Newton's Laws - Mission NL9 Detailed Help

A 2400-kg car is moving at 20 m/s when it slams on the brakes and skids to a stop with a leftward acceleration of 7 m/s/s. Fill in all blanks in the diagram below and determine the force of friction. (Use the approximation that g ~ 10 N/kg.) ... 
(Note: Numbers are randomized numbers and likely different from the numbers listed here.)

The big idea in this problem is to use the acceleration of the object to determine the net force; and then to use the net force to determine the value of an individual force - Ffrict. The following method will assist your solution to the problem.
  • The mass and acceleration of the object are explicitly stated. The net force can be determined using Newton's second law equation: Fnet= m • a.
  • None of the individual forces are explicitly stated. But the force of gravity can be determined from the object's mass (see Formula Fix section; use g = 10 N/kg). Since there is no vertical acceleration, the two vertical forces must balance; thus, the normal force is equal to the force of gravity. Now two of the three individual force values have been determined; all that is left to be determined is the Ffrict value.
  • The net force is the vector sum of all the forces. It has a value (which is the m•a product) and a direction (which is the same direction as the acceleration). The net force tells who wins the tug-of-war between individual forces (that's the direction) and the winning margin in the war (that's the value). Since there is only one horizontal force in this tug-of-war, it is the outright winner. Under this condition of being the only horizontal force, the friction force is the net force.

The mass of an object is mathematically related to its weight by the equation: 
Weight = Fgrav = mass • g

where g is the gravitational field strength. The value of g on Earth is 9.8 N/kg (approximately 10 N/kg).


The relationship between net force (Fnet), mass (m) and acceleration (a) is expressed by the equation:
a = Fnet / m