Force and Momentum

Question 1

What is the equation for momentum?

Answer 1

momentum = mass x velocity

Question 2

What is the principle of the conservation of momentum?

Answer 2

The total momentum, if no external forces are acting on the objects involved, stays the same in explosions and collisions.

Question 3

What is Newton’s second law?

Answer 3

Force = rate of change of momentum

Question 4

What is Newton’s third law?

Answer 4

• For every action, there is an equal and opposite reaction.
• In an interaction of two objects there is a pair of forces acting on each object (e.g. a cat sitting on a table) and the size of the force acting on the first object is equal to the size of the force acting on the second object.

Question 5

What features are there in cars to make them safer?

Answer 5

• The air bags increase the time it takes to decelerate, which makes the force acting on the passenger smaller.
• Crumple zones are at the front and back of cars, and they collapse when a car collides with something. By collapsing, the time it takes to decelerate increases, so the impact force is smaller.
• Seat-belts are slightly stretchy, meaning that the passenger is able to move slightly forward in a collision, but without them the passenger would continue to go forward until they hit something, and that would be the front windscreen most likely. Seat-belts increase the deceleration time, so reduce the force.

Question 6

How would you find the centre of gravity of a lamina object?

Answer 6

• Cut out the shape.
• Pierce a hole in the object with a pin so that the object can move more freely.
• Hold the pun in a clamp stand and place the object on the pin - it must be able to hang without touching anything other than the pin.
• Attach a plumb line to the pin.
• Without moving the object, mark a vertical line passing through the pinhole onto the object.
• Repeat the procedure for a second hole in the object.
• Centre of gravity is where the two lines cross.
• Check by repeating for a third hole in the object.

Question 7

What links equation links force and the rate of change of momentum?

Answer 7

force = rate of change of momentum