Unit 3: Dynamics — Short Questions

A force can produce the following changes in motion:
i. Change in Speed: A force can make an object move faster (acceleration) or slower (deceleration).
ii. Change in Direction: A force can change the direction of a moving object, such as in circular motion.
iii. Change in Shape: Applying a force can deform or change the shape of an object (like stretching a rubber band).
iv. Starting or Stopping Motion: A force can set a stationary object in motion or bring a moving object to a stop.

Examples of contact forces i. Frictional Force: The force that opposes the motion of an object sliding or moving across a surface (e.g., rubbing your hands together).
ii. Tension Force: The pulling force exerted by a string, rope, or cable when it is attached to an object (e.g., pulling a bucket up with a rope).
iii. Normal Force: The support force exerted by a surface perpendicular to an object resting on it (e.g., a book on a table).
iv. Applied Force: A force applied directly to an object by a person or another object (e.g., pushing a car).
v. Air Resistance: The force that opposes the motion of an object moving through the air (e.g., parachuting).

An object moving with constant velocity in free space will continue to move with that velocity forever unless an external force acts on it. This is because, in free space, there is no friction or air resistance to slow the object down, and according to Newton's First Law of Motion (law of inertia), an object in motion stays in motion at constant velocity unless acted upon by an external force.

Impulse of force is the product of a force (F) and the time duration (Δt) over which it acts. It is equal to the change in momentum (Δp) of an object.

Formula Impulse=F×Δt=Δp

Its unit is Newton-second (N·s) or kilogram meter per second (kg·m/s). It measures how a force changes an object's momentum over time.

Newton's First Law of Motion states that an object will remain at rest or move with constant velocity unless acted upon by an external force. This law has not been directly proved on Earth because of the presence of external forces like friction and air resistance, which constantly affect the motion of objects.

When a car suddenly accelerates from rest, you feel pushed back into the seat due to inertia, which is explained by Newton's First Law of Motion. Inertia is the tendency of objects to resist changes in their state of motion.

In Newton's Second Law of Motion, the force is called the net force because it is the total force acting on an object after considering all the individual forces acting on it. The net force is the sum of all these forces. It is the overall force that causes the object to accelerate.

Example If a car is being pushed forward while friction resists it, the net force is the total forward force after subtracting friction. This net force determines how much the car will accelerate.

Rolling friction is less than sliding friction because, in rolling, only a small area of contact touches the surface at a time, reducing resistance. In contrast, sliding friction involves direct contact between two surfaces, creating more resistance due to greater surface interaction. This is why less force is needed to move an object on wheels than to slide it across a surface.

Terminal velocity is the constant velocity achieved by an object when the upward force of air resistance balances the downward force of gravity, causing the object to stop accelerating and continue falling at a constant speed.

The astronaut should fire the hand rocket in the opposite direction to the spaceship. This expels gases in one direction, and by Newton's Third Law, the equal and opposite reaction pushes the astronaut back toward the spaceship.

Sir Isaac Newton was born in Lincolnshire on January 4, 1643, and the name of his famous book is "Principia Mathematica."

This happens because of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.

Given data m=100g=0.1kg
g=10ms^-2

To find w=?

Solution The weight of the body can be calculated by using formula:
w=mg
w=0.1×10=1N

Result Hence the weight of the body is 1N.

The water does not form a wet layer between the tyre surface and the road surface due to the spaces in the tread pattern on the tyre. These spaces allow the water to be displaced, improving grip and reducing the chances of skidding on wet roads.

The surface temperature of the shuttle rises to over 950°C when it re-enters the Earth's atmosphere due to the friction caused by the atmosphere. This friction generates intense heat, increasing the shuttle's surface temperature.

Friction in human joints is very low because our bodies contain a natural lubricating system. As a result, even though our bones rub against each other at the joints when we move, they do not normally wear out.

The contact point of the wheel is not perfectly circular; under pressure, it becomes flat. This flat portion slides against the surface, producing a frictional force.

A hovercraft moves over both water and ground by ejecting air underneath through powerful fans, creating a cushion of air. This cushion offers very small resistance, allowing the hovercraft to move smoothly.

A cricketer draws his hands back to reduce the impact of the ball by increasing the time over which the force is applied. This decreases the force experienced by the hands.

When the arrow penetrates into the apple, the momentum of the apple changes in response to the arrow's force. Conversely, the apple applies an opposing force to the arrow, causing a change in the arrow's momentum.

Soft packing materials, like Styrofoam, corrugated cardboard sheets, and bubble wrap, reduce the effect of quick changes in momentum. This reduces the force acting on the fragile objects, preventing them from breaking due to jerks or direct impacts with hard objects.

Crumple zones are designed to compress during an accident, absorbing deformation energy from the impact. They manage crash energy by absorbing it in the outer parts of the vehicle, rather than transmitting it to the occupants. This is done by making the outer parts weaker, like plastic bumpers, while making the passenger cabin stronger.

A seatbelt is a safety feature in a car that prevents passengers from moving forward suddenly when the car stops. It reduces the chances of passengers hitting the windshield or steering wheel.

This is due to the second law of motion according to which the acceleration is directly proportional to the applied force. As the person falls with large acceleration and hence with greater force, he is hurt due to fall on hard cemented floor

(a = v/t, a ∝ F)

This is also due to the second law of motion. As we fall on the sand, the sand being soft is pressed and time increases. That is why a person falling on sand is not hurt much.

This is due to the Newton's second law of motion, by pulling or drawing the hands downward the time of motion increase. Therefore, the force decreases and hands are not hurt.

(a = v/t, a ∝ F)

This is due to the property of inertia. On shaking the branch of tree while the fruits tend to remain at rest and therefore fall down. This is due to the law of inertia.

The passengers sitting in stationary bus are also at rest but if the bus suddenly starts moving, the lower part of the body comes into motion while the upper part tends to remain stationary due to inertia. That is why the passengers fall in the backward direction.

The dust which is stuck with the carpet is initially at rest. When we beat a carpet with a stick, the carpet is set into motion suddenly while the dust particles are at rest and tends to remain at rest due to inertia. Consequently, the dust particles get removed from the carpet.