P10 - Forces and Motion

• Momentum is a property of any moving object that is the product of its mass and velocity.
• Momentum is conserved in a closed system.
• The total momentum before an event is equal to the total momentum after the event.
• Example of conservation of momentum

  the boat is stationary at the bank, meaning its momentum is 0 kg m/s. When the person jumps out, they have a velocity and therefore a momentum. The boat must move away from the bank, since momentum is conserved (so must add up to 0 after the event too) so the boat has momentum in the opposite direction to the person – the boat moves away from the bank.
• Stopping distance is thinking distance + braking distance.
• Thinking distance is the distance travelled during the driver's reaction time.
• Breaking distance is distance travelled under braking force.
• The greater the speed of the vehicle, the greater the stopping distance with any braking force. This is because there is a greater change in speed to reach 0 m/s so braking force takes longer to reduce velocity.
• Thinking distance is longer at higher speed because reaction time stays the same while speed is increased - you'd go further in the same amount of time.
• When force is applied to the brakes, work is done by the friction force between the brake pads and the wheel. The kinetic energy of the vehicle is transferred to thermal energy.
• To stop a vehicle in a certain distance, a larger force is needed for a faster vehicle since larger deceleration needed. F = ma.
• Force is the rate in change of momentum.
• If time taken for momentum to change is longer, the resultant force is smaller. Air bags used to increase time taken for body to impact car therefore reducing force felt by body as the momentum takes longer to change.