Theme A - Space, Time & Motion

Unraveling Newton's Third Law: Momentum Conservation Explored

633 words

4 mins read

Last edited on 5th Nov 2024

Newton’s third law states that when two objects A and B interact, if A produces an impulse on B, then B will produce an equal and opposite impulse on A.
Let's take an example. If two toy cars collide head-on and then rebound, the forces exerted by each car on the other are equal and opposite. The time of contact (Δt) is identical for both cars, and therefore the magnitude of the product F × Δt (force x time) is the same for both. However, since the forces are in opposite directions, the signs of F × Δt will be different.
This product (F × Δt) is essentially the change in momentum of each car. Hence, the change of momentum of car A is equal in magnitude and opposite in direction to the change of momentum of car B.
When we think of both cars together as a single system, we can see that there's no change in momentum of this system during the collision. In other words, the system's momentum is conserved.

Momentum is always conserved when no external forces act on the system. This principle is called the conservation of linear momentum. It's called "linear" because the objects are moving in a straight line.
Experiments can be conducted to test this principle. For instance, by measuring the speed of a cart of known mass hitting another stationary cart of known mass, and comparing the total momentum before and after the collision.
The experiment can be conducted in different scenarios - when the carts don't stick together after collision, when they are both moving before the collision, when the masses are not the same, etc.
Make sure to take into account factors like friction and air resistance. You can compensate for them by raising the track end so that a cart runs at a constant speed when pushed.

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Physics SL

633 words

4 mins read

Last edited on 5th Nov 2024

Newton’s third law states that when two objects A and B interact, if A produces an impulse on B, then B will produce an equal and opposite impulse on A.
Let's take an example. If two toy cars collide head-on and then rebound, the forces exerted by each car on the other are equal and opposite. The time of contact (Δt) is identical for both cars, and therefore the magnitude of the product F × Δt (force x time) is the same for both. However, since the forces are in opposite directions, the signs of F × Δt will be different.
This product (F × Δt) is essentially the change in momentum of each car. Hence, the change of momentum of car A is equal in magnitude and opposite in direction to the change of momentum of car B.
When we think of both cars together as a single system, we can see that there's no change in momentum of this system during the collision. In other words, the system's momentum is conserved.

Momentum is always conserved when no external forces act on the system. This principle is called the conservation of linear momentum. It's called "linear" because the objects are moving in a straight line.
Experiments can be conducted to test this principle. For instance, by measuring the speed of a cart of known mass hitting another stationary cart of known mass, and comparing the total momentum before and after the collision.
The experiment can be conducted in different scenarios - when the carts don't stick together after collision, when they are both moving before the collision, when the masses are not the same, etc.
Make sure to take into account factors like friction and air resistance. You can compensate for them by raising the track end so that a cart runs at a constant speed when pushed.

Dive deeper and gain exclusive access to premium files of Physics SL. Subscribe now and get closer to that 45 🌟