Homework in physics (average force and momentum)

[yek_zzz]

Homework Statement

A 1000 kg car strikes a tree at 30 km/hr & comes to a stop in 0.15s. Find its initial momentum and the average force on the car while it is being stopped.


Attempt at a solution

For velocity,

I tried converting 30 km/hr to m/s

30 km/hr * 1hr/3600s*1000m/1km

=30000 m/ 3600 s = 8.33 m/s

For the momentum...

p = mv

p = 1000 kg (8.33 m/s)

p = 8330 kg * m/s

Is this correct? How do I find the average force? Please help me. My teacher says I'm wrong at some equation.

 

[Doc Al]

Everything you've done so far is correct. To find the average force, consider the impulse delivered by the tree.

 

[yek_zzz]

how do I solve for that? We are only given the mass, velocity and the time.

 

[Doc Al]

You have all you need. What's the definition of impulse? Look up the impulse-momentum theorem. You'll set up an equation and solve for the force.

 

[rwisz]

Your calculation for the initial momentum is correct. To find the average force on the car, we can use the equation F = Δp/Δt, where F is the average force, Δp is the change in momentum, and Δt is the time interval. In this case, the change in momentum is equal to the initial momentum, since the car comes to a complete stop. So, we can rewrite the equation as F = p/Δt. Plugging in the values, we get F = 8330 kg * m/s / 0.15 s = 55,533 N. This is the average force exerted on the car while it is being stopped by the tree. I hope this helps clarify the concept. Keep up the good work in physics!