# Newton's Laws -- Your best tip

Tags:
1. Mar 9, 2016

### Ameer Bux

Hi guys. I am planning to get 100 percent for my upcoming physics test in two days time. I am i grade 11. The test is based on newtons 3 laws and universal law of gravitation.

What are some of the best tips you can offer from your experience to help me achieve my goal?

2. Mar 9, 2016

### Staff: Mentor

Do lots of practice problems.

3. Mar 9, 2016

### PhanthomJay

> master the art of drawing free body diagrams
> identify all forces acting in that diagram and apply Newton 1 for bodies in equilibrium (at rest or moving at constant velocity) or Newton 2 for accelerating bodies, with the wisdom to know the difference
> Do not confuse Newton 3 with Newton 1 or Newton 2. Newton 3 refers to equal and opposite forces acting on different bodies, whereas Newton 1 and 2 refer to net forces acting on the same body.

(I like your Avitar. Sort of like "Now that my house has burned down, I have a better view of the moon").

4. Mar 10, 2016

### Ameer Bux

Did 40 problems so far on newtons first 3 laws. Going to start with universal law of gravitation in 30 mins.

5. Mar 10, 2016

### Ameer Bux

Thanks a lot. Do you guys know a quick way to solve a proportionality problem in newtons law of gravitation? I need some steps/method to follow. I don't know how to approach these kind of problems.

Example of a problem like that is as follows : A large planet has a radius 10x that of the earth and a mass of 300x that of the earth.
a.) if the weight of a body is 500N on earth what will be the weight on this planet?
b.) compare strenght of gravitational fields at the surface of the 2 planets.

6. Mar 10, 2016

### PhanthomJay

The weight of an object on the surface of a particular planet is GmM/r^2, where G is the universal gravitational constant, m is the mass of the object, M is the mass of the planet, and r is the radius of the planet. So weight varies directly linearly with the planet mass, and inversely as the square of its radius. So in comparison to earth, if the planet mass increases 300 fold, and the planet's radius increases by 10 fold, then the object's weight increases by 300/10^2 = 300/100, that is, by a factor of 3. So the 500 N body weight on earth will weigh 1500 N on that planet. So the gravitational field increases by 3.

You should note that since Weight = mg, then using the above formula yields g = GM/r^2, which , on planet earth, works out to g = 9.8 m/s^2, the acceleration of gravity. On the massive planet, g would be 9.8(3) or 29.4 m/s^2.