M017-24

Question 1

1.What is the formula for momentum?
2. Explain linear momentum
3. Explain the principle of conservation of linear momentum

Answer 1

1. p = mv
   Where: p = momentum, measured in kg m s−1
   m - mass, measured in kg
   v = velocity, measured in m s−1
2. Linear momentum is the momentum of an object that is moving in only one dimension. The linear momentum of an object remains constant unless the system is acted upon by an external resultant force.
3. The principle of conservation of linear momentum states: The total momentum before a collision is equal to the total momentum after a collision, provided no external force acts.

Question 2

1. How do you calculate the momentum in collisions?
2. Explain internal and external forces in a system
3. What is the average force equal to?

Answer 2

1. Equate the total momentum before to the total momentum after. If the object is moving in the opposite direction to the way you are reading arrows, then the velocity must be negative, so you may have a negative momentum.
2. Systems with no external forces may be described as ‘closed’ or ‘isolated’. External forces are forces that act on a structure or system from outside e.g. friction and weight. Internal forces are forces exchanged by the particles in the system e.g. tension in a string.
3. The average force is equal to the change in kinetic energy, initial kinetic energy - final kinetic energy.

Question 3

1. What is the explanation for force and formula for force involving momentum?
2. What is impulse?
3. What is the area under a force time graph?

Answer 3

1. Force can also be defined as the rate of change of momentum on a body. The force that is equal to the rate of change of momentum is still the resultant force
   The positive direction is taken to be the direction of the initial motion; therefore
   a force on an object will be negative if the force opposes its initial velocity the opposing force is exerted by the object it has collided with the forces will be of equal magnitude and opposite in direction, in accordance with Newton’s Third Law.
2. Impulse is the product of the force applied and the time for which it acts
   Impulse = F (change in)t
   Where:
   Impulse is measured in newton seconds (N s)
   F = resultant external force applied, measured in newtons (N)
   (Change in) t = change in time over which the force acts, measured in seconds (s)
3. The area under a force-time graph is the impulse.

Question 4

1. How do you find the direction of impulse?
2. How is momentum conservation considered in product transport design?

Answer 4

1. To find the direction of the impulse, The impulse is always in the direction of the resultant force, and the resultant force is equal to the change in momentum. Impulse acts in the opposite direction to an object’s direction of motion after the force is applied.
   Eg. If the left is taken as positive and therefore the right as negative, an impulse of 20 N s to the right is equal to −20 N s.
2. Packaging, especially for fragile items, uses bubble wrap or polyester packaging to reduce the impact forces that items experience in transit.
   These help cushion the items by increasing the time over which they experience a force, which reduces the risk of damage.

Question 5

1. What is an elastic collision and how do you check if the collision is elastic?
2. Explain some safety features present in cars

Answer 5

1. A collision or explosion is elastic if the kinetic energy is conserved. Elastic collisions are common when colliding objects do not stick together and move in opposite directions, while objects commonly stick together in inelastic collisions. To check if a collision is elastic or inelastic, compare the kinetic energy before and after the collision, and see if they are the same.
2. The force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs. Vehicles have safety features such as crumple zones, seat belts and airbags. The safety features absorb energy from the impact , increasing the time taken for the change in momentum of the passenger to occur, and therefore reducing the force exerted on the passenger and reducing the risk of injury.

Question 6

1. What is the formula for work done?
2. What are the formulae for power?
3. What is work done equal to in a force-displacement graph?
4. Explain the change in power output when an object (with a driving force, such as a cyclist) moves at a higher, constant speed.

Answer 6

1. If a constant force is applied parallel to the direction of the object’s displacement:
   W = Fs, where w = work done (J), F = average force applied and s = displacement (m)
   If a constant force is applied at an angle to the direction of the object’s displacement:
   W = Fscosθ, where θ = the angle between the direction of the force and the object’s motion.
2. Power is the rate of doing work or the rate of energy transfer
   Power = change in work done / change in time
   If an object is moving at constant velocity with a constant force, the power can also be calculated by: power = force x velocity
3. Work done is equal to the area under a force-displacement graph.
   Work done = force x displacement
4. The power output will be higher as a larger velocity results in the object providing a large force, as force = mass x acceleration

Question 7

1. Define efficiency, and what is the equation for efficiency?
2. What are the different ways efficiency can be expressed as?
3. State the principle of conservation of energy

Answer 7

1. Efficiency is the ratio of the useful power output from a system to its total power input.
   Efficiency = (useful power output / total power input) x 100%
   or Efficiency = (useful energy output / total energy input) x 100%
2. Efficiency can be expressed as a decimal, percentage or fraction. If a question asks for the efficiency to be displayed as a ratio, then you should write it as a decimal (the exam board do not accept fractions for ratios).
   3.The Principle of Conservation of Energy states that: Energy cannot be created or destroyed, it can only be transferred from one form to another.

Question 8

What are common energy transformations for the following:
1. A falling object (in a vacuum)
2. A battery
3. Horizontal mass on a string
4. What does loss in kinetic energy equal when an object travels up a rough inclined surface?

Answer 8

1. A falling object (in a vacuum): gravitational potential energy ➝ kinetic energy
2. A battery: chemical energy ➝ electrical energy
3. Horizontal mass on a spring: elastic potential energy ➝ kinetic energy
4. Loss in kinetic energy = Gain in gravitational potential energy + Work done against friction

Question 9

What are the six types of energy?

Answer 9

Kinetic : the energy of a moving object,
Gravitational potential: the energy something gains when you lift it up, and which it loses when it falls.
Elastic: the energy of a stretched spring or elastic band (sometimes called strain energy.)
Chemical: the energy contained in a chemical substance.
Nuclear: the energy contained within the nucleus of an atom.
Internal: the energy something has due to its temperature or state, somethimes referred to as thermal or heat energy.