Chapter 2 - Force, Motion and Gravitation

Question 1

Define the equation for force

Answer 1

Force = mass x acceleration
F = ma
Force is a vector, it has magnitude and direction

Question 2

What is the force of gravity

Answer 2

F = G (m1 x m2) / r^2

Question 3

What is the equation for uniformly accelerated motion

Answer 3

x = xo + voT + 1/2 gT^2

Question 4

Define the equation for velocity

Answer 4

V = Vo + at

Question 5

A ball shot vertically has initial velocity 45m/s. Gravity pulls it back at 10m/s. How long will it take to return?

Answer 5

V = Vo + at
Vo = 45m/s
V = - 45
-45 = 45 + (10) t
-90 = 10t
9 = t

Question 6

Acceleration due to gravity is 32, what force is needed to lift object of 16 vertically with acceleration of 16?

Answer 6

w = m x g
m = w / g
m = 16 / 32 = 1/2
*the sum of forces is equal to te weight mg of object plus the force ma causing the acceleration. Have to add both because 2 values of acceleration here
sum of F = ma + mg
= (1/2 x 16) + (1/2 x 32)
= 24

Question 7

An object at rest is dropped. Velocity on impact is 10m/s. What height was it dropped from?

Answer 7

V = Vo + at
10 = 0 + (10) t
1 = t
to get length
X = Vot + at^2
= 1 + 1/2 x 10 = 5

Question 8

A rock is thrown at 60 degrees (cos 60 = 0.5). Has a velocity of 20m/s. How far after 1 sec?

Answer 8

x= (20 x 0.5)(1) = 10 m/s (1s) = 10 m

Question 9

A planet has twice Earth’s radius and is twice the mass. How would your weight differ?

Answer 9

Your weight on the planet will be halved compared to your weight on earth since:
F = (m1 m2) / r^2
where m1 is mass of planet and m2 is the mass of you
r is the distance from centre of mass of planet to you
So F is proportional to m1/r^2. So if m is doubled, so is F but if r is doubled F would be quartered but M1 and r are doubled so f is halved

Question 10

A 10kg block accelerates at 2m/s up frictionless plance of 30 degrees horizontal. What is the force acting parallel? given sin30 = 0.5

Answer 10

weight = m x g = 10 x 10 = 100
sin 30 x 100 = 50 (the force of gravity pulling down)
net force of accel on the block
m x a
10 x 2 = 20
therefore 20 + 50 = 70 is net force parallel

Question 11

When you bump a vase and it tilts before returning upright what happened to the centre of gravity and the potential energy when the vase was initially bumped?

Answer 11

The centre of gravity moved up and the potential energy increased. PE must increase so it tilts back in the direction that reduced potential energy as quickly as possible, returning it to equilibrium. Lower centre of gravity is the most stable

Question 12

A 1kg and 2kg cart are puled apart by a spring. The force of the 1kg cart is 1N. What is the force on the 2kg cart?

Answer 12

1N. Since objects exert an equal but opposite force on each other.
eg. Punching a wall with force of 10N then the wall exerts a fist of 10N on your fist.

Question 13

A cannon shell travels 4 feet horizontally for every 3 feet vertically. If the firing speed is 500m/s how far does it travel before hitting the ground?

Answer 13

its a triangle where 5 is the hypotenuse hence the ratio is 3:4:5
The vertical component is 3/5 x 500 = 300
Time to reach the ground 300 / 10 = 30s
Time shell travels in the air = 30 x 2 = 60 s
Horizontal range = 4/5 x 500 x 60 = 24,000m

Question 14

Two planets triple the distance between them, what will the ratio of the new gravitational force to the old gravitational force be between them?

Answer 14

When distance increases there will be less and less force. Meaning the gravity is 1/9th of what it was when the planets were close.
F = G (m1 m2) / r^2

Question 15

How far will a brick starting from rest fall freely in 3 seconds

Answer 15

d = vi x t + 1/2 at^2
since initial velocity is 0 the equation is 
d = 1/2 a t^2
    = 1/2 x 10 x 3^2
    = 9 x 5 = 45m

Question 16

A force of 100N pushes a 300N container across the floor. What is the force of friction acting by the floor on the container?

Answer 16

100N.