Forces and Motion 2

Question 1

What is momentum

Answer 1

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

Question 2

What is the equation for momentum

Answer 2

Momentum = mass * velocity

Question 3

What are the units for momentum

Answer 3

kg m/s

Question 4

Is momentum a scalar or a vector quantity

Answer 4

Vector

Question 5

What does negative momentum mean

Answer 5

It means that the object is moving in the opposite direction

Question 6

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

Answer 6

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

Question 7

What is the equation relating force and momentum

Answer 7

Force = change in momentum / time

Question 8

How is force related to the rate of change of momentum

Answer 8

Force is directly proportional to the rate of change in momentum

Question 9

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

Answer 9

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

Question 10

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

Answer 10

By increasing the time it takes to bring it to rest

Question 11

List some car safety features

Answer 11

Crumple zones
Seat belts
Air bags

Question 12

How do crumple zones protect passengers during a crash

Answer 12

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

Question 13

Why do parachutists bend their knees and roll on landing

Answer 13

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

Question 14

What is a collision

Answer 14

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

Question 15

State the principle of conservation of momentum

Answer 15

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

Question 16

What is the equation for the principle of conservation of momentum

Answer 16

m1u1 + m2u2 = m1v1 + m2v2

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

What happens in an elastic collision

Answer 19

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

Question 20

Give an example of an elastic collision

Answer 20

Collisions between the molecules in a gas

Question 21

What happens in an inelastic collision

Answer 21

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

Question 22

What is an explosion

Answer 22

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

Question 23

How is Newton’s 3rd law related to collisions

Answer 23

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

Question 24

Define a moment

Answer 24

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

Question 25

Are moments scalar or vector

Answer 25

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

Question 26

What is the equation for moments

Answer 26

moment = force * perpendicular distance from pivot

Question 27

What are moments measured in

Answer 27

Newton meters (Nm)

Question 28

What is the principle of moments

Answer 28

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

Question 29

Steps for solving problems related to the principle of moments

Answer 29

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

Question 30

Define the center of gravity

Answer 30

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

Question 31

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

Answer 31

The point where the axes of symmetry cross

Question 32

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

Answer 32

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

Question 33

When do objects topple over when tilted

Answer 33

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

Question 34

Why are objects with a wider base more stable

Answer 34

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

Question 35

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

Answer 35

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