Average force in stopping a block

[ride4life]

Homework Statement

A 300kg block is suspended by a cable.
a) what is the weight of the motor block?
b) the cable snaps and the block falls 3 metres to the ground below.
i) calculate the time taken for the block to reach the ground.
ii) calculate the impact velocity of the block with the ground
c) if the block comes to rest having caused a 1cm dent in the block (ie traveled 1cm in coming to rest), calculate the average force exerted by the ground in stopping the block.

Homework Equations

a) weight = mass x gravity
b) i) s = ut + 0.5at^2
ii) v = u + at
c) ?

The Attempt at a Solution

a) weight = 300x9.81 = 2943N down
b) i) 3 = 0.5 x 9.81 x t^2 t = 0.78s
ii) v = 9.81 x 0.78 = 7.65m/s down
c) no idea

 

[rock.freak667]

Work done against the block = change in gravitational potential energy of the block.
(Conservation of energy)

 

[ride4life]

work done = potential energy
w = m.g.h
w = 300x9.81x3
w = 8829 joules

is that right?

 

[rock.freak667]

work done = potential energy
w = m.g.h
w = 300x9.81x3
w = 8829 joules

is that right?

Right, so if you have the work done on the block. How would you find the force if you are given the distance?

 

[ride4life]

work = force x displacement

foce = 8829/3

force = 2943N

 

[rock.freak667]

work = force x displacement

foce = 8829/3

force = 2943N

The block travels 1cm before coming to rest.

 

[ride4life]

2943 x 0.01?

 

[songoku]

2943 x 0.01?

the unit of 2943 x 0.01 is not Newton (the question is about force)

rock.freak667 talked about what the force is if the block traveled 1 cm with some amount of work done.

 

[ride4life]

so it's 2943x0.01 J?

 

[songoku]

calculate the average force exerted by the ground in stopping the block.

What is the unit of force?

 

[ride4life]

What is the unit of force?

Newtons

 

[songoku]

2943 x 0.01?

will the unit of this calculation be Newton?

 

[ride4life]

will the unit of this calculation be Newton?

Newton seconds?

 

[songoku]

lol

yes it's Newton seconds and that's unit of momentum, not force.
Since the question is about force, no doubt the answer is wrong.

you have the distance and work, find F

 

[ride4life]

s = 0.01m
W = 300x9.81x3
F = W/s
F = 882900N

 

[Redbelly98]

Looks good.

I recommend getting in the habit of including units during your calculations, and not just sticking them on at the end. Including units should help you understand better what's going on, and will help you catch errors like thinking that "2943 x 0.01" represents a force.

 

[ride4life]

Looks good.

I recommend getting in the habit of including units during your calculations, and not just sticking them on at the end. Including units should help you understand better what's going on, and will help you catch errors like thinking that "2943 x 0.01" represents a force.

Ok, thanks for the advice!