Momentum is a measure of inertia in motion Momentum is a measure of inertia in motion. Momentum is equal to mass multiplied by velocity. p = mv
A 2250 kg pickup has a velocity of 25 m/s east A 2250 kg pickup has a velocity of 25 m/s east. What is the momentum of the truck?
A moving object can have a large momentum if it has a large mass, a high speed, or both.
Can you think of a case where a roller skate and a truck would have the same momentum?
If momentum changes, either the mass of the object or its velocity or both change.
In most cases the velocity changes, not the mass In most cases the velocity changes, not the mass. A change in velocity is acceleration, which is caused by only one thing:
an unbalanced force. The greater the force, the greater the acceleration and the greater the change in momentum.
The length of time the force is applied is also important The length of time the force is applied is also important. The longer the force is applied the greater the change in momentum.
Force times time interval is called impulse. Impulse = Ft
Remember F = ma, and a = v/t. Therefore: F = m v/t, or: Ft = mv
Ft = mv, or Ft = mv Impulse is equal to change in momentum.
A 1400 kg car moving west with a velocity of 15 m/s collides with a utility pole and is brought to rest in 0.30 s. Find the magnitude of the force exerted on the car during the collision.
To increase the momentum of an object, one should apply a large force for a long period of time.
Decreasing momentum also requires an impulse. mv = Ft
A great mv can be accomplished with little force (F) if the force is applied over a large time (t).
If you lost control of your car, would you rather run into a brick wall or a stack of hay?
Other examples of extended “t”: a padded dashboard, airbags, bent-knee landings, mats for gymnasts, a glass dish dropped on a carpeted floor.
When the time of impact is extended, it reduces the force of impact.
Why is hail more damaging than rain?
Bouncing objects require a greater force to change their momentum than an object that doesn’t bounce.
The change in momentum is greater when an object bounces, therefore a greater impulse is needed.
To change the momentum of an object, an outside unbalanced force must be applied.
Internal forces are always in balanced pairs and have no effect on the total momentum of the system.
If no net force or impulse acts on a system, the momentum of that system cannot change.
This is the law of conservation of momentum: In the absence of an external force, the momentum of a system remains unchanged.
A 76 kg boater is at rest in his 45 kg boat A 76 kg boater is at rest in his 45 kg boat. If he steps out of the boat at a velocity of 2.5 m/s to the right, what is the final velocity of the boat?
Newton’s Second Law states that if no net force is exerted on a system, no acceleration occurs. Does it follow that no change in momentum occurs?
Newton’s Third Law states that the force a cannon exerts on a cannonball is equal and opposite to the force the cannonball exerts on the cannon. Does it follow that the impulse the cannon exerts on the cannonball is equal and opposite to the impulse the cannonball exerts on the cannon?
The forces involved in impulse are not constant The forces involved in impulse are not constant. The force starts small, increases to a maximum, then decreases. In calculations we use the average force.