3. Dynamics Flashcards
Define mass.
The mass of a body is a measure of its resistance to change in state of motion.
Define momentum.
Momentum is a product of its mass and its velocity.
Define force.
The force exerted by X on Y is defined as the rate of change in momentum of Y produced by X.
Define impulse.
Impulse of a force is defined as the product of the force and the time of impact of the force.
State Newton’s first law of motion.
In the absence of a resultant external force acting on a body, a body at rest will remain at rest, and a body in motion will continue its motion in constant velocity.
State Newton’s second law of motion.
The rate of change of momentum of a body is proportional to the resultant external force that acts on it, and the change in momentum is in the direction of the force.
State Newton’s third law of motion.
When a body A exerts a force on body B, body B exerts a reaction force on A that is equal in magnitude and opposite in direction to the action force.
For some makes of car, the front and the rear are made with flexible material so that they collapse on impact, but the passenger compartment is made of rigid material so that they resist impact. Give a brief physical explanation of how this design may help protect passengers from serious injury in the event of a collision.
The shell of the passenger compartment is rigid to protect the passenger from being impaled by external objects or other parts of the car. The front and rear sections that collapse on impact form a cushioning that allows the change in momentum for the passenger to take place over a longer time interval, hence reducing the impact force on the passenger.
A conveyor belt is used to transfer luggage at an airport. It consists of a continuous horizontal belt running over driving rollers, moving at a constant speed of 1.5 m s-1.
Describe the change in the horizontal velocity of the baggage when it is loaded onto the belt.
The horizontal velocity of the baggage increases from zero to the velocity of the belt.
A conveyor belt is used to transfer luggage at an airport. It consists of a continuous horizontal belt running over driving rollers, moving at a constant speed of 1.5 m s-1.
State the relationship between the force exerted by the conveyer belt and the horizontal momentum of the baggage using Newton’s second law of motion.
The frictional force of the conveyer belt on the suitcase is proportional to the rate of increase in horizontal momentum of baggage.
Three blocks connected by two inextensible strings of negligible mass are being pulled horizontally. Weight of each block is ascending from left to right. If the blocks are being pulled leftwards, why is the string towards the left more likely to break?
The tension in that string is high enough to accelerate the total mass of the 2 other blocks of mass, whereas the tension in the other string is only high enough to accelerate 1 block of mass at the same rate.
Define weightlessness.
The feeling of weight or the apparent weight of an object is the sum of the upward contact forces on the body. When free-falling, only weight, a non-contact force, acts on the mass and there are no contact forces on it. Thus there is no ‘feeling’ of weight.
Two identical billiard balls moving towards each other from opposite directions have speeds of 1.50 m s-1 and 0.40 m s-1 just before they meet in a head-on elastic collision.
State qualitatively what can be deduced about the subsequent motion of the balls as a result of the collision being head-on.
The direction of the final velocities of the two particles will be along the same line as the initial velocities of the ball.
Because the collision of two balls is elastic, both momentum and kinetic energy of the two balls are conserved in the collision. Discuss the validity of his statement.
Total momentum of the two balls is conserved (constant) throughout the collision. However, total kinetic energy changes throughout the collision, but it’s value before and after the collision remains equal.
