Question

17 How large an average force is required to stop a 1400-kg car in 5.0 s if the car’s initial speed is 25 m/s?
2000 N
3500 N
9000 N
7 000N

18 Which of these is NOT a statement of Newton’s law of universal gravitation?
gravitational force between two particles is attractive as well as repulsive
gravitational force acts along the line joining the two particles
gravitational force is directly proportional to the product of the masses of the particles
gravitational force is inversely proportional to the square of the distance of the particles apart

Answer 1

Answer on question #58100, Physics / Mechanics | Relativity

Question 17 How large an average force is required to stop a 1400-kg car in 5.0 s if the cars initial speed is 25 m/s?

2000 N

3500 N

9000 N

7000 N

18 Which of these is NOT a statement of Newtons law of universal gravitation? gravitational force between two particles is attractive as well as repulsive

gravitational force acts along the line joining the two particles

gravitational force is directly proportional to the product of the masses of the particles

gravitational force is inversely proportional to the square of the distance of the particles apart

Solution 17.Energy of car is

$E=mv^{2}/2=1400\cdot 25^{2}/2=437500\,J$

Length of stopping is

$L=vt-at^{2}/2$

where t=5 s and a=25/5 =5 m/s^{2}.

$L=5\cdot 25-5\cdot 25^{2}/2=62.5\,m$

Then force is

$F=E/L=437500/62.5=7000\,N$

18. This is not a statement of Newtons law of universal gravitation:

gravitational force between two particles is attractive as well as repulsive