Momentum & Impulses

SPH4U – Grade 12 Physics

Unit 4

Momentum

Momentum is a commonly used term in our every day lives. When a sports announcer says that a team has momentum they mean that the team is really on the move and is going to be hard to stop. The term momentum is a physics concept. Any object with momentum is going to be hard to stop.

Momentum

Momentum is oomph!

Momentum is NOT the same as inertia

Momentum

A force acting for a given amount of time will change an object’s momentum- it will change how difficult the object is to stop.

Recall, a net force always accelerates an object – either speeding it up, or slowing it down.

Momentum

If the force acts opposite the object’s motion, it slows the object down. If a force acts in the same direction as the object’s motion then the force speeds the object up. Either way, a force will change the velocity of an object. And if the velocity of the object is changed, then the momentum of the object is changed.

Momentum is dependent on both mass and velocity. Therefore we have the following relationship:

Momentum

Note: Momentum is measured in kgm/s We will consider linear momentum in this

class (objects moving in straight lines). Angular momentum is another matter and

will be discussed in upper year physics.

Impulses

“Change in Momentum” is different from Momentum itself.

To change the momentum of something, (to change how difficult it is to stop), you have to supply a force, and you have to apply that force over a certain period of time.

Impulses

Ex: If you push a ball for 5 seconds, not much happens to the ball’s momentum. If you push on a ball for 1 minute, a greater change in momentum happens.

Change in momentum is equal to = (Force) x (Change in time)

Impulses

These concepts are merely an outgrowth of Newton’s second law:

Impulses

When combined with the definition of acceleration (a = change in velocity / time), the following equalities result.

Impulses

This tells us that a force applied over an amount of time, t, will equal the mass times the change in velocity, which is a change in momentum.

Impulses

A change in momentum, or the force multiplied by the time, is called ‘impulse’.

Impulses

In a collision, objects experience an impulse; the impulse causes and is equal to the change in momentum.

Impulses cause a change in momentum. Just the same as a force causes a change in velocity, not velocity itself.

Practical Implications

Long barrels of cannons increase the velocity of cannonballs. The force of the exploding gunpowder in the long barrel acts on the cannon ball for a long period of time

If we decrease momentum over a longer period of time, a smaller force results.

Catching a base-ball with your bare hands will hurt less if you move your hand backward after you make contact with the ball.

Since the ball always approaches you at the same speed, its change in momentum as you stop it is always the same p = mv.

Practical Implications

Thus if you allow your hands to move with the ball as you catch it, Δt will be larger, and the force will be smaller so your hands will hurt less.

Change in momentum = force over time.

Example

Example - Solution

Homework

Read section 5.1 and supplement your notes

Answer the following questions:

Pg. 227 # 3, 4, 5, 9, 10