NEWTON'S 1st LAW OF MOTION
As a reminder, here is Newton's first law:
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"Objects at rest will stay at rest and objects in constant motion in a straight line will remain in constant motion in a straight line, unless acted on by an unbalanced force."
"At rest" or "In constant motion"
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In the definition of Newton's first law, we use the phrases "at rest" and "in constant motion".
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To help you visualise this, we can discuss some examples.
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If a car is parked in a driveway, it is said to be at rest, since it is not moving.
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The more interesting case is when the car is in motion. A car driving down a motorway at a constant speed of 60 miles per hour is said to be in constant motion in a straight line. The car is in constant motion because the speed is not changing, meaning the car doesn't speed up or slow down, it remains at 60 miles per hour.
"Balanced Forces" or "Unbalanced Forces"
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Balanced forces:
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When the forces acting on an object are all equal, the forces are said to be balanced. When we have balanced forces, the object will remain at rest.
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For example, when an airplane is sitting on a runway, the forces acting on the plane are balanced. This is because the downwards force = the upwards force, meaning gravity is equal to the normal force. Here, gravity is the force pushing down on the plane due to its weight and the normal force comes from the ground pushing back up on the plane. These two forces are equal and therefore the airplane is at rest.
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An object at rest is not the only case where the forces are balanced. An object in constant motion in a straight line will also have balanced forces. If we return to the example of the car driving down a motorway at a constant speed of 60 miles per hour and we think about the forces acting on this car:
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Forward force - the force from the engine that makes the car go
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Backward force - the drag from air resistance and the friction from the road
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Since the car is moving at a constant speed, we know that these two forces have to be equal.
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If the car was slowing down, we would know that the backward force from the air resistance and friction would have to be the larger force since the force from the engine is being reduced, meaning the car is beginning to decelerate (slow down).
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If the car was speeding up, we would know that the forward force from the car's engine would have to be the larger force since it is accelerating forward (speeding up).
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Here, the car is moving at a constant speed, so we know that the forward force must = backward force.
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Unbalanced Forces
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We use the term "unbalanced force" in the definition of this law. This describes a situation where the forces acting on an object are not equal in size. [1]
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When the forces acting on an object are unbalanced, the object will no longer be at rest or in constant motion.
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An example of this would be if you were to kick a ball that was sitting stationary on the ground. Before you kick the ball, the forces on the ball are balanced, since it is at rest. However, when you kick the ball, you apply an unbalanced force. As we have learned, when we apply an unbalanced force to an object, the object will no longer remain at rest. This is exactly what we see in this example, after we have kicked the ball, it begins to move in the direction from which we kicked it.
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If we look at the forces acting on the ball as it moves off:
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Forward force - from the kick
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Backward force - from friction of the ground and air resistance
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Immediately after the ball is kicked, the forward force is much larger than the backward force, so the ball accelerates forward.
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However, after a short period of time, the forward force from the kick diminishes due to the backward force slowing the ball down. The backward force from the friction of the ground and air resistance will become larger and larger, meaning the force from the kick will become smaller and smaller, until eventually, the ball stops. Once the ball is at rest, the forces are balanced again.
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Normal force
Gravity
Drag and friction from the road
Force from car's engine
Ball is initially at rest
How does Newton's First Law Apply to Rockets? [1]
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We have briefly mentioned how Newton's 1st law applies to rockets but let's go over it again now that we have learned about some of the important terms in the definition of this law.
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Newton's first law is very applicable to rockets as the forces on a rocket are both balanced and unbalanced at different stages in a rocket's journey. This can help us understand the launching and the motion of rockets as they embark on their journey into space!
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When a rocket is sitting on the launchpad before it is launched, the forces acting on the rocket are balanced. Let's have a look at the forces acting on the rocket here:
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Downward force - force due to gravity pushing the rocket down towards the ground
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Upward force - normal force from the launchpad pushing up on the rocket
The downwards force = the upwards force, meaning gravity is equal to the normal force. Here gravity is the force pushing down on the rocket due to its weight and the normal force comes from the launchpad pushing back up on the rocket. These two forces are equal and therefore the rocket is at rest.
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When it is time for the rocket to be launched into space, we must apply an unbalanced force to allow the rocket to begin to move, since Newton's first law tells us that an object at rest will stay at rest unless acted on by an unbalanced force.
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The unbalanced force that we apply is the force of thrust from the engines. When the engines are switched on, the thrust creates a large force that unbalances the forces acting on the rocket and allows the rocket to be launched!
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We will discuss how the thrust causes the rocket to move upwards into the sky in the Newton's third law section.
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This is shown in this picture here.
The forces are balanced and therefore the rocket is at rest.
