1. Forces
In the Rough

2. R mg The block sits on the table. What forces are there ?
The normal reaction
The weight
m is the mass of the block mg
g is gravity (9.8 ms-2)

3. R mg If we start to push it what forces are there then?
The normal reaction
Friction
A pushing force
The weight
m is the mass of the block mg
g is gravity (9.8 ms-2)

4. R mg And if we start to pull it what forces are there then?
The normal reaction
Friction
A pulling force
The weight
m is the mass of the block mg
g is gravity (9.8 ms-2)

5. Worth Knowing
F≤R
Where  is a constant, which depends only on the roughness of the surface, and is known as the coefficient of friction.

6. Some worked examples
1. A particle of mass 10kg is placed on a rough horizontal plane. A horizontal force P acts on the particle. P is increased until the particle is on the point of sliding, which occurs when P=10N. Find the coefficient of friction between the particle and the plane Click to reveal answer
1. Draw a diagram
Resolve vertically
R = 98 N
Resolve horizontally
F = 10 N
Motion is just about to occur so
F = R
10 = 98 
 = R F
10N
98N

7. And the next example is
In the following situations a body of mass 10kg is placed on a rough horizontal plane. If  = ½ in each case determine whether motion will occur.
Click to reveal answers R R
50 N R
60 N F
30 N F
70 N F
30º
98N
98N
98N
Resolve vertically
R = = 148 N
Fmax= 0.5 x 148 = 74 N
F = 70 < 74
So no motion will occur
Resolve vertically
R + 60sin30º = 98
R = 68 N
Fmax = 0.5 x 68 = 34 N
F = 60cos30º = 52 > 34 N
So sliding occurs
Resolving vertically
R = 98 N
F = 30 < 49 (R)
So no motion takes place

8. So off you go and do some yourselves
Mrs. G’s book pg 64 exercise 4F
Big Blue pg 182 q 1, 2, 3, 5 pg 183
enjoy