Momentum, Forces and Motion

• The equation used to calculate an object’s momentum is Momentum = Mass x Velocity.
• The unit used for momentum is kg m/s kilogram metres per seconds.
• In a closed system, the total momentum before a collision is equal to the total momentum afterwards.
• An equation linking change in momentum, force and time is Force x Time = Change in Momentum F Δt = mΔv.
• The quantity equal to the force experienced in a collision is the rate of change of momentum.
• If an object’s change of momentum is fixed, the only way to reduce the force that the object experiences is to increase the length of time over which the change of momentum occurs.
• Seatbelts improve a passenger’s safety during a collision by increasing the time over which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced.
• A distance quantity does not require a specific direction, making it a scalar quantity.
• If an object moves 3 metres to the left and then 3 metres back to its initial position, the object has zero displacement.
• Displacement is a vector quantity, involving both distance and direction.
• The object starts and ends at the same point.
• A typical value for the speed of sound is 330 m/s.
• A typical value for human walking speed is 1.5 m/s.
• A typical value for human running speed is 3 m/s.
• Work is done by the friction force between the brakes and wheel.
• Another consequence of a vehicle undergoing very large decelerations is loss of control of the vehicle.
• Consequences of a vehicle undergoing very large decelerations include kinetic energy converted to heat, causing brakes to overheat.
• To stop a car in a given distance, if its velocity is increased, the braking force must also be increased.
• Kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs.
• A typical value for human cycling speed is 6 m/s.
• The equation linking distance, speed and time is: distance = speed × time.
• If the resultant force on a stationary object is zero, the object will remain at rest.
• The defining equation for Newton’s Second Law is Resultant force = Mass x Acceleration F = ma.
• The resultant force acting on an object when it is falling at terminal velocity is zero.
• The braking forces are equal to the driving forces when a car is travelling at constant velocity.
• An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass.
• If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction).
• Inertia is the tendency of an object to continue in its state of rest or uniform motion.
• If an object changes direction but remains at a constant speed, there must be a resultant force.
• Appropriate units for the equation linking distance, speed and time are: distance in metres, speed in metres per second, and time in seconds.
• An approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface can be given as: -9.8 m/s².
• To calculate speed at a given time from a distance-time graph for an accelerating object, draw a tangent to the curve at the required time and calculate the gradient of the tangent.
• The equation for the average acceleration of an object is: Acceleration = (Change in Velocity)/(Time Taken).
• Speed can be calculated from a distance-time graph by finding the gradient of the graph.
• The distance travelled by an object can be calculated from a velocity-time graph by finding the area under the graph.
• In circular motion, the direction is continuously changing.
• Distance is calculated using Speed and Time.
• Speed is a scalar quantity.
• Velocity is a vector quantity which means it can only be constant if the direction is constant.
• Braking distance may be affected by adverse (wet/icy) road conditions and poor tyre/brake conditions.