# Average force in stopping a block

1. Jul 17, 2009

### ride4life

1. The problem statement, all variables and given/known data
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 travelled 1cm in coming to rest), calculate the average force exerted by the ground in stopping the block.

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

3. 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

2. Jul 17, 2009

### rock.freak667

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

3. Jul 17, 2009

### ride4life

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

is that right?

4. Jul 17, 2009

### rock.freak667

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

5. Jul 17, 2009

### ride4life

work = force x displacement

foce = 8829/3

force = 2943N

6. Jul 17, 2009

### rock.freak667

The block travels 1cm before coming to rest.

7. Jul 17, 2009

### ride4life

2943 x 0.01?

8. Jul 17, 2009

### songoku

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 travelled 1 cm with some amount of work done.

9. Jul 18, 2009

### ride4life

so it's 2943x0.01 J?

10. Jul 18, 2009

### songoku

What is the unit of force?

11. Jul 18, 2009

### ride4life

Newtons

12. Jul 18, 2009

### songoku

will the unit of this calculation be Newton?

13. Jul 18, 2009

### ride4life

Newton seconds?

14. Jul 18, 2009

### 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

15. Jul 18, 2009

### ride4life

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

16. Jul 18, 2009

### Redbelly98

Staff Emeritus
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.

17. Jul 18, 2009