# Physics P2 (Forces, Momentum etc.)

• Physics P2
• Forces
• SPEED = DISTANCE / TIME.  Example : A person runs 200 M in 20s. Whats his speed? Answer : Speed = D/T Speed = 200/20. Speed = 10 M/s
• RESULTANT FORCES. Example - A car is driving at a force of 1000N. The air resistance is 400N. What is the resultant force? Answer : 1000 - 400 = 600N
• NEWTONS SECOND LAW OF MOTION  : F = ma. F - Force (Newtons) M - Mass (Kilograms) A- Acceleration (Meters per second)
• Example - A person is running at 1000 N and the air resistance is 400N. What is the acceleration? Answer : A= F / M = 600/70 = 8.5 M/s.
• GPE Equation :  GPE = Mass x Gravitational Field Strength (10 N) x Height. GPE = Joules (J) Mass = Kilograms (Kg). Gravitational Field Strength = Newtons per kilogram (N/Kg) Height = Metres (M)
• Example : A person has a mass of 60 kg. How much do they weigh on Earth, if the gravitational field strength is 10 N/kg. Answer : Weight = mass × gravitational field strength Weight = 60 kg × 10 N/kg Weight = 600 N
• Car Safety Features
• Anti-lock braking system (ABS) – prevents skidding allowing the driver to remain in control. The vehicle stops more quickly as there's more friction between the road and tyres
• Modern cars also have safety features that absorb kinetic energy in collisions. These typically include: seat belts, air bags, crumple zones
• Acceleration and Deceleration
• Acceleration - When speed increases. Deceleration - When speed decreases.
• A = V-U / T. Final Velocity - Initial Velocity / Time.
• Example : A car accelerates from STANDSTILL to 19 m/s in 6 s. What is the acceleration. Answer : 18 - 0 (Standstill) /6 = 3 M/s.
• Stopping Distance
• Stopping Distance = Thinking Distance + Braking Distance
• Thinking Distance = Speed x Reaction Time
• Braking Distance = Speed x Braking Time
• Tiredness/Alcohol/Drugs - Affects braking distance as it affects your body
• Speed - Affects your braking distance, it will be longer
• Road Conditions - Affects braking distance, it will be longer
• Poorly Maintained brakes - Affects braking distance, increases braking distance. (Reduces Friction)
• Factors affecting Stopping Distance - Speed of vehicle and Mass of vehicle.
• Momentum
• Momentum Equation - P = Mass x Velocity, P = Momentum (Kg M/s) Mass = Kilograms (Kg) Velocity = Metres per second (M/s).
• Example : what is the momentum of a 5 kg object moving with a velocity of 2 m/s. Answer = P = Mass x Velocity. 5 x 2 = 10  kg m/s
• Example : Two railway carriages collide and move off together. Carriage A has a mass of 12,000 kg and moves at 5 m/s before the collision. Carriage B has a mass of 8,000 kg and is stationary before the collision. What is the velocity of the two carriages after the collision?
• Answer : Step 1 -Work out the total momentum before the event (before the collision) p = m × v Momentum of carriage A before = 12,000 × 5 = 60,000 kg m/s Momentum of carriage B before = 8,000 × 0 = 0 kg m/s    Total momentum before = 60,000 + 0 = 60,000 kg m/s
• Answer Part 2 : Step 2 Work out the total momentum after the event (after the collision) Because momentum is conserved,total momentum afterwards = 60,000 kg m/s. Momentum Before = Momentum After
• Answer Part 3 : Step 3 Work out the total mass after the event (after the collision) Total mass = mass of carriage A + mass of carriage B = 12,000 + 8,000 = 20,000 kg
• Answer Part 3 : Step 4 Work out the new velocity:p = m × v, but you rearrange this equation so that v = p ÷ m Velocity (after the collision) = 60,000 ÷ 20,000 = 3 m/s
• Momentum Before = Momentum After
• Explosion Calculations
• Two trolleys A & B collide and bounce off each together. Momentum Of A - Mass of A x Velocity Of A. Momentum Of B - Mass Of B x Velocity Of B.
• "Bob" and the gun have a total mass of 2000 Kg. The shell fired has a mass of 20 kg. It traveled at a velocity of 120 m/s. Mass of gun x Recoil velocity of gun = - Mass of shell x Velocity of shell.
• 2000 x V (?) = - 20 x 120. 2000 x V (?) = -2400. V = 2400/2000 = - 1.2 m/s
• The minus sign - = they move in different directions