# Force questions?

I'm not sure what equation is used on this particular problem.

How much time would be requird to stop a 2,000 kg car that is moving at 80.0 km/hr if the braking force is 8,000 N?

I know.....
Mass 2000 kg
Acceleration is 80.0 km/hr
Breaking force is 8,000 N

What tension must a 50.0 cm length of string support in order to whirl an attached 1,000.0 gram stone in a circular path at 5.00 m/s?

I used the formula F= mv^2/ r

= (1,000.0 g) (5.00 m/s)/ 50.0 cm
= 1,000.0 x 5.00 g*m/s
____________________
50.0

I wanted to know if I was tackling this question correctly..if not what am I doing wrong????

THX!

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terpsgirl said:
I'm not sure what equation is used on this particular problem.

How much time would be requird to stop a 2,000 kg car that is moving at 80.0 km/hr if the braking force is 8,000 N?

I know.....
Mass 2000 kg
Acceleration is 80.0 km/hr
Breaking force is 8,000 N
80 km/hr is the velocity, not the acceleration. Just use Newton's second law and some kinematics and you should solve this problem easily.

What tension must a 50.0 cm length of string support in order to whirl an attached 1,000.0 gram stone in a circular path at 5.00 m/s?

I used the formula F= mv^2/ r

= (1,000.0 g) (5.00 m/s)/ 50.0 cm
= 1,000.0 x 5.00 g*m/s
____________________
50.0

I wanted to know if I was tackling this question correctly..if not what am I doing wrong????
You might want to check the units on that again. I suggest you stick to kg, m, and s.

terpsgirl...for the first problem....use the equation that states that impulse = change in momentum.....the equation is:
Force * time = change in velocity * mass.

Think of what the original velocity is, and the final velocity...and u should be able to find the change in the velocity. Basically plug in your results...and solve for time.

for the second problem.....i would check ur units....notice what unit ur expressing r in....and look at the units ur measuring speed in. Remeber that u should measure everything in kg's, meters and seconds.

Last edited:
terpsgirl said:
How much time would be requird to stop a 2,000 kg car that is moving at 80.0 km/hr if the braking force is 8,000 N?
First convert to the correct units.
Car = 2000kg.
Speed = 22.22... m/s.
Braking force = 8000N.

Using F=MA. 8000=2000A, the 8000N force acting against the car will create a deceleration of magnitude 8000/2000, thus, a = -4m/s/s.

U = 22.22....., V = 0, a = -4, t = ?
V = u + at
0 = (200/9) + (-4t)
(200/9) = 4t
t = 5, 5/9 seconds
= 5.56 seconds (2dp)

What tension must a 50.0 cm length of string support in order to whirl an attached 1,000.0 gram stone in a circular path at 5.00 m/s?

I used the formula F= mv^2/ r

= (1,000.0 g) (5.00 m/s)/ 50.0 cm
= 1,000.0 x 5.00 g*m/s
____________________
50.0

I wanted to know if I was tackling this question correctly..if not what am I doing wrong????

THX!
I haven't studied the concept used in question 2, but the equation looks like it would be effective. However, [always] convert to more useful units. You should be working in m/s, kgs, Ns and M.