Forces and Motion 2 Flashcards
What is momentum
Momentum is the product of the mass and the velocity of an object
What is the equation for momentum
Momentum = mass * velocity
What are the units for momentum
kg m/s
Is momentum a scalar or a vector quantity
Vector
What does negative momentum mean
It means that the object is moving in the opposite direction
Calculate the momentum of a 300g ball moving at 10 m/s
p = m * v
p = 0.3 * 10
p = 3 kg m/s
What is the equation relating force and momentum
Force = change in momentum / time
How is force related to the rate of change of momentum
Force is directly proportional to the rate of change in momentum
A 1500 kg car is traveling with a speed of 20 m/s. A braking force is applied to bring the car to a complete stop within 10 seconds. Calculate the braking force
force = change in momentum / time
change in momentum = final p - initial p
initial p = 1500 * 20 = 30 000 kg m/s
change in momentum = 0 - 30 000 = -30 000
force = -30 000 / 10 = -3000
How can you reduce the size of the force acting on a car during deceleration
By increasing the time it takes to bring it to rest
List some car safety features
Crumple zones
Seat belts
Air bags
How do crumple zones protect passengers during a crash
Crumple zones are areas of a vehicle that are designed to collapse during a collision, increasing the time it takes for the car and the driver to come to a stop. Since the change in momentum remains the same, the rate of change of momentum is reduced. According to Newton’s 2nd law, the force acting on the object and the rate of change of its momentum are equal. Thus the force is also reduced, leading to passengers suffering less severe injuries
Why do parachutists bend their knees and roll on landing
Parachutists have a lot of momentum, and thus a lot of force is exerted on them when landing. By bending their knees and rolling, the time for which the force is acting on them increases, leading to the rate of change of their momentum decreasing. Since, the force acting on the parachutist is proportional to the rate of change of momentum, the force is also reduced, allowing for a smoother landing
What is a collision
A collision is an event in which two or more bodies exert forces on each other
State the principle of conservation of momentum
The total momentum before a collision is equal to the total momentum after the collision
What is the equation for the principle of conservation of momentum
m1u1 + m2u2 = m1v1 + m2v2
How do u tackle problems regarding the principle of conservation of momentum
1) Determine the total momentum before the collision (remember that one object might have a negative momentum)
2) Write an equation for the total momentum after
3) Equate the total momentum before and after the collision and solve for the unknown
Two cars are involved in a head-on collision. Car A has a mass of 1,200 kg and is traveling at 20 m/s towards Car B. Car B has a mass of 1,500 kg and is traveling at 15 m/s in the opposite direction of Car A. After the collision, Car A reverses direction and moves at a speed of 10 m/s. Calculate the velocity of Car B
total momentum before = (1200 * 20) + (1500 * -15)
= 24 000 - 22 500 = 1500
total momentum after = (1200 * - 10) + (1500* -v)
= -12000 + 1500v
-12 000 + 1500v = 1500
1500v = 13500
v = 9
What happens in an elastic collision
The total kinetic energy of the objects before the collision is the same as the total kinetic energy after the collision
Give an example of an elastic collision
Collisions between the molecules in a gas
What happens in an inelastic collision
The total kinetic energy is not conserved, as some kinetic energy was used to cause permanent deformation of the objects
What is an explosion
An explosion is an event involving a release of energy that causes bodies to move apart
How is Newton’s 3rd law related to collisions
In a collision, two objects exert forces on each other. According to Newton’s 3rd law, every action has an equal and opposite reaction. Thus, this two forces have the same size but are opposite in direction
Define a moment
A moment is the turning effect of a force about a pivot
Are moments scalar or vector
Vector. They act about a point in a clockwise or anti-clockwise direction
What is the equation for moments
moment = force * perpendicular distance from pivot
What are moments measured in
Newton meters (Nm)
What is the principle of moments
For an object to be balanced, the sum of the clockwise moments must be equal to the sum of the anti-clockwise moments
Steps for solving problems related to the principle of moments
- Identify the forces
- For each force, find the distance to the pivot
- For each force identify whether it causes a clockwise or anti-clockwise moment
- Calculate the total clockwise and anticlockwise moment (use a symbol for the unknown)
- Equate the total clockwise and the total anticlockwise moment
Define the center of gravity
The center of gravity is the point where the weight of an object appears to act
How can you find the center of gravity of regularly shaped objects
The point where the axes of symmetry cross
How can you find the center of gravity of irregularly shaped objects
Hang the object from a point so that it can move freely. Then, by using a plumb line, draw a vertical line through the point of support. Repeat that from another hanging point. The point where the two lines intersect is the center of gravity
When do objects topple over when tilted
Objects topple over when a vertical line from their center of gravity falls outside their base
Why are objects with a wider base more stable
An object having a wider base makes it more difficult for it to topple over, as the vertical line from its center of gravity is less likely to fall outside the base
Which two conditions must be true for a beam to be in equilibrium
- The sum of upward forces must be equal to the sum of downward forces
- The sum of clockwise moments acting on the beam at any point must be equal to the sum of anti clockwise moments acting on the beam
