Question

friction
up until this point, its been convenient to ignore one of the most useful and most troublesome forces in everyday life... a force that has tremendous ap - plication in transportation, machinery, and all parts of mechanics, yet people spend tremendous amounts of effort and money each day fighting it. this force, friction, is a force that opposes motion.
4.24 q: a projectile launched at an angle of 45° above the horizontal trav - els through the air. compared to the projectiles theoretical path with no air friction, the actual trajectory of the projectile with air friction is
(a) lower and shorter
(b) lower and longer
(c) higher and shorter
(d) higher and longer
4.25 q: a box is pushed toward the right across a classroom floor. the force of friction on the box is directed toward the
(a) left
(b) right
(c) ceiling
(d) floor
4.28 q: the diagram below shows a 4.0 - kilogram object accelerating at 10 meters per second² on a rough horizontal surface.
acceleration = 10 m/s²
frictional force = f₁
applied force = 50 n
m = 4.0 kg
(not drawn to scale)
what is the magnitude of the frictional force f, acting on the ob - ject?
(a) 5.0 n
(b) 10 n
(c) 20 n
(d) 40 n

Explanation:

Step1: Apply Newton's second law

Newton's second law is \(F_{net}=ma\). Here, the net force \(F_{net}\) is the applied force \(F_{applied}\) minus the frictional force \(F_f\), so \(F_{net}=F_{applied}-F_f\).

Step2: Substitute values into the formula

We know \(m = 4.0\space kg\), \(a=10\space m/s^{2}\), and \(F_{applied}=50\space N\). Substituting into \(F_{applied}-F_f=ma\), we get \(50 - F_f=4\times10\).

Step3: Solve for \(F_f\)

First, calculate \(4\times10 = 40\). Then, from \(50 - F_f=40\), we can solve for \(F_f\) by rearranging the equation: \(F_f=50 - 40\).

Answer:

B. 10 N