Calculating the force (Homework question)

[Tony09_]

Hello users,

Im a boy whos in high school and has physics as a school subject. I'm not the best in physics at all and next week I'm going to have a 3-chapter test about physics. The teacher gave us some practice material and I struggle with this question, anyone that could help me?

1. Homework Statement

A car (920kg) with a driver (80kg) drives with a constant speed of 36km/h. The car crashes into a tree. The car hood is dented with a length of 50cm. Consider the crash uniformily retarded.

a) What is the force that the tree practises on the car?

(I already know the answer, which is 1.0 * 10^5N

2. Homework Equations

F(res)= m(kg) * a(m/s2)??

3. The Attempt at a Solution
Honestly, I don't have any idea where to start (I'm not the best in physics)

 

[lewando]

So you are trying to find F, you know m. You need to figure out what a is. Do you know of any relevant equations involving a?

 

[Tony09_]

So you are trying to find F, you know m. You need to figure out what a is. Do you know of any relevant equations involving a?

Yes a = delta V / delta T but since there is no time mentioned it has no use i think. The formula above is what i thought what was needed to solve it but it seems that it isn't the one I'm looking for

 

[lewando]

Do you have any relevant kinematic equations that do not involve t?

 

[Tony09_]

Do you have any relevant kinematic equations that do not involve t?

No, sadly not. The only other equations mentioned in the chapter (where the question is based on) are :
F ( force ) gravity = mass (kg) * gravity (9.81 based on the gravity of the netherlands in m/s2)
P(rho) = mass / volume (m3)
W (work) = F * s (distance made in direction of force in meters)

I study in the netherlands so all the equations are originally stated in dutch so there may be bad translation. But there are a few more but those are difficult to translate in english

 

[haruspex]

W (work) = F * s (distance made in direction of force in meters)

Try that one. You will need another equation relating motion to energy.

By the way, you can delete "in meters".

 

[lewando]

If the phrase "consider the crash uniformily retarded" implies that the velocity during the impact period is changing uniformly with time (which I think it does), then, for this special case, there is a useful formua for average velocity: v_x,average = (v_x,initial + v_x,final)/2. The equation: distance = rate * time can used with v_x,average as the rate to find t.

 

[haruspex]

If the phrase "consider the crash uniformily retarded" implies that the velocity during the impact period is changing uniformly with time (which I think it does), then, for this special case, there is a useful formua for average velocity: v_x,average = (v_x,initial + v_x,final)/2. The equation: distance = rate * time can used with v_x,average as the rate to find t.

As you indicated in post #4, it is not necessary to find t.

@Tony09_, the five SUVAT equations each involve four of a certain five variables. So one of them only uses s, u, v and a. This one is effectively the work conservation equation (see post #6), but with mass canceled out.

I was never one for learning formulas, much better at remembering the underlying principles and how to derive formulas. But I always found it worth knowing the five SUVAT equations. See https://en.wikipedia.org/wiki/Equat...translational_acceleration_in_a_straight_line.