Newton's third law

Title : Newton's third law

Apparatus :

Science Workshop Interface 750

Motion Sensor ( PASCO Model CI 6742 ) 2

Dynamics Cart 2

Laboratory jack

Runway

Mass ( 1 set )

Metre rule

Triple Beam Balance or Digital Balance

Objective :

1. How does the change of momentum of cart 1 compare to the

change of momentum of cart 2 during collision ?

2. To show that the forces acting on each object are action -

reaction forces during collision.

Theory :

Newton's third law of motion

When two objects interact, they exert equal and opposite forces on one another

The third law expresses the idea that a force always involves two bodies and the " third-law pair " of forces act on two different bodies..

For two-object collision, momentum conservation is easily stated mathematically by the equation :

Rearranging the above equation,

In Collision, the change of momentum of each object is equal

and opposite to each other.

Suppose the time of contact during collision is

.......................... (1)

Newton’s second law of motion states that

The rate of change of momentum of an object is proportional to the resultant force which acts on the object and occurs in the direction of the resultant force.

Equation (1) becomes

It shows that the interacting forces, action and reaction, are equal and opposite and act on different objects.

In collision, the forces acting on each object are action - reaction forces.

Another method to show Newton’s third law from conservation of momentum

From Newton's second law of motion :

The slope of velocity against time graph will give the variation of acceleration during collision.

Rearranging equation (1),

It shows that

Use Motion Sensors to measure the motion of two carts before and after collision.

Compare the total momentum of the two carts before collision to the total momentum of both carts after collision.

Procedure :

Basic setup :

Connect the interface to the computer, turn on the interface, and turn on the computer.

Connect one Motion Sensor into Digital Channels 1 and 2 on the interface. Connect the yellow plug into channel 1 and the other plug into channel 2.

Connect the second Motion Sensor into Digital Channels 3 and 4 on the interface. Connect the yellow plug into channel 3 and the other plug into Channel 4.

Sensor Calibration

Place one Motion sensor exactly one meter away from a target. Arrange the sensor and target so the pulses from the sensor can reflect from the target and be detected by the sensor.

Set Calibration distance =1 m. Click the 'Calibrate' button in the Motion Sensor Window.

( For the second Motion Sensor, set the Calibration distance = -1 m )

Equipment Setup :

Place the runway ( track ) on a horizontal surface.

Level the track by placing a cart on the track. ( Use the adjustable feet at one end of the track to raise or lower that end until the cart will not roll one way or the other. )

Use the Triple Beam Balance to find the mass of each cart.

Place a cart at each end of the track. Let the cart on the left be 'cart 1' and the cart on the right be 'cart 2'.