Impact Force Help: 70lb Girl Rollerblading & Knee Impact

In summary, the conversation discusses a physics problem involving a girl roller blading and tripping, and the impact force on her knee after hitting the ground. The participants suggest using concepts like conservation of energy, potential and kinetic energy, impact velocity, and work to solve the problem. The question of whether the girl slides or sinks into the ground is also addressed.
  • #1
lettam
1
0
I need help on how to approach this problem and which equations I can use.
It was a problem that we were supposed to think about, but even that's difficult. Here it is:

If there is a 70 lb. girl roller blading at a constant speed of 5 mph and she suddenly hits a rock and trips downward, what would be the impact force on her knee if it is 1 ft above the ground and the distance traveled after impact is 0.25 in?

We had a similar problem without the girl traveling at an initial speed which was simple enough using conservation of energy, potential and kinetic energy, impact velocity, and work. But I'm not sure how the rotational momentum from her fall downward comes into play. Can anyone point me in the right direction? PLEASE!
 
Physics news on Phys.org
  • #2
What do you mean by the distance traveled after impact ?
Any data on the center of mass ?
 
  • #3
Welcome to PF!

An interesting problem. I would simplify it by assuming that the tripping changes the girl's velocity from horizontal to vertical. So she essentially falls from a height of 1 foot with an initial speed of 5 mph. So what's her KE at the time of impact?

When she hits the ground, the ground exerts a force on her that brings her to rest over a distance of 0.25 inches. So, how much work does that force do? And what's the average force on her knee?
 
  • #4
Doc Al said:
When she hits the ground, the ground exerts a force on her that brings her to rest over a distance of 0.25 inches. So, how much work does that force do?
Is the force the ground exerts on her knee friction? So the change in KE would be the amount of work done by this friction?
 
  • #5
I am assuming that the girl falls straight down and sinks into the ground. So the force of the ground on her knee would be a normal force, not friction. But, yes, the change in KE will equal the work done by this normal force.
 
  • #6
She sinks into the ground!? Ouch. I thought she would slide 0.25 in. across the ground, which is why I asked the question.
 

What is impact force?

Impact force is the force exerted by an object when it collides with another object. In this scenario, it is the force exerted on the girl's knee when she falls while rollerblading.

How is impact force calculated?

The formula for impact force is force = mass x acceleration. In this case, the mass would be the girl's weight (70lbs) and the acceleration would be the speed at which she falls or collides.

What factors affect the impact force?

The impact force can be affected by several factors, including the mass and velocity of the object, the surface area of impact, and the elasticity of the objects involved.

Why is impact force important to consider?

Impact force is important to consider because it can cause damage or injury to both the object and the person or surface it collides with. Understanding the magnitude of the impact force can help in designing safety measures and protective gear.

How can impact force be reduced?

Impact force can be reduced by increasing the collision time, which can be achieved by using materials with greater elasticity or by increasing the surface area of impact. Wearing protective gear such as knee pads can also help in reducing the impact force on the girl's knee.

Similar threads

What is the formula for determining the force of impact in a physics problem?
    • Introductory Physics Homework Help
Replies
9
Views
935
Determining position of an object after inelastic collision
    • Introductory Physics Homework Help
Replies
1
Views
1K
Modeling impact forces in rotary motion
    • Mechanical Engineering
Replies
5
Views
1K
Equivalent Bending Moment due to Impact Formula?
    • Introductory Physics Homework Help
Replies
2
Views
1K
Finding the height of a fall given force on impact
    • Introductory Physics Homework Help
Replies
15
Views
4K
Impact force calculation due to a rotating object in Overspeed Trip Valve Design
    • Mechanical Engineering
Replies
11
Views
3K
Jackhammer on Vehicle Suspension, Unsprung Mass, Tire patch load
    • General Engineering
Replies
13
Views
1K
What Was the Force of Impact on My Mom During a Biking Accident?
    • Introductory Physics Homework Help
Replies
1
Views
5K
A Motorcycle, the force exerted upon from collision
    • Other Physics Topics
Replies
1
Views
2K
Girl on Slide: Solving for Normal Force, Energy Transfer, and Final Speed
    • Introductory Physics Homework Help
Replies
4
Views
2K

Hot Threads

Recent Insights

Back
Top