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Forces and Newton’s Laws
1. State Newton’s 3 laws.
2. On a certain planet of radius 8900 km a 45 kg laser gun weighs 350 N.
a) Find the acceleration due to gravity.
b) Find the weight of the laser on earth
c) Find the mass of the planet
d) Find the height above the planet’s surface at which the laser weighs 300 N
3. A 100 kg man weighs 625 N on the surface of Ganeymede
a) What is the gravitational field strength on Ganymede?
b) What is the mass of someone who weighs 495 N on Ganymede?
4. 60000 Km from the center of planet X, a space ship experiences a gravitational force
of 295 N.
What will the gravitational force on the same space ship be
a) 30000 km from planet X’s center?
b) 180000 km from planet X’s center?
c) 65000 Km from planet X’s center?
5. A bathroom scale has a spring which is compressed when a person stands on the scale,
and is connected to the needle which indicates the weight. When a 67 kg person steps on
the scale the spring compresses 1.8 mm.
a) Find the spring constant
b) Find the compression for a 34 kg person
c) Find the weight of a person who causes the spring to compress 3.2 mm
6. A 50 kg sled is pulled at a constant speed by a 70 N force.
a) What is the force of friction? How do you know?
b) What is the coefficient of friction?
c) How much force is required to pull the sled with a 20 kg box on it?
7. An elastic with spring constant 89 N/m is used to pull a 2.6 kg block along a rough
surface. If the block moves at a constant speed the extension of the elastic is 7.9 cm.
a) Find the coefficient of friction.
b) Find the force required to pull the block with an acceleration of 2.3 m/s2
c) Find the acceleration if the extension of the elastic is 12 cm
d) Find the extension if the acceleration is 3.8 m/s2
8. A 873 kg dragster attains a speed of 26.3 m/s in 0.59 s
a) find the average acceleration of the car in this interval
b) find the average net force on the car
c) find the average net force on the 68 kg driver
9. A 100 kg sled is pulled along an icy surface by a force of 300 N and it accelerates at a
rate of 2 m/s2
a) Find the force of friction
b) find the coefficient of friction
10. a) How much force is needed to accelerate a 300 kg crate across a cement floor with
a coefficient of friction equal to 0.65 from rest to 10 m/s in 2.5 seconds?
b) How far does the crate get pushed in this time?
11. A 60 kg boy and a 70 kg boy are standing side by side on a frictionless ice surface.
The lighter boy pushes the heavier one with a force of 300 N for 0.1 seconds.
a) Find the acceleration of each boy
b) find the speed of each boy just after the shove
12. An astronaut finds herself stranded 10 m from the space shuttle, her 30 kg jet pack
having run out of fuel. Using Newton’s laws explain how she might return to the ship.
13. A 800 kg car traveling at 108 km/h north brakes and comes to a stop 50 m later.
Assuming the brakes locked and the car slid to a stop, find
a) the acceleration
b) the force of friction including direction
c) the coefficient of friction between the tires and the road
14. A fisherman hooks a 1.7 kg fish on a line that can only sustain a force of 38 N before
breaking. At one point the fish fights back with a force of 40 N. What is the minimum
acceleration with which you must play out the line to prevent it from breaking?
15. A 4500 kg helicopter accelerates upwards from rest gaining 25 m of elevation in 2.4
seconds. Find the lift force exerted by the rotor blades.
16. The maximum force that a grocery bag can withstand and not break is 250 N. If 20 kg
of groceries are lifted from the floor to the table with an acceleration of 5 m/s2, will the
bag hold?
17. A student stands on a bathroom scale calibrated in newtons. The scale reads 864 N
when the elevator is at rest.
a) As the elevator begins to move up, the scale reads 935 N. find the acceleration.
b) As the elevator approaches a higher floor the scale reading drops to 782 N. Find the
acceleration.
c) Explain the changes in the scale you would expect on the ride back down.
18. A 2.3x10-4 kg spider spins a web that can withstand 2.9x10-3 N. What is the
maximum acceleration with which the spider can climb the web?
19. A sled of mass 50 kg is pulled across a flat snow-field. The coefficient of static
friction is 0.10 and the coefficient of kinetic friction is 0.10
a) What minimum force is needed to get the sled moving?
b) What force is needed to keep it moving at 4.0 m/s?
c) What force is needed to accelerate it from 4.0 m/s to 6.0 m/s in 6.0 seconds?
20. What is the acceleration of a 200 kg crate which is pushed with a force of 700 N on s
surface where u = 0.20?
21. A 500 kg elevator is designed to hold 20 people of average mass 75 kg. The
maximum tensile strength of the cable holding it up is 29 600 N. What is the largest safe
acceleration?