= contact force - 20x9.8N
2.7x20kg=contact force - 196N
contact force=250N

 

[PeroK]

so the two forces are normal force and weight?
is my solution correct?:

net force= contact force - 20x9.8N
2.7x20kg=contact force - 196N
contact force=250N

That's correct.

The intuitive way to look at is. If the lift is not accelerating, then the upward force on the block is $mg$, which is needed to support the block's weight. If you want to accelerate the block upwards, you need to add to this force by an amount $ma$ where $a$ is the upward acceleration. The upward force on the block from the lift is then $m(g + a)$.

 

[jamiebean]

That's correct.

The intuitive way to look at is. If the lift is not accelerating, then the upward force on the block is $mg$, which is needed to support the block's weight. If you want to accelerate the block upwards, you need to add to this force by an amount $ma$ where $a$ is the upward acceleration. The upward force on the block from the lift is then $m(g + a)$.

thank you!

 

[jamiebean]

That's correct.

The intuitive way to look at is. If the lift is not accelerating, then the upward force on the block is $mg$, which is needed to support the block's weight. If you want to accelerate the block upwards, you need to add to this force by an amount $ma$ where $a$ is the upward acceleration. The upward force on the block from the lift is then $m(g + a)$.

I'm confused with other physics questions too, could you also help me with that?

 

[PeroK]

I'm confused with other physics questions too, could you also help me with that?

I'll be offline for a bit now.