Discuss centripetal acceleration.
Centripetal acceleration is the rate of change of velocity. It is caused by the external force that act at a tangent and is needed to make a body move in a circular path at constant speed. For instance if a ball is swung on a string, it requires tension and the ball will go off in a tangent line if the string breaks. Thus, centripetal acceleration can be derived when a body moves in a circular path.
EXAMPLE: If a rock of 2 kg is swung around with a 3 m rope. It is swung at speed of 5 m/s, then the force acting on the rope is the tension T as shown and the centripetal acceleration can be calculated as:
Discuss centripetal force.
Centripetal forces, is the force that acts towards the centre and keeps the body moving in a circular path. It acts perpendicular to the direction of the movement of the body. For instance if a ball is attached to a string and whirled round in a circle the force acting on it towards the centre is known as centripetal force. It can change the motion of the ball by changing its direction
EXAMPLE: If your car turns on a roundabout, the force of friction on the wheels provide a centripetal force needed for the circular motion as shown
Discuss circular motion.
Circular motion refers to moving of objects in a circular path. Large objects like planets go around the sun and even small objects like electrons move around the magnetic field. Other examples that can be applied include rides at fun fair or clothes spun in a washing machine.
Discuss the conservation of energy.
The principle of conservation of energy states that energy can neither be created nor destroyed in any process. However, it is converted from one form to another or transferred from one body to another, but the total quantity of energy remains same. For instance, chemical energy stored in fuels is converted by a process of burning from heat energy to light energy.
EXAMPLE: If a car is moving up a hill the energy of the car will change from potential energy to kinetic energy to heat energy as the car starts, moves and then applies brakes.
Discuss conservative and non-conservative forces.
The forces that store energy are known as conservative forces. For instance, if you are lifting a book the work done against gravity is available for kinetic energy. A neoconservative force is a force that does not store energy. For instance air resistance.
EXAMPLE: If a ball is thrown up in air the gravitational force against the movement of the ball. As it acts against the ball the ball slows down and falls back. As the force is conserved till the ball reaches the ground gravitational force is a conservative force.
Whereas, if you are pushing a book the work done against friction, is lost. It is not conserved as kinetic energy. Thus, friction is a neoconservative force.
Discuss the dot product.
The dot product also known as scalar product, takes two vectors over the real numbers and in turn gives a real number scalar quantity. It is used for multiplication of vectors. EXAMPLE: The diagram below shows two vectors a and b.
A person is pulling the brick with a constant force a along the horizontal surface. In order to calculate the work done in moving the brick along a distance b is found by multiplying the distance and the magnitude of the force. Thus the dot product can be stated as:
a.b = |a||b| cos t
(t is the angle between a and b)
State and discuss Newton’s Universal Gravitational Law.
Newton’s law of universal gravitation describes the gravitational attraction between two bodies of mass. It points out that every body with a mass attracts other body with a mass by a force pointing along the line intersecting the bodies. The force exerted is proportional to the masses of the bodies and inversely proportional to the square of distances between them. It can be stated as:
F = Gm1m2/r2
(F is gravitational force, G is gravitational constant, m1 and m2 are mass of first and second body respectively and r is the distance between bodies.)
EXAMPLE: To determine the gravitational attraction force between the earth of mass = 5.98 x 1024 kg and a 70-kg student standing at a distance of 6.38 x 106 m from earth’s center. The gravitational constant G is 6.673 x 10-11 N m2/kg2, by substituting values in the formula we get F = 686 N
Discuss internal and external forces.
Internal and external forces are classified based on their ability to change an object’s total mechanical energy as work is done on it. External forces are capable of changing the total mechanical energy of an object. Examples include tension force and friction force. Internal forces are capable of changing the form of energy but not the total amount of mechanical energy. Examples include magnetic force and gravity force.
Define the Joule.
One joule is defined as the work done by a force of one Newton which moves a body or object through a distance of one metre in the direction of the force.
Discuss momentum and impulse.
Momentum is a useful quantity to consider when bodies are involved in collisions and explosions. Momentum can be calculated as:
Momentum = mass x velocity
For instance, we can calculate pressure of gas molecules that collide with the walls of the container using momentum.
If a person kicks a football, his foot is in contact with the ball for some time t during which the kick force F acts on the ball. When F x t it is known as impulse applied on the ball. Momentum and impulse are related as:
Impulse = change of momentum
EXAMPLE: A tennis ball collides with the wall with some velocity and bounds back with some velocity. Momentum will depend on the velocity change and impulse will depend on the impulse change.
State and discuss Newton’s third law.
Newton’s third law states: if body A exerts a force on body B, then body B exerts an equal but opposite force on body A. This indicates that forces never exist alone but always in pairs as result of action between bodies. For example if you walk forward your foot pushes the Earth backward and the Earth exerts an equal and opposite force forward on you. Another application could include that when you sit on chair you exert a force downwards and the chair exerts an upward force on your body
Define period and frequency.
Period is the time taken to complete one wave or oscillation. It is measured in seconds. Frequency is the number of waves or oscillations in one second. It is measured in Hertz. Period and frequency are related by the equation:
T = 1/f
(T is period and f is frequency)
Discuss potential energy.
Potential energy is the energy possessed by an object because of its position or condition. For instance a body above the Earth’s surface has gravitational potential energy because of its raised position and a stretched rubber band has elastic potential energy because of change in its condition.
EXAMPLE: A stone on the edge of a cliff has potential energy due to its position in the earth’s gravitational field. If it falls, the force of gravity acts on it till it reaches ground; thus the stone’s potential energy is equal to its weight times the distance it can fall.
Discuss projectile motion.
Projectile motion is defined as the motion of an object projected into the air at a certain angle. During projectile motion the only force that acts is gravity. If any other force acts on the object then it is not a projectile. For instance, a firework on a particular occasion.
EXAMPLE: A football kicked in air at a velocity of 5 m/s. After five seconds the ball would have travelled a distance of 25 m (5 s x 5 m/s).
Chew C., Cheng L. S. and Foong C. S. (2000) Physics, Marshall Cavendish Education