# What is the magnitude of the average stopping force?

• vshah
In summary, the conversation was about two physics problems - one involving a man dropping onto a concrete patio and the other involving a landing craft approaching the surface of a moon. The expert provided equations and hints to solve both problems, emphasizing the need to understand the concepts and use them to solve the equations. The conversation ended with the person thanking the expert for their help.
vshah

1. A 77 kg man drops to a concrete patio from a window only 0.45 m above the patio. He neglects to bend his knees on landing, taking 2.0 cm to stop.
(a) What is his average acceleration from when his feet first touch the patio to when he stops?

(b) What is the magnitude of the average stopping force?

2. Imagine a landing craft approaching the surface of the moon of a distant planet. If the engine provides an upward force (thrust) of 2910 N, the craft descends at constant speed; if the engine provides only 2200 N, the craft accelerates downward at 0.47 m/s2.
(a) What is the weight of the landing craft in the vicinity of moon's surface?

(b) What is the mass of the craft?

(c) What is the magnitude of the free-fall acceleration near the surface of the moon?

A couple of hints:

for the first problem : use the eqn: 2ax=Vf^2-Vi^2

Note that vf = 0
Just need to compute Vi, the speed at which he strikes the ground. X above is the displacement during deceleration, 0.02m

Second problem: Need to understand that in the first case, he is coasting, and the rocket is providing just enough thrust to offset gravity of moon.

F1-m*g=0 where g is the moon's gravity constant and F1 is the force of the rocket engine

Second case there is acceleration

F2-mg=m*a where a is the acceleration given. Pay attention to signs.

Need to solve these simultaneously--subtracting the top from the bottom eqn seems easy.

Last edited:
thanks but i cannot get the first problem or part b for the second one can you please explain?

I'll try but you may need to spend some time looking at your book/notes as well.

For the first: The equation to compute a, the acceleration, is given above.

It simplifies to a(2*d)=v^2 where d is the displacement during which he stops= 2.0cm

The same eqn can be used to compute how fast he is going when he strikes the ground:

here 9.8(2*0.45m) = v^2

since V^2 is common to both equations: set the other quantities as equal and solve for a.

Problem two: If you solve the 2 simultaneous equations, you need to figure out what g is. Note both m and g are unknown, but you have 2 eqns so both can be figured out. As I hinted subtracting the first eqn from the second will get you F2-F1 = m*a and a is given to you. Now go back using that info using either equation to compute g.

i got it

thank you

great, and you are welcome.

## 1. What is the definition of stopping force?

The stopping force is the force applied to an object to bring it to a complete stop.

## 2. How is the average stopping force calculated?

The average stopping force is calculated by dividing the change in velocity of an object by the time it takes to come to a complete stop.

## 3. What are the units of measurement for stopping force?

The units of measurement for stopping force are typically Newtons (N) in the metric system or pound-force (lbf) in the imperial system.

## 4. How does the magnitude of the average stopping force affect an object's motion?

The magnitude of the average stopping force is directly related to an object's motion. A larger stopping force will result in a quicker decrease in velocity and a shorter stopping distance, while a smaller stopping force will result in a longer stopping distance and slower decrease in velocity.

## 5. What factors can impact the magnitude of the average stopping force?

The magnitude of the average stopping force can be impacted by various factors, such as the mass and velocity of the object, the surface it is traveling on, and any external forces acting on the object.

• Introductory Physics Homework Help
Replies
2
Views
2K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
1
Views
2K
• Introductory Physics Homework Help
Replies
5
Views
5K
• Introductory Physics Homework Help
Replies
4
Views
2K
• Introductory Physics Homework Help
Replies
7
Views
7K
• Introductory Physics Homework Help
Replies
14
Views
8K
• Introductory Physics Homework Help
Replies
4
Views
1K
• Introductory Physics Homework Help
Replies
5
Views
3K
• Introductory Physics Homework Help
Replies
1
Views
1K