Please help, Resistive Forces, Initial Acceleration.

In summary, the conversation discusses the calculation of the initial acceleration of a sports car after shifting into neutral and coasting at a velocity of 100 km/h. The mass of the car, the aerodynamic drag coefficient, and the frontal area are given, and the formula R=.5*D*p*A*v^2 is used to calculate the resistive force. After some confusion and incorrect calculations, it is determined that the deceleration of the car is -0.02 m/s^2.
  • #1
yb1013
56
0

Homework Statement



The mass of a sports car is 900 kg. The shape of the car is such that the aerodynamic drag coefficient is 0.250 and the frontal area is 2.00 m2. Neglecting all other sources of friction, calculate the initial acceleration of the car ,if it has been traveling at 100 km/h and is now shifted into neutral and is allowed to coast. (Take the density of air to be 1.295 kg/m2.)

Homework Equations



R=.5 *D*p*A*v^2

The Attempt at a Solution



I calculated it out and got that R is equal to 9, however I am not quite sure what to do with the Resistive Force.
 
Last edited:
Physics news on Phys.org
  • #2
Doesn't F = m*a still?
 
  • #3
I guess it would, so does that mean that i would just do 9=ma?
because i did that but then you get like .01 and that's not correct. Plus i know the answer has to be negative..
 
  • #4
yb1013 said:
I guess it would, so does that mean that i would just do 9=ma?
because i did that but then you get like .01 and that's not correct. Plus i know the answer has to be negative..

Well of course it's decelerating.

Apply minus sign accordingly.
 
  • #5
Btw, I didn't calculate your number. I suppose you converted your units correctly?
 
  • #6
i believe i did,
the drag coeff. is still .25
the frontal area is 2 m^2
100 km/h converts to 27.8 m/s
and i think the density of the air is still 1.295 km/m^2

After i did that plugged them into the equation i got that the ressistance force is 17.99

so 17.99=(900) (a)?

a= .02

Where did i go wrong?
 
  • #7
im sorry, density of air was kg/m^2
 
  • #8
That would be different from your first answer of 9.

And it is (-) because the force is resistive to the direction of motion.
 
  • #9
o I am sorry, i forgot to plug 1/2 in the beginning of the equation, so it really is 9, and then that divided by 900 equals .01.

but -.01 isn't the correct initial velocity..
 
  • #10
yb1013 said:
o I am sorry, i forgot to plug 1/2 in the beginning of the equation, so it really is 9, and then that divided by 900 equals .01.

but -.01 isn't the correct initial velocity..

You mean deceleration don't you?
 
  • #11
yes, that's what i meant, but do you have any idea what I am doing wrong?
 
  • #12
yb1013 said:
R=.5 *D*p*A*v^2

R = .5*(.25)(1.295)*(2)*(27.8)2

and that only comes out to 9?

I get a different answer.
 
  • #13
oo thank you, i seemed to forget to square the velocity for some reason?
but anyways, thank you very much!
 

1. What are resistive forces?

Resistive forces are forces that act in the opposite direction of motion and oppose the movement of an object. They can include friction, air resistance, and drag.

2. How do resistive forces affect initial acceleration?

Resistive forces can decrease the initial acceleration of an object. This is because they act in the opposite direction of the applied force, reducing the net force and therefore the acceleration.

3. What factors affect the strength of resistive forces?

The strength of resistive forces depends on several factors, including the surface area of the object, the speed at which it is moving, and the properties of the medium it is moving through (e.g. air, water).

4. How can we reduce the impact of resistive forces?

There are a few ways to reduce the impact of resistive forces. One way is to decrease the surface area of the object, as this will decrease the amount of friction or air resistance it experiences. Another way is to increase the speed of the object, as this will help it overcome the resistive forces.

5. How do resistive forces affect the motion of an object in the long run?

In the long run, resistive forces can cause an object to come to a stop. This is because they act continuously to oppose the motion of the object, eventually reducing its speed to zero. However, if a constant force is applied to counteract the resistive forces, the object can maintain a constant velocity.

Similar threads

  • Introductory Physics Homework Help
Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
883
  • Introductory Physics Homework Help
Replies
10
Views
2K
  • Classical Physics
3
Replies
95
Views
4K
  • Introductory Physics Homework Help
Replies
18
Views
2K
  • Introductory Physics Homework Help
Replies
6
Views
1K
  • Introductory Physics Homework Help
Replies
7
Views
2K
  • Introductory Physics Homework Help
Replies
3
Views
2K
  • Introductory Physics Homework Help
Replies
15
Views
1K
Back
Top