Friction DYNAMICS KUS objectives BAT solve DYNAMIC problems
Friction: DYNAMICS • KUS objectives BAT solve DYNAMIC problems with moving objects using Friction, F=ma and the suvat model Starter: A ball of mass 0. 4 kg is thrown at an angle of 26° with a force of 46 N Work out the vertical and horizontal components of the Force Work out the vector of the initial acceleration of the ball Work out the magnitude of the initial acceleration of the ball
Limiting Equilibrium: The magnitude of the frictional force is just sufficient to prevent relative motion Friction is a Force on objects caused by a touching surfaces resistance to a direction. ‘Rougher’ surfaces cause greater friction The coefficient of friction is a measure of this ‘roughness’ of different surfaces. There is a direct relationship between friction and the reaction force when an object is touching a surface when a particle is in equilibrium when a particle is moving Remember: An object moving at a constant velocity will still be in equilibrium An accelerating object is subject to F=ma
WB 1 a Finding the coefficient of friction The same 8 kg sledge is put on two different surfaces and pulled by a 100 N force at angle of 200. Given the values of Friction, find the coefficient of friction and the acceleration of the sledge for each surface a) Icy surface, friction is 5 N R Ice Fr 5 100 200 W = 8 9. 8 = 78. 4
WB 1 b. Finding the coefficient of friction b) Wet grassy surface, friction is 15 N Grass Fr 15 R 100 200 W = 8 9. 8 = 78. 4
WB 1 c. Finding the coefficient of friction c) Dry surface, friction is 25 N Tarmac. R 100 15 200 Fr W = 8 9. 8 = 78. 4
WB 2 a Finding the acceleration 35 R Fr W = 25 g = 245
WB 2 b Find the acceleration R 35 Fr • The vertical component of the force is now downwards • The Reaction force goes up • So Friction is greater • So the acceleration will be less since the forward horizontal force is the same as in a) W = 25 g = 245
WB 3 a moving down a slope, find acceleration A parcel of mass 3 kg is sliding down a rough slope of inclination 300 The coefficient of friction between the parcel and the slope is 0. 35. Find the acceleration of the particle R Fr
R Fr
WB 5 two-step problem, change of angle A 3 kg particle rests in limiting equilibrium on a plane inclined at 300 to the horizontal. Determine the acceleration with which the particle will slide down the plane when the angle of inclination is increased to 400 Step 1 Equilibrium R Fr
WB 5 Parcel two-step problem, change of angle (cont) A 3 kg particle rests in limiting equilibrium on a plane inclined at 300 to the horizontal. Determine the acceleration with which the particle will slide down the plane when the angle of inclination is increased to 400 R Step 2 Bigger slope Fr Coefficient of Friction is the same
WB 6 moving up a slope, find acceleration R 25 Fr
WB 7 down slope, find coefficient friction, suvat A parcel of mass 5 Kg is released from rest on a rough ramp of inclination θ = arcsin 3/5 and slides down the ramp. After 3 secs it has a speed of 4. 9 ms-1 Treating the parcel as a particle, find the coefficient of friction between the parcel and the ramp R Fr
WB 8 Parcel find acceleration and distance, suvat R Fr use F=ma to find the acceleration
WB 8 Parcel find acceleration and distance, suvat (cont)
WB 9 Parcel: finding the angle of inclination A parcel of mass 3 kg is sliding down a rough inclined plane with an acceleration of 4 ms-2. Find the angle of inclination of the plane if the coefficient of friction between the parcel and plane is 0. 6 R Fr This is solvable using the addition formula
WB 9 Parcel: finding the angle of inclination (solution cont) A parcel of mass 3 kg is sliding down a rough inclined plane with an acceleration of 4 ms-2. Find the angle of inclination of the plane if the coefficient of friction between the parcel and plane is 0. 6 addition formula
• KUS objectives BAT solve DYNAMIC problems with moving objects using Friction, F=ma and the suvat model self-assess One thing learned is – One thing to improve is –
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WB 7 find coefficient friction solution A parcel of mass 5 Kg is released from rest on a rough ramp of inclination R θ = arcsin 3/5 and slides down the ramp. After 3 secs it has a speed of 4. 9 ms-1 Treating the parcel as a particle, find the coefficient of friction between the parcel and the ramp Suvat to find a resultant force in the i direction Equilibrium in the j direction Coefficient of friction Fr
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