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Physics > Forces and Motion 2 > Flashcards

Forces and Motion 2 Flashcards

1
Q

What is momentum

A

Momentum is the product of the mass and the velocity of an object

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2
Q

What is the equation for momentum

A

Momentum = mass * velocity

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3
Q

What are the units for momentum

A

kg m/s

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4
Q

Is momentum a scalar or a vector quantity

A

Vector

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5
Q

What does negative momentum mean

A

It means that the object is moving in the opposite direction

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6
Q

Calculate the momentum of a 300g ball moving at 10 m/s

A

p = m * v
p = 0.3 * 10
p = 3 kg m/s

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7
Q

What is the equation relating force and momentum

A

Force = change in momentum / time

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8
Q

How is force related to the rate of change of momentum

A

Force is directly proportional to the rate of change in momentum

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9
Q

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

A

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

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10
Q

How can you reduce the size of the force acting on a car during deceleration

A

By increasing the time it takes to bring it to rest

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11
Q

List some car safety features

A

Crumple zones
Seat belts
Air bags

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12
Q

How do crumple zones protect passengers during a crash

A

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

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13
Q

Why do parachutists bend their knees and roll on landing

A

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

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14
Q

What is a collision

A

A collision is an event in which two or more bodies exert forces on each other

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15
Q

State the principle of conservation of momentum

A

The total momentum before a collision is equal to the total momentum after the collision

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16
Q

What is the equation for the principle of conservation of momentum

A

m1u1 + m2u2 = m1v1 + m2v2

17
Q

How do u tackle problems regarding the principle of conservation of momentum

A

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

18
Q

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

A

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

19
Q

What happens in an elastic collision

A

The total kinetic energy of the objects before the collision is the same as the total kinetic energy after the collision

20
Q

Give an example of an elastic collision

A

Collisions between the molecules in a gas

21
Q

What happens in an inelastic collision

A

The total kinetic energy is not conserved, as some kinetic energy was used to cause permanent deformation of the objects

22
Q

What is an explosion

A

An explosion is an event involving a release of energy that causes bodies to move apart

23
Q

How is Newton’s 3rd law related to collisions

A

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

24
Q

Define a moment

A

A moment is the turning effect of a force about a pivot

25
Q

Are moments scalar or vector

A

Vector. They act about a point in a clockwise or anti-clockwise direction

26
Q

What is the equation for moments

A

moment = force * perpendicular distance from pivot

27
Q

What are moments measured in

A

Newton meters (Nm)

28
Q

What is the principle of moments

A

For an object to be balanced, the sum of the clockwise moments must be equal to the sum of the anti-clockwise moments

29
Q

Steps for solving problems related to the principle of moments

A
  1. Identify the forces
  2. For each force, find the distance to the pivot
  3. For each force identify whether it causes a clockwise or anti-clockwise moment
  4. Calculate the total clockwise and anticlockwise moment (use a symbol for the unknown)
  5. Equate the total clockwise and the total anticlockwise moment
30
Q

Define the center of gravity

A

The center of gravity is the point where the weight of an object appears to act

31
Q

How can you find the center of gravity of regularly shaped objects

A

The point where the axes of symmetry cross

32
Q

How can you find the center of gravity of irregularly shaped objects

A

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

33
Q

When do objects topple over when tilted

A

Objects topple over when a vertical line from their center of gravity falls outside their base

34
Q

Why are objects with a wider base more stable

A

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

35
Q

Which two conditions must be true for a beam to be in equilibrium

A
  1. The sum of upward forces must be equal to the sum of downward forces
  2. 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

Physics (7 decks)

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