La Vida

Study Guide
Topics to study: Vector characteristics. Vector examples: displacement, velocity, etc. The three Newton’s Laws.

Physics II

Inertia. (remember that we said the inertia is relatedwith the mass of the object)

Problems: A box is pulled over a surface has shown in the figure 1. The force exerted over the box is 350 N and is applied with an angle of 29º with the horizontal. Ifthe box has a weight of 200N, what acceleration will occur over the box.

Solution: 1.- The force exerted over the box has an specific direction. In this case, it means that the acceleration it is notdirectly related with that force, because the acceleration we are looking for is horizontal. Has we are only concerned about the horizontal components; we have to use equation 9, which will obtain,only the horizontal component of that Force.

Fx  F cos

Fx = (350 N)*cos(29º)= 306.11 N

Has the second Newton‟s Law states: an object with a certain mass „m‟, that receives a force F willaccelerate with a magnitude of „a‟. So we need to use equation 5; nevertheless, we actually don‟t know the value of that mass.

However, we know the value of the Weight, 200N. And knowing that, andusing equation 10, where g is the value of gravity, we can obtain the value of that mass: W=m*g m=W /g m = 200N / 9.81 m/s2 = 20.39 kg

We can now use equation 5 where: F=m*a a = F / m a = 306.11 N /20.39 kg = 15.01 m/s2 , which is the acceleration felt in the box horizontally.

Problem 2.- An automobile goes over Gonzalitos with a velocity of 130 km/h in a “straight” road and then applies thebreaks and it stops in a distance of 350 meters. If the automobile weights 9810 N, a)What is the magnitude of the force for the automobile to stop? b)How many time it takes to the car to stop?Solution: Again, the second Newton‟s Law relates the force (in this case applied by the breaks) felt by the car to stop, the mass of the car and the acceleration needed to stop. Neither the mass nor the...