Back to P5 Home
P5 P) Terminal Velocity
P5 P) Terminal Velocity
Resistive Forces
Resistive forces can also be known as frictional forces. There are a variety of different resistive forces that act in the opposite direction to the direction of travel. For example, let’s have a look at a car that is travelling towards the right.
Resistive forces can also be known as frictional forces. There are a variety of different resistive forces that act in the opposite direction to the direction of travel. For example, let’s have a look at a car that is travelling towards the right.
The car above is travelling towards the right, which means that the resistive forces will act towards the left (the opposite direction to the direction of travel). The resistive forces occur due to friction; friction happens when two surfaces are in contact with one another. Drag is the resistance that you get in a gas or liquid; one type of drag is air resistance.
When the driving forces and the resistive forces are the same, the resultant force is zero and the car (or any other object) will continue at a constant velocity (Newton’s first law).
The resistive forces for an object increase as the speed of the object increases. The resistive forces are greater when a car is travelling at 30 m/s compared to when the same car is travelling at 15 m/s. This means that in order for a car to travel at a faster constant speed, the driving force will need to be greater to balance out the greater resistive forces.
When the driving forces and the resistive forces are the same, the resultant force is zero and the car (or any other object) will continue at a constant velocity (Newton’s first law).
The resistive forces for an object increase as the speed of the object increases. The resistive forces are greater when a car is travelling at 30 m/s compared to when the same car is travelling at 15 m/s. This means that in order for a car to travel at a faster constant speed, the driving force will need to be greater to balance out the greater resistive forces.
Reducing Frictional Forces
We can reduce drag by making objects as streamlined as possible so that the fluid can easily flow over/ around the object. For example, some lorries have a curved top so that it is easier for the air to flow over it. The curved top on the lorry reduces the amount of air resistance that the lorry experiences.
We can reduce drag by making objects as streamlined as possible so that the fluid can easily flow over/ around the object. For example, some lorries have a curved top so that it is easier for the air to flow over it. The curved top on the lorry reduces the amount of air resistance that the lorry experiences.
Terminal Velocity
Terminal velocity is the maximum velocity that an object will reach when falling through a fluid (usually air). Terminal velocity occurs when the driving force is the same as the resistive forces (air resistance or drag). When the driving force and resistive forces are the same, the resultant force will be zero, which means that the object will no longer accelerate; the object will have reached it maximum velocity, which we call its terminal velocity.
We are going to have a look at an example whereby I drop a ball out of a helicopter. When the ball is first dropped, the force of gravity (weight) is considerably greater than the resistive forces (air resistance). This means that there is a large resultant force downwards, which results in the ball accelerating rapidly. An increase in the velocity of the ball results in the resistive forces increasing (air resistance increases). This decreases the size of the resultant force as the force of gravity (weight) pulling the object down remains the same and the resistive forces have increased. A decrease in the resultant force results in the acceleration rate decreasing (the velocity of the ball still increases, but at a slower rate). The ball will continue accelerating at a decreasing rate until the resistive forces are the same as the force of gravity (weight). When the force of gravity (weight) and the resistive forces are equal, the velocity of the ball will remain the same – the ball has reached its terminal velocity.
The velocity time graph for a ball being dropped out of a helicopter is shown below.
Terminal velocity is the maximum velocity that an object will reach when falling through a fluid (usually air). Terminal velocity occurs when the driving force is the same as the resistive forces (air resistance or drag). When the driving force and resistive forces are the same, the resultant force will be zero, which means that the object will no longer accelerate; the object will have reached it maximum velocity, which we call its terminal velocity.
We are going to have a look at an example whereby I drop a ball out of a helicopter. When the ball is first dropped, the force of gravity (weight) is considerably greater than the resistive forces (air resistance). This means that there is a large resultant force downwards, which results in the ball accelerating rapidly. An increase in the velocity of the ball results in the resistive forces increasing (air resistance increases). This decreases the size of the resultant force as the force of gravity (weight) pulling the object down remains the same and the resistive forces have increased. A decrease in the resultant force results in the acceleration rate decreasing (the velocity of the ball still increases, but at a slower rate). The ball will continue accelerating at a decreasing rate until the resistive forces are the same as the force of gravity (weight). When the force of gravity (weight) and the resistive forces are equal, the velocity of the ball will remain the same – the ball has reached its terminal velocity.
The velocity time graph for a ball being dropped out of a helicopter is shown below.
The gradient of the curve on a velocity time graph is the acceleration rate; (for positive gradients) steeper curves mean greater acceleration, and flatter curves mean lower acceleration. From the above velocity time graph, we can see that the acceleration is the greatest at the start because the curve is the steepest at the start. Over time the curve becomes flatter, which tells us that the acceleration rate is decreasing. The terminal velocity is where the curve becomes horizontal – this happens at time T with a velocity of V.
Shapes of Objects Affecting Terminal Velocity
Different objects have different shapes and surface areas, which affects the terminal velocities of the objects. Generally, the less streamlined an object is, the lower the terminal velocity will be for that object. This is because a less streamlined object will have greater resistive forces for a given speed, which means that the resistive forces will equal the force of gravity (weight) at a lower speed.
We will have a look at a skydiver jumping out of an aeroplane to show that this is the case. The force of gravity pulling the skydiver will be the same irrespective of whether the parachute is in their backpack or open.
When the parachute is in the skydiver’s backpack, the skydiver is very streamlined and the air resistance is very low. This means that the skydiver will have a high terminal velocity. A skydiver’s terminal velocity when the parachute is in their backpack is more than 100 mph.
When the parachute is open, the skydiver is less streamlined, which results in air resistance increasing dramatically. The terminal velocity when a skydiver’s parachute is open is around 15 mph.
The velocity time graph below shows how the velocity of a skydiver changes as they jump out of an aeroplane.
Different objects have different shapes and surface areas, which affects the terminal velocities of the objects. Generally, the less streamlined an object is, the lower the terminal velocity will be for that object. This is because a less streamlined object will have greater resistive forces for a given speed, which means that the resistive forces will equal the force of gravity (weight) at a lower speed.
We will have a look at a skydiver jumping out of an aeroplane to show that this is the case. The force of gravity pulling the skydiver will be the same irrespective of whether the parachute is in their backpack or open.
When the parachute is in the skydiver’s backpack, the skydiver is very streamlined and the air resistance is very low. This means that the skydiver will have a high terminal velocity. A skydiver’s terminal velocity when the parachute is in their backpack is more than 100 mph.
When the parachute is open, the skydiver is less streamlined, which results in air resistance increasing dramatically. The terminal velocity when a skydiver’s parachute is open is around 15 mph.
The velocity time graph below shows how the velocity of a skydiver changes as they jump out of an aeroplane.
I am now going to explain what happens between the terminal velocity when the parachute is in their backpack and the terminal velocity when the parachute is open. When a skydiver opens their parachute the resistive forces (air resistance) are greater than the force of gravity. This means that the resultant force will be upwards and the skydiver will start to decelerate. The skydiver will continue to decelerate at a decreasing rate until they reach the terminal velocity with the parachute open.