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P5 D) Resultant Force
P5 D) Resultant Force
Free Body Diagrams
We can draw a free body diagram which shows all of the forces acting on an isolated object or system; a free body diagram will show all of the forces acting on an object or system, and none of the forces that are acting on the rest of the world. We show the forces on a free body diagram by using arrows. The direction of the arrow shows us the direction of the force, and the length of the arrow shows us the magnitude of the force (a longer arrow means a greater/ stronger force).
There is a free body diagram below of a ball that has just been knocked off of a table.
We can draw a free body diagram which shows all of the forces acting on an isolated object or system; a free body diagram will show all of the forces acting on an object or system, and none of the forces that are acting on the rest of the world. We show the forces on a free body diagram by using arrows. The direction of the arrow shows us the direction of the force, and the length of the arrow shows us the magnitude of the force (a longer arrow means a greater/ stronger force).
There is a free body diagram below of a ball that has just been knocked off of a table.
We can see on the above diagram that the weight is pointing downwards, and air resistance is pointing upwards. The arrow for weight is longer than the arrow for air resistance, which tells us that the force of weight is greater than air resistance.
Resultant Force
The resultant force for an object is the overall force acting on an object in one direction. For the above free body diagram of the ball that has just been dropped, we can see that the weight force (downwards force) is greater than air resistance (upwards force). This means that the resultant force will be downwards.
I am now going to add some values onto the free body diagram for weight (7 N) and air resistance (5 N).
The resultant force for an object is the overall force acting on an object in one direction. For the above free body diagram of the ball that has just been dropped, we can see that the weight force (downwards force) is greater than air resistance (upwards force). This means that the resultant force will be downwards.
I am now going to add some values onto the free body diagram for weight (7 N) and air resistance (5 N).
In order to work out the resultant force acting on an object, we need to choose one direction to work out all of the forces for. The resultant force on the ball will be downwards because the downwards force (weight) is greater than the upwards force (air resistance). Therefore, I am going to have all of the forces going down as positive (positive 7 N for weight) and all of the forces going up as negative (negative 5 N for air resistance). We can now work out the resultant force by using the calculation below.
The resultant force acting on the ball is 2 N downwards.
Another Example
A cyclist is riding a bike. The cyclist turns the pedals, which results in a driving force of 300 N (the driving force is rightwards). The cyclist and bike experiences air resistance of 200 N. Add these forces to the free body diagram below and work out the resultant force.
A cyclist is riding a bike. The cyclist turns the pedals, which results in a driving force of 300 N (the driving force is rightwards). The cyclist and bike experiences air resistance of 200 N. Add these forces to the free body diagram below and work out the resultant force.
Free Body Diagram
The question tells us that the driving force is 300 N and this force is going rightwards. So, we draw an arrow going rightwards labelled as the driving force with 300 N on.
The question tells us that the driving force is 300 N and this force is going rightwards. So, we draw an arrow going rightwards labelled as the driving force with 300 N on.
We are also told in the question that the cyclist and bike experience air resistance of 200 N. Air resistance is a resistive force, which means that it will act in the opposite direction to the direction of travel; this means that the air resistance will act in a leftwards direction. The air resistance is 200 N, which is less than the driving force. Therefore, when we draw the leftwards air resistance arrow on the diagram, the arrow needs to be shorter than the driving force arrow.
Resultant Force
The resultant force for an object is the overall force acting on an object in one direction. I am going to work out the resultant force towards the right. This means that the driving force will be positive (300 N) and the air resistance force will be negative (-200 N). The calculation for working out the resultant force is shown below.
The resultant force for an object is the overall force acting on an object in one direction. I am going to work out the resultant force towards the right. This means that the driving force will be positive (300 N) and the air resistance force will be negative (-200 N). The calculation for working out the resultant force is shown below.
This gives us a resultant force of 100 N, which is positive and this means that the direction of the resultant force is to the right (as we took the right to be positive). Therefore, the resultant force is 100 N to the right.
Acceleration
If the resultant force acting on an object is zero, the object will remain in its same state of motion:
If the resultant force acting on an object is non-zero, the object will start to accelerate in the direction of the resultant force. This acceleration can take 5 different forms:
We will be looking at acceleration and resultant force in a lot more detail in Newton’s first & second law section (click here to be taken to this section now).
The Bike Example
For the cyclist example, the resultant force was 100 N to the right.
If the resultant force acting on an object is zero, the object will remain in its same state of motion:
- If the object is stationary, the object will remain stationary
- If the object is moving, the object will continue moving at a constant velocity
If the resultant force acting on an object is non-zero, the object will start to accelerate in the direction of the resultant force. This acceleration can take 5 different forms:
- Starting
- Stopping
- Speeding up
- Slowing down
- Changing direction
We will be looking at acceleration and resultant force in a lot more detail in Newton’s first & second law section (click here to be taken to this section now).
The Bike Example
For the cyclist example, the resultant force was 100 N to the right.
A resultant force of 100 N towards the right means that the cyclist will accelerate towards the right. As the cyclist is already moving rightwards, an acceleration to the right means that the velocity of the cyclist will increase.
Normal Reaction Force
When we place an object on a surface, the surface will exert an equal and opposite force on the object. This equal and opposite force from the surface to the object is why the object does not fall through the surface.
For example, let’s suppose that we place a cup onto a table. Weight is the force pulling the object towards the earth (downwards). The table exerts a normal reaction force that is equal and opposite to the weight of the object (this force is upwards). There is a diagram of this taking place below with some values for weight and the normal reaction force.
Normal Reaction Force
When we place an object on a surface, the surface will exert an equal and opposite force on the object. This equal and opposite force from the surface to the object is why the object does not fall through the surface.
For example, let’s suppose that we place a cup onto a table. Weight is the force pulling the object towards the earth (downwards). The table exerts a normal reaction force that is equal and opposite to the weight of the object (this force is upwards). There is a diagram of this taking place below with some values for weight and the normal reaction force.
The downwards force (weight) and upwards force (normal reaction force) are equal and opposite to one another. This means that the resultant force acting on the cup is zero (0 N). A resultant force of zero means that the object will stay in its same state of motion. Our cup was originally stationary, so a resultant force of zero means that the cup will remain stationary.