Grade 11 Physics Review Help!?
I'm in grade 12 but we have a test on Monday on everything we learned in grade 11 Physics, and I had previously thrown out all of my notes... and so, I would love some help!
An object weighing 15N is thrown forward on ice with a speed of 20m/s and after 5.0s it has half the speed. What is the coefficient of friction?
I know that I have to use...
Fnet=ma
f=(mu)N
F= F-f (I don't understand this equation... what does each big F stand for?)
Also, what is the difference between applied force, net force, and normal force?
Update:I did the work out and this is what I get...
m=15N/9.81m/s
m=1.53kg
a=v/t
a=(20m/s)/5s
a=4m/s^2
Fnet=ma
Fnet=(1.53kg)(4m/s^2)
Fnet=6.12N
and now I don't know what to do to find friction!
Comments
I think your last equation should be Fnet = F- f
F is the applied force which is how hard something is pushed or pulled.
f is the friction force which is how much drag there is. As you note f = μN where
N is the normal force (how much force is pushing the on the object perpendicular to the surface it rests on) and
μ is the coefficient of friction. The coefficient of friction is a measure of how rough the touching surfaces are. The harder it is for an object to slide on the surface, the higher the coefficient of friction. μ for a block on a concrete sidewalk would be higher than if the block were on smooth ice.
The net force is the force that is available to accelerate the object. It is what is left of the applied force once you subtract off the loss due to the force of friction.
So now let's get to your problem.
The object lost 10m/s in 5 s as it slid across the ice. Its acceleration, a, is 10/5 = 2 m/s^2
The normal force is the weight (we assume the ice surface is horizontal). There is no applied force so the net force is just the friction force.
The mass of the object is (weight)/g =15/9.8 = 1.53 kg
f = μN = ma
μ = ma/N = 1.53 * 2 / 15 = 0.204
Grade 11 Physics Notes
Applied force implies that a human, other animal, engine, or motor, is causing the application of the force. I typically refer to such a force as a human force.
Normal forces are forces perpendicular to a surface, which prevents the object of interest from sinking through the surface. Think of these as contact forces or forces of constraint. Normal force is just a name to remind you that they are perpendicular to the surface (unlike friction which is along the surface).
Net force means the vector sum of all the forces acting on an object. In the equation Fnet = m*a, Fnet is the net force. The resulting acceleration will be in the same direction as the net force.
"F= F-f" is a poorly written equation. Perhaps, "F-f" was intended to mean F subscript f, which should be written as "F_f" instead. Perhaps the author of that equation intended to write "Fnet=F_f", which means that the net force is equal to the friction force.
When you are finding the net force acting on a freely sliding object (meaning without a human pushing it), only the object's weight, the normal force and the frictional force are acting on it. In the case of a horizontal surface, the frictional force equals the net force.